The ecology of the deep-sea benthic and benthopelagic fish on the slopes of the Rockall Trough, Northeastern Atlantic

Prog. Oceanog. Vol. 15, pp. 037-069, 1985.

0079-6611/85 $0.00 + .50 Copyright © 1985 Pergamon Press Ltd.

Printed in Great Britain. All rights reserved.

The Ecology of the Deep-sea Benthic and Benthopelagic Fish on the Slopes of the Rockall Trough, Northeastern Atlantic J. D. M. G O R D O N and J. A. R. D U N C A N

Scottish Marine Biological Association, P.O. Box No. 3, Oban, Argyll, Scotland Abstraet-A total of over 28,000 benthic and benthopelagic fish belonging to 34 families and comprising at least 85 species were collected from the Hebridean Terrace in the Rockall Trough between soundings of 500 and 2000 m. Commercial type trawls (20.6 m Granton or 140 foot German bottom trawls) fished on paired warps at 33 stations accounted for 89% of all individuals caught, the remainder being caught by a 16.4 m prawn trawl fished on a single warp (22 stations) and a 3 m Agassiz trawl (12 stations). The stations sampled, with a few exceptions, fell into discrete bathymetric zones separated by increments of approximately 250 m and different combinations of nets were used at each of these zones. The catch composition of the commercial trawls differed from those of the other nets. The most obvious difference was that squalid sharks, the alepocephalid Alepocephalus bairdii and the black scabbard fish Aphanopus carbo were important in the commercial type trawls but were absent or poorly represented in the other nets. Net size and towing speed were considered to be important factors influencing the catch composition. Net selectivity was most apparent on the upper and mid slopes but less apparent on the lower slopes. Relatively few families contribute to the total biomass at a given bathymetric zone and because the families Squalidae and Alepocephalidae contribute significantly to the biomass on the upper and mid slope it is therefore concluded that small nets must grossly underestimate the biomass at these depths. The greatest biomass occurred at mid slope depths (750-1000 m).

CONTENTS 1. Introduction 2. Materials and Methods 2.1. Sampling area 2.2. Sampling methods 3. Results and Discussion 3.1. Catch composition by bathymetric zone 3.2. Bathymetric changes in biomass using different trawls Acknowledgements References

37 39 39 39 41 49 59 67 68

1. I N T R O D U C T I O N C O M P A R E D w i t h other areas o f the eastern N o r t h Atlantic such as o f f West Africa, a r o u n d Madeira, the Bay o f Biscay and o f f the s o u t h west o f Ireland c o m p a r a t i v e l y little is k n o w n o f the deep-water bottom-living fish o f the Rockall Trough. This is all the m o r e surprising since s o m e o f the earliest deep-sea dredgings were carried o u t in the n o r t h e r n part o f the Rockall Trough by H.M.S. Lightning in 1868. A further series o f dredgings were carried o u t by H.M.S. Porcupine in 1869 across the RockaU T r o u g h and later in the same year on either side o f the W y v i U e - T h o m s o n Ridge, a l t h o u g h at this t i m e the existence o f the ridge was u n k n o w n (THOMSON, 1874). Interest in the separation o f w a r m and cold water in this n o r t h e r n part o f the Rockall Trough led to further e x p e d i t i o n s by H.M.S. Knight Errant in 1880 and H.M.S. Triton 37

38

J.D.M. GORDON and J. A. R. DUNCAN

in 1882 which established the existence of the Wyville-Thomson Ridge and also afforded the opportunity for further dredgings on both sides of the ridge. MURRAY (1886) has summarized the fauna obtained from all the dredgings on these expeditions and lists some of the fish specimens identified at that time. The stations worked on the northern part of the Rockall Trough ranged in depth from 374 to 555 fathoms (684 to 1015 m). GONTHER (1887)in his report on the fishes collected by the CHALLENGER Expedition has included the fish caught by H.M.S. Knight Errant and H.M.S. Triton. The next expedition was that of the Michael Sars in 1910 (MURRAY and HJORT, 1912) but although a number of stations were fished in the vicinity of the Faroes only one trawling was made in the Rockall Trough at a depth of 1853 m (KOEFOED, 1927). Meanwhile considerable progress was being made on the deep-water fish populations of the Irish Atlantic Slope (HOLT and CALDERWOOD, 1895; HOLT and BYRNE, 1906, 1908, 1910a, b, 1911, 1913; and FARRAN, 1924). Although these surveys were in the Porcupine Sea Bight and other areas to the south west of Ireland they were important in as much that the fish caught have much in common with the fish of the Rockall Trough. In 1927 exploratory fishing for hake on the upper slopes of the Rockall Channel prompted HICKLING (1928) to record the deep-water fishes encountered. The Scottish Atlantic slope was surveyed between depths of 118 and 520 fathoms (215 and 951 m), the Rockall Bank between 132 and 285 fathoms (241 and 521 m) and the Faroe-Shetland Channel between 89 and 560 fathoms (162 and 1024m). Similarly BLACKER (1962)recorded some of the rarer fish encountered during exploratory fishing down to depths of 280 fathoms ( 5 1 2 m ) i n the Rockall Trough and other areas. A further survey of the slopes of the Rockall Trough and other areas to the west of the British Isles by the Ministry of Agriculture, Fisheries and Food (MAFF) was carried out in 1973 and 1974 at depths from 300 to 600 fathoms (548-1097m) and BRIDGER (1978) has published some information on species which may have some commercial importance. The Institut fiir Seefischerei (ISH) in Hamburg has also been interested in the commercial exploitation of the upper slopes of the Rockall Trough and the surrounding banks and some information on the fish catches of dominant species has been published by FREYTAG and MOHR (1974); WAGNER and STEHMANN (1975); MOHR and FREYTAG (1975); EHRICH and CORNUS (1979) and EHRICH (1980). The overall results of these surveys have been summarized by EHRICH (1983). The transfer of the laboratories of the Scottish Marine Biological Association (SMBA) to Oban on the west coast of Scotland and the availability of the research vessel R.R.S Challenger for frequent cruises led to a seasonal study of the slope fishes to the west of Scotland. The area chosen was within the area designated by BRIDGER (1978) as potentially productive. The survey began in 1975 with stations being worked at approximately bi-monthly intervals at depths corresponding to 700 and 1000 m. From 1976 until 1978 additional stations at 500 and 1250 m were fished. It soon became apparent that many of the life stages of some of the important species were not being sampled because the large paired warp trawl was limited to a depth of 1250 m. Accordingly, in 1977 the depth range was increased to 2000 m by the use of a small prawn trawl fished on a single warp. Occasionally an Agassiz trawl was used for benthic sampling or for fishing in poor weather; fish from these samples have also been included in this paper. Beam trawls and small otter trawls fished on a single warp have been the most common method of sampling deep-sea bottom living fish e.g. DAY and PEARCY (1968); MARKLE and MUSICK (1974); HAEDRICH, ROWE and POLLONI (1975, 1980); MERRETT and

Ecology of deep-sea fish of the RockaU Trough

39

MARSHALL (1981) and PEARCY, STEIN and CARNEY, (1982). All these authors have been aware of the limitations of small trawls but the only investigation which compares the catches of these nets with commercial type trawls is that of PEARCY, STEIN and CARNEY (1982). They compared the biomass of fish on the slope off Oregon obtained by beam trawls with that obtained by large commercial trawls fished both on paired warps and single warps (ALVERSON et al., 1964; and ALTON, 1972). They concluded that on the shelf and the upper slope beam trawls greatly underestimated the biomass, but with increasing depth the differences between the two nets became less. They attributed these differences to the change in species composition with depth and speculated that the larger, faster swimming fish of the shelf and upper slope tend to avoid capture by small nets while the largest deepwater fish may for a variety of reasons be more susceptible to capture. In the northeastern Atlantic large commercial trawls fished on paired trawl warps have been used to at least 1200 m (BRIDGER, 1978; EHRICH, 1983) but the published results have only been concerned with the species of commercial or potentially commercial interest. In this paper we report on the total catch of large trawls at depths from 500 to 1750m and compare them with the catches obtained by small otter trawls and Agassiz trawls fished at depths from 750 to 2000 m. 2. MATERIALS AND METHODS 2.1. Sampling area The Rockall Trough (Fig. 1) is generally considered to be the area of the northeast Atlantic bounded by the eastern slope of the Rockall and George Bligh Banks, the southern slopes of the Lousy, Bill Bailey and Faroe Banks, the Wyville Thomson Ridge, the western slopes of the Scottish mainland and the western slopes of the Porcupine Bank. The bathymetry of the Rockall Trough has been described by ROBERTS, HUNTER and LAUGHTON (1979), the marine geology by ROBERTS (1975) and LONSDALE and HOLLISTER (1979) and the hydrography by ELLETT and MARTIN (1973). The trawling area was on the Hebridean Terrace and Barra Fan bounded by latitude 56o20 ' to 56°50'N and longitude 09o05 ' to 10°05'W. Although temperature was not recorded at the time of trawling, ELLETT (1978) has published sub-surface temperatures to approximately 1400m depth over a period from March 1975 to November 1978 for a station at 57°06'N 09°25'W, a little to the north of the working area. ROBERTS (1975) has described the bottom sediment of the Barra Fan as being of terrigenous origin and the fan is formed because the shelf sediment transport paths are perpendicular to the slope and there are no basins on the shelf such as occur further north to act as sediment traps. PEARCE (1980) has analysed the sediment structure of undisturbed bottom cores obtained by the SMBA multiple corer from a transect of stations corresponding to the fishing stations on the Hebridean Terrace and the Barra Fan. The surface sediments down to 1000 m are clastic and have a predominance of fine sand. From 1250 to 2500 m the sediments are of Globigerina ooze with an increasing abundance of fine sand with depth. Spot temperature and salinity measurements for these stations are also given. 2.2. Sampling methods Full details of the trawls and the station positions have been given by GORDON and DUNCAN (1983). The Granton trawl which had a headline and footrope of 20.6m was fished on

40

J.D.M. GORDON and J. A. R. DUNCAN

lO°W

% 60°N

©

M~'PLING ~J.7~REA

~ ~,'~:)

8

~1/' ~ ,/'@

~IRELAND

)/l)"c I I,

Isob~h~i i~ rnetres

FIG. 1. The bathymetry of the Rockall Trough and its surrounding banks. The sampling area is shaded. paired trawl warps. The central section of the footrope (7.2 m) was fitted with 380 mm solid rubber bobbins. The mesh size decreased from 140 mm in the wings to 40 mm in the codend. The codend was lined with a fine mesh blinder of 12 mm mesh and this was used for all hauls except 1/79/38 and 1/79/39. Standard 9 foot (2.7x 1.4m) single keel "Fearnought" otter boards were used. The bridles between the otter boards and the dan lenos were 5 0 m but on some earlier cruises were reduced to 12 m for ease of handling the trawl. Wherever possible a

Ecology of deep-sea fish of the Rockall Trough

41

warp to depth ratio of 3 : 1 was used and the net was towed at 3.5 to 4 knots (1.86 to 2.0 m/ sec). This trawl could only be fished to a depth of 1250m by R.R.S. Challenger and further information at 1500 and 1750 m depth was obtained by a 140 foot Bottom trawl (BT)fished with paired warps by the German Fishery Research Ship 'Walther Herwig'. The single warp trawl was an '8 fathom' box trawl with a headline of l l . 4 m and a footrope of 16.4m. No flotation was used on the headline and the footrope was covered with 76 mm rubber discs interspersed with 152 mm rubber discs. The mesh size decreased from 88 mm in the wings to 70 mm in the codend, but after initial trials a 5.5 m codend extension of 16mm mesh was added. Standard 1525 x 907 mm "V" doors weighing 118 kg were connected to a swivel and the main warp by a pair of 50 m bridles. It was towed at between 2 and 2.5 knots (1.03 to 1.28 m/sec) after allowing it to settle on the bottom at a ship's speed of about 0.5 to 1 knot (0.26 to 0.51 m/sec) for about 0.5 hr. The Agassiz trawl had a width of 3 m and a mesh size of approximately 10 mm. As far as possible the trawls were fished at bathymetric zones of approximately 250 m intervals between 500 and 2000 m on the Hebridean Terrace. Some hauls were only partially successful either because they did not conform to a specific bathymetric zone or because insufficient warp was paid out. The results presented in this paper are based on 31 Granton trawls, 2 trawls by the 140 foot German trawl, 22 single warp trawls and 12 Agassiz trawls. 3. RESULTS AND DISCUSSION Full data on the number and weight of each species at each station has been given by GORDON and DUNCAN (1983). The nomenclature follows HUREAU and MONOD (1979) except for the macrourids which follow IWAMOTO and STEIN (1974). The following species, which are considered to be principally mesopelagic or bathypelagic in their ecology, have been excluded from further consideration; Gonostoma bathyphilum; Argyropelecus olfersi; Boro-

stornias antarcticus; Chauliodus sloani; Stomias boa ferox; Bathylagus euryops; Myctophum punctatum ; Benthoserna glaciale ; Lampanyctus crocodilus ; L. macdonaldi ; Semvomer beani ; Melanonus zugmayeri; Scopelogadus beanii; Poromitra crassiceps; Anoplogaster cornuta; Chiasmodon niger and Melanostigma atlanticum. The remaining collection amounts to some 28,150 specimens belonging to 34 families and comprising at least 85 species. Table 1 shows the depth distribution of each species. It could be argued that some of the species included in this list are more representative of the pelagic fauna and indeed any such division must be arbitrary. The inclusion of species such as Aphanopus carbo, Micromesistius poutassou, Xenodermichthys copei is because their presence on the bottom has been documented elsewhere (EHRICH, 1983; MARKLE and WENNER, 1979). Others are included on the basis of factors such as diet, predation and body size. These fish were caught by a number of different nets and while some bathymetric zones were sampled by several of the nets, others were only sampled by one net. Before attempting any discussion of changes in species associations and btomass with depth or season, it is first necessary to consider the effectiveness of the sampling techniques and how far it is valid to compare catches by the different nets. The Granton trawl and the 140 foot BT, because of their large size and high towing speeds, are likely to be the most effective nets for catching the large, mobile fishes although the large meshes must also mean that they are selective against small species and the juveniles of larger species. In fishery biology catch rates are often expressed in terms of number or weight of fish

Alepocephalus rostratus Risso, 1820 Alepocephalus agassizi Goode and Bean, 1883 Alepocephalus bairdii Goode and Bean, 1879

Alepocephalidae

Rhinochimaera atlantica Holt and Byrne, 1909 Harriotta raleighana Goode and Bean, 1895

Rhinochinlaeridae

Chimaera monstrosa Linnaeus, 1758 Hydrolagus affinis (Capello, 1867) Hydrolagus mirabilis (Col/ett, 1904)

Chimaeridae

Raja clavata Linnaeus, 1758 Ra/a ? nidarosiensis Storm, 1881 Ra]a circularis Couch, 1838 Raja fyllae Liitken, 1888 Ra/a bigelowi Stehmann, 1978 Raja bathyphila Holt and Byrne, 1908 Ra]a kreffti Stehmann, 1977

Rajidae

Squalus acanthias Linnaeus, 1758 Centroscyllium fabricii (Reinhardt, 1825) Centroscymnus crepidater (Bocage and CapeUo, 1864) Centroscymnus coelolepis (Bocage and Capello, 1864) Deania calcea (Lowe, 1839) Etmopterus spinax (Linnaeus, 1758) Etmopterus princeps Collett, 1904 Lepidorhinus squamosus (Bonnaterre, 1788) Scymnodon ringens Bocage and Capello, 1864 Scymnorhinus licha (Bonnaterre, 1788)

Squalidae

Apristurus spp. Galeus melastomus Rafinesque, 1810 Galeus murinus (CoUett, 1904)

Scyliorhinidae

+

q-

+

+ +

+

+

+

500

+

+

+

-b

+

+

+

+

+

+ +

+ + +

1000

+

+ +

+ + + + + + + +

+

750

+

+

+ +

-I-

+

4-

+

+ + +

+

1250

B a t h y m e t r i c Zone

+ +

+

+ -t-

+

+ + +

+

1500

+ +

+

+

+

1750

+

+

+

2000

T A B L E I . THE DEPTH DISTRIBUTION O F T H E B O T T O M LIVING FISH O F T H E R O C K A L L T R O U G H BY PRESENCE O R A B S E N C E I N B A T H Y M E T R I C ZONES

Z ¢3

~7

o 0 ~Z

0

Narcetes stomias (Gilbert, 1890) Rouleina attrita (Vail/ant, 1888) Xenodermichthys copei (Gill, 1884) Searsidae Normiehthys operosus Parr, 1951 Sagamichthys schnakenbecki (Krefft, 1953) Argentinidae Argentina silus (Ascanius, 1775) Chlorophthalmidae Bathypterois dubius Vaillant, 1888 Scopelosauridae Scopelosaurus lepidus (Krefft and Maul, 1955) Paralepididae Notolepis rissoi (Bonaparte, 1840) Congridae Conger conger ([Artedi, 1738] Linnaeus, 1758) Synaphobranchidae Synaphobranchus kaupi Johnson, 1862 llyophis blachei Saldanha and Merrett, 1982 Halosauridae Halosauropsis macrochir (Giinther, 1878) Notacanthidae Notaeanthus chemnitzii Bloch, 1788 Notacanthus bonapartei, Risso, 1840 Polyacanthonotus rissoanus (Filippi and Verany, 1859) Macrouridae Trachyrincus murrayi (Giinther, 1887) Nezumia aequalis (Giinther, 1887) Malacocephalus laevis (Lowe, 1843) Coelorinchus coelorinchus (Risso, 1810) Coelorinchus occa (Goode and Bean, 1885) Coryphaenoides rupestris Gunnerus, 1765 Coryphaenoides guentheri (Vaillant, 1888) Coryphaenoides (Nematonurus ) armatus (Hector, 1875) Coryphaenoides (Chalinura) brevibarbis (Goode and Bean, 1896) Coryphaenoides (Chalinura) mediterraneus (Giglioli, 1893) +

+

+

+

+

750

+

500

TABLE 1. CONTINUED

+

1000

+

+

4+

+

1250

Bathymetric Zone

+

+

+

1500

+

+

1750

+

2000

4~

e~

,-q

O

Merlucciidae Merluccius merluccius (Linnaeus, 1758) Gadidae Gadiculus argenteus thori J. Schmidt, 1914 Micromesistius poutassou (Risso, 1826) Trisopterus esmarki (Nitsson, 1855) Brosme brosme (Ascanius, 1772) Molva molva (Linnaeus, 1758) Molva d. dypterygia (Pennant, 1784) Phycis blennoides (Briinnich, 1768) Antonogadus macrophthalmus (Giinther, 1867) Moridae Antimora rostrata Giinther, 1878 Halarygyreus/ohnsonii Giinther, 1862 Lepidion eques (Giinther, 1887) Mora moro (Risso, 1810) Berycidae Beryx decadac tylus Cuvier, 1829 [ April] Trachichthyidae Hoplostethus atlanticus Collett, 1889 Apogonidae Epigonus telescopus (Risso, 1810) Sparidae Pagellus bogaraveo (Briinnich, 1768) Gempylidae Nesiarchus nasutus Johnson, 1862 Trichiuridae Aphanopus carbo Lowe, 1839 Zoarcidae Lycodes spp. Reinhardt, 1831 Ophidiidae Spectrunculus grandis (Giinther, 1877) Bythitidae Cataetyx laticeps Koefoed, 1927 + +

+ +

÷ +

-I+ + "t-

+ +

+

+

-t-

+ +

+

+

+

+ +

+ +

+ +

+

"t+

+

+

+

+ +

+

+

+

1000

+

+

+

750

+

500

TABLE 1. CONTINUED

+

+

+

-I-

-t-

1500

+

-I-

+

"t+

1250

Bathymetric Zone

+

+

-I-

1750

+

+

+

2000

t~

0

~r

-~

4~.

Scorpaenidae Helicolenus d. dactylopterus (Delaroche, 1809) Sebastes mentella Travin, 1951 Sebastes viviparus KrCyer, 1845 Cottunculidae Cottunculus thomsoni (GUnther, 1882) L~aridae Paraliparis hystrix Merrett, 1983 Scophthalmidae Lepidorhombus whiffiagonis (Walbaum, 1792) Lepidorhombus boscii (Risso, 1810) Pleuronectidae Glyptocephalus cynoglossus (Linnaeus, 1758) Lophiidae Lophius piscatorius Linnaeus, 1758. + -I-t-b

+ -I+

+

+

-t-

+

+

-I-

750

+

500

TABLE 1. CONTINUED

-k

-I-

+

+

+

+

+

1000

-I-

+

1250

Bathymetric Zone 1500

1750

2000

~r

o

O O

46

J.D.M. GORDONand J. A. R. DUNCAN

trawled over a given time interval while in deep-sea studies they have frequently been expressed as number or weight per area of sea floor swept (HAEDRICH, ROWE and POLLONI, 1975; MERRETT and MARSHALL, 1981 ; PEARCY, STEIN and CARNEY, 1982). The latter is easy to calculate for a net with a fixed opening such as a beam trawl or an Agassiz trawl provided the towing speed and the time on the bottom is known. The calculation of the area swept by an otter trawl (e.g. Granton trawl) is more difficult since, in addition to the width of the path swept by the net itself, the bridles have an important role in herding fish into the path of the net. These problems have been discussed by TRESCHEV (1978) who has defined the area swept by the net alone as the fished area and that by the net and bridles as the covered area. Since the herding action of the bridles is more species-specific than that of the net itself he advocates the use of fished area in calculating catch rates. The path width used for the calculation of the fished area of an otter trawl is often assumed to be 5/8th of the headline length (MERRETT and MARSHALL, 1981). A. Corrigal of the Marine Laboratory of the Department of Agriculture and Fisheries for Scotland, has supplied the following information on the Granton trawl used in this study. When correctly rigged with 50 m bridles between the otter doors and the dan leno, the width of the path swept by the net and the net and bridles was 12.6 and 36.6 m respectively. The angle of attack of the bridles would be close to the optimum for efficient herding and the headline height would be approximately 1.8 m. The shorter bridles used at the start of the survey would increase the angle of attack of the bridles and as a consequence would increase the path width of the net to about 15 m, reduce the width between the otter doors to 30.5 m and lower the headline height to about 0.9 m. The distance travelled by the trawl on the bottom was calculated as the product of time on the bottom and the speed. The time on the bottom for the Granton trawl was simply the time from when paying out ceased until the start of hauling, which with heavy gear, was considered to be a fair approximation of the true time on the bottom. The only estimate of the speed over the bottom was the speed of the ship through the water which is subject to error caused by sea conditions and tides. Figure 2 shows the catch rates of each of the Granton trawls expressed as kg/1000m 2 plotted against depth. The swept area was calculated using path width of the net and a mean speed of 3.75 knots (1.9 m/sec). The solid lines connect trawls from the same cruise using the net with short bridles, and there were considerable differences between cruises. All the trawls were of approximately 90 minutes duration. The largest catch at the 500 m bathymetric zone was almost entirely due to the blue whiting (Micromesistius poutassou) which accounted for 97.4% of the total number of individuals and 95.6% of the total weight. The dashed lines connect trawls of the same cruise using the 50m standard bridles and a towing time of 45 minutes. One cruise in April/May 1978 differed considerably from the others because a large catch of spawning Alepocephalus bairdii was taken at 1000m and accounted for 51.5% of the total number of individuals and 73.9% of the total weight of the catch. A. bairdii was absent from the catch at the 750m bathymetric zone and present in above average numbers at the 1250 m bathymetric zone. There is obviously great variability between trawls at the same bathymetric zone and it is perhaps useful to speculate on some of the reasons for this variability. Seasonal events such as the spawning aggregations of blue whiting or A. bairdii may be an important factor (see above). Annual cycles of abundance cannot be excluded but there are insufficient data in this survey to even speculate on this aspect. It is clear from Fig. 2 that with a few exceptions the catches made by the net with the short bridles are smaller than when long bridles were used, despite the fact that in the former the net was towed on the bottom for twice as long and

Ecology of deep-sea fish of the Rockall Trough

kg. I 0 0 0

47

m -2

5

I0

[5

]

I

I

11/711- ":

r

ooo ' " "

/

""

I

(f/~/p

,s

..

., ,,.o

28,3

c s ~" ~

1500

FIG. 2. The catch rate expressed as kg/1000m 2 for all the Granton trawls by bathymetric zone. The solid lines and circles connect trawls of the same cruise with short bridles and the dashed lines and open circles connect trawls with standard bridles.

the path width between the wing ends was greater. There is obviously a chronological difference between the use of the two rigs of the nets and it is interesting to speculate that ship handling may provide a partial explanation. At the start of the survey the normal trawling practice of towing with the ship's head to wind, as far as was compatible with following a depth contour, was adopted. It soon became apparent that R.R.S. Challenger was slightly underpowered for towing the Granton trawl into strong winds or a heavy sea and had difficulty maintaining the desired speed of 3.5 to 4 knots (1.8 to 2.0 m/sec). On later cruises, the strategy of towing stern to wind in all but the calmest conditions was adopted. Underestimating the ship's speed could therefore have an effect on the estimate of the area of sea bed covered by a trawl and hence the estimate of biomass. It is probable, however, that the differences in the rig of the net are responsible for the differing catch rates. Shortening the bridles, even though it increases the path width will decrease the number of fish herded into the net. The herding action will be species-specific since it is well known from shelf studies that some species are more susceptible to herding than others. Another effect of shortening the bridles is to reduce the headline height and indeed by recalculating the biomass in terms of kg/1000 m a, based on the estimates of headline height given above, it can be shown that differences between the two rigs are reduced. MARSHALL and MERRETT (1977) have shown that a substantial proportion of bottom-living deep-sea fish are benthopelagic and support for this comes from feeding studies in the Rockall Trough (MAUCHLINE and GORDON, 1983a, b, 1984a, b, in press). Increased headline height while having little or no influence on the catches of truly benthic species will selectively favour the capture of benthopelagic species. Another approach to assess whether there are differences between the two methods of fishing the Granton trawl is to compare individual catches using the Bray-Curtis or Czekanowski coefficient which when standardized by percentage becomes the percentage similarity (BOESCH, 1977). Examples of the use of percentage similarity in deep-sea fish studies are DAY and PEARCY (1968), MARKLE and MUSICK (1974), HAEDRICH, ROWE and POLLONI (1975, 1980) and PEARCY, STEIN and CARNEY (1982). Table 2 shows in matrix form the JP0 15:I-D

48

J . D . M . GORDON and J. A. R. DUNCAN

TABLE 2. THE PERCENTAGE SIMILARITY FOR ALL THE GRANTON TRAWL CATCHES GROUPED IN BATHYMETRIC ZONES TO COMPARE TRAWLS WITH SHORT AND LONG BRIDLES [Short bridles (S), Long bridles (L)] 500 m Bathymetric zone (458-542 m)

1 2 3 4 5

Station No.

Month

5B/76/13 9/76/19 16/76/23 16/76/25 7/77/29

Apr. June Oct. Oct. May

1

2

3

4

5

10

5 50

10 62 65

6 54 44 55

S S L L L

2

3

4

5

6

7

8

9

10

11

57

54 55

46 51 54

56 61 54 50

57 56 60 66 54

45 40 67 60 39 62

45 50 36 40 52 40 33

46 44 64 73 47 70 71 35

66 56 41 40 59 50 31 51 33

64 58 51 49 64 62 36 46 47 70

2

3

4

5

6

7

8

9

10

11

73

89 75

73 80 77

69 79 65 79

66 72 75 76 65

82 79 81 83 74 73

66 68 66 76 70 69 71

77 80 76 75 70 73 82 71

43 47 45 49 47 50 48 70 55

55 64 60 63 55 61 62 80 66 77

2

3

4

86

78 77

83 83 86

750mBathymetric zone (689-761m)

1 2 3 4 5 6 7 8 9 10 11

Station No.

Month

4B/75/1 10B/75/6 12B/75/7 14B/75/9 5B/76/14 6/76/15 16/76/20 7/77/28 16/77/30 7/78/35 1/79/38

Mar. Jul. Sept. Nov. Apr. Jun. Oct. May Oct. Apr. Jan.

1

S S S S S S L L L L L

1000 m Bathymetric zone (958-1064 m)

1 2 3 4 5 6 7 8 9 10 11

Station No.

Month

4B/75/2 7B/75/4 10B/75/5 12B/75/8 14B/75/10 5B/76/11 9/76/16 16/76/21 7/77/27 7/78/36 1/79/39

Mar. May Jul. Sept. Nov. Apr. Jun. Oct. Apr. May Jan.

1

S S S S S S S L L L L

1250 m Bathymetric zone (1237-1273 m)

1 2 3 4

Station No.

Month

9/76/17 16/76/22 7/77/26 7/78/34

June Oct. Apr. Apr.

1

S L L L

p e r c e n t a g e similarity c o e f f i c i e n t s b e t w e e n all t h e G r a n t o n trawls g r o u p e d in b a t h y m e t r i c zones. In the 5 0 0 m b a t h y m e t r i c zone, s t a t i o n 5 B / 7 6 / 1 3 has a very l o w p e r c e n t a g e similarity w i t h all t h e o t h e r c a t c h e s because o f t h e large c a t c h o f blue w h i t i n g r e f e r r e d t o earlier. A l t h o u g h t h e p e r c e n t a g e similarity b e t w e e n t h e o t h e r 4 s t a t i o n s was fairly low, t h e r e was n o suggestion t h a t

Ecology of deep-sea fish of the Rockall Trough

49

the catch obtained from the net with short bridles was markedly different from the others. Similarly, although there was considerable variability between stations at the 750 m bathymetric zone, there was no suggestion of a difference associated with the rig of the net. At the 1000 and 1250m bathymetric zones the percentage similarity between stations was of a higher order, with the exception of station 7/78/36 and to a lesser extent 1/79/39 which had large catches of Alepocephalus bairdii. It would, therefore, appear that when the Granton trawl is rigged with short bridles it has a deleterious effect on the total quantity of fish caught, but little influence on the species composition as determined by percentage similarity. The lower levels of similarity between stations at the 500 and 750 m bathymetric zones probably result from seasonal events, which will be discussed later. The single warp trawl was used primarily to yield information on the fish populations at depths greater than 1250 m which could not be sampled by the Granton trawl. Unfortunately, there were few opportunities to carry out comparisons between this net and the Granton trawl at shallower depths. A semi-balloon otter trawl (OTSB) which differs in its dimensions and mesh sizes, but is fished on an identical rig at the same towing speed, has been used extensively by the Institute of Oceanographic Sciences in the Porcupine Sea Bight together with the same SMBA Granton trawl as was used in this survey. Details of the comparisons between the two nets will be published on completion of the joint survey, but many of the comments about the catches of the single warp trawl which follow have been influenced by a preliminary knowledge of these results. The details of the differences between the catches of the single warp trawl and the Granton trawl at upper and mid slope depths will be dealt with later in more detail, but at this point it is necessary to state that the single warp trawl is noticeably less effective at catching the larger sharks, such as Centroscymnus coelolepis, the alepocephalid Alepocephalus bairdii and the black scabbard fish Aphanopus carbo. There may be many reasons for these differences, but probably the three most important are the small size of the net, the slower towing speed and the configuration of the bridles in advance of the net. It was noticed that for a successful catch the swivel attachment to the main warp had to show evidence of bottom contact which implies that the 50 m bridles will create a disturbance in the path of the net which would tend to deflect fish susceptible to herding out of the path of the net. Undoubtedly because of its small size and low towing speed the Agassiz trawl must be regarded as a fairly poor sampler especially for large mobile fish. Having discussed in general terms the fishing characteristics of the nets used in this survey it is now appropriate to consider the catches by bathymetric zone. 3.1. Catch composition by bathymetric zone The top ten species by number of individuals and the percentage that each contributes to the total combined catch of each net and at each bathymetric zone are given in Table 3. In each category, except the 750 m Granton trawls, one or two species account for 50% or more of the total number of individuals. At most bathymetric zones the same species dominates all the individual samples from a given net but at the 500 m bathymetric zone, which was only sampled by the Granton trawl on 5 occasions, there were considerable differences between the samples as is shown in Table 4, It is well known that the blue whiting is very seasonal in its distribution in the Rockall Trough (BAILEY, 1982) and this probably accounts for the greater dissimilarity of the April catch (Table 2). Two catches which were taken within 8 days of each other on the same cruise in October only show 65% similarity. While this might be within the range expected by patchiness

Coryphaenoides rupestris Alepocephalus bairdii Lepidion eques Nezumia aequalis Halargyreus johnsonii Trachyrincus murrayi Hydrolagus mirabilis Chimaera monstrosa Xenodermich thys copei Centroscymnus coelolepis

1000m Granton Trawls (n = 11)

Coryphaenoides rupestris Lepidion eques Nezumia aequalis Epigonus telescopus Halargyreus johnsonii Chimaera monstrosa Alepocephalus bairdii Deania calcea Aphanopus carbo Hydrolagus mirabilis

750 Granton Trawls (n = 11)

Micromesistius poutassou Chimaera monstrosa Argentina silus Helicolenus d. dactylopterus Lepidion eques Phycis blennoides Etmopterus spinax Gadiculus argenteus thori Epigonus telescopus Coelorinchus coelorinchus

5 0 0 m Granton Trawls (n = 5)

47.45 16.67 8.81 6.14 3.79 3.45 2.16 2.15 1.64 1.05

5.24 5.02 2.07

5.69

5.82

6.13

25.93 13.29 8.84 6.30

1.02 0.80

1.30

2.90 2.75 2.35 2.02

3.33

73.04 6.52

Coryphaenoides rupestris Lepidion eques Synaphobranchus kaupi Nezumia aequalis Alepocephalus bairdii Mora moro Chimaera monstrosa Molva d. dypterygia Centroscymnus crepidater Deania calcea

1000 m Single warp Trawls (n ~- 2)

Lepidion eques Helicolenus d. dactylopterus Brosme brosme Epigonus telescopus Nezumia aequalis Coryphaenoides rupestris Phycis blennoides Mora moro Chimaera monstrosa Synaphobranchus kaupi

750 Single warp Trawls (n = 3)

54.85 29.38 3.76 3.04 2.46 2.03 1.16 1.16 0.58 0.43

65.43 6.51 4.65 3.90 3.53 3.35 2.42 2.42 2.23 1.12

Coryphaenoides rupestris Lepidion eques Synaphobranchus kaupi Alepocephalus bairdii Chimaera monstrosa Trachyrincus murrayi

1 0 0 0 m Agassiz Trawls (n = 2)

Coryphaenoides rupestris Lepidion eques Synaphobranchus kaupi Nezumia aequalis Alepocephalus bairdii Coelorinchus occa Glyptocephalus cynoglossus

750 m Agassiz Trawls (n = 4)

TABLE 3. THE TEN TOP RANKING SPECIES BY NUMBER IN EACH OF THE BATHYMETRIC ZONES AND BY D I F F E R E N T TRAWLS

51.19 22.62 7.14 5.95 3.57 2.38

35.16 31.87 15.38 9.89 4.40 2.20 1.10

Z t"l

0

0

0.99 0.85

1.41 1.17

41.70 24.94 15.88 3.33 1.70 1.63

0.59 O.59

1.01

2.35

2.86

3.19 3.11

46.81 23.78 7.14 5.97

Coryphaenoides rupestris Synaphobranchus kaupi Coryphaenoides guentheri Alepocephalus bairdii Coelorinchus occa Etmopterus princeps Coryphaenoides (Chalinura) mediterraneus Polyacanthonotus rissoanus Alepocephalus agassizi Antimora rostrata Cataetyx laticeps 1.01 0.60 0.60 0.60

6.65 5.64 4.23 3.43 2.22 2.22

71.77

1 7 5 0 m 'Walter Herwig' Trawl (n = 1)

Coryphaenoides rupestris Alepocephalus bairdii Trachyrincus murrayi Coryphaenoides guentheri Coelorinchus occa Coryphaenoides (Chalinura) mediterraneus Harriotta raleighana Synaphobranchus kaupi Apristurus spp. Centroscyllium fabricii Polyacanthono tus rissoanus

1500 m 'Walter Herwig' Trawl (n = 1)

Coryphaenoides rupestris Alepocephalus bairdii Trachyrincus murrayi Coelorinchus occa Synaphobranchus kaupi Chimaera monstrosa Coryphaenoides guenth eri Hoplosteth us atlanticus Apristurus spp. Notacanthus bonapartei

1 2 5 0 m Granton Trawls (n = 4)

Coryphaenoides guentheri Coryphaenoides rupestris Coryphaenoides (Chalinura) mediterraneus Antimora rostrata Coelorinchus occa Alepocephalus agassizi Coryphaenoides (Chalinura) brevibarbis Synaphobranchus kaupi Etmopterus princeps Alepocephalus bairdii Hoplostethus atlanticus

1750 m Single warp Trawls (n = 4)

Coryphaenoides rupestris Coryphaenoides guentheri Trachyrincus murrayi Coelorinchus occa Coryphaenoides (Chalinura) mediterraneus An timora rostrata Alepocephalus bairdii Synaphobranchus kaupi Harriotta raleighana

1500 m Single warp Trawls (n = 5)

Coryphaenoides rupestris Trachyrincus murrayi Coelorinchus occa Alepocephalus bairdii Coryphaenoides (Chalinura) mediterraneus Hoplostethus atlanticus Lepidion eques Chimaera monstrosa Apristurus spp.

1250 m Single warp Trawls (n = 3)

1.89 0.94 0.94 0.94

11.32 4.72 3.30 2.36

39.15 16.04 15.57

1.65 1.18 0.94 0.71

70.99 7.78 7.08 4.95 1.18

3.38 2.03 1.58 1.13

36.26 30.18 8.11 5.41 3.60

Synaphobranchus kaupi Coryphaenoides guentheri Coryphaenoides rupestris Antimora rostrata Coelorinchus occa Coryphaenoides (Chalinura) mediterraneus Alepocephalus bairdii Polyacanthonotus rissoanus Trachyrincus murrayi

1500 m Agassiz Trawls (n = 2)

Lepidion eques Coryphaenoides rupestris Synaphobranchus kaupi Trachyrincus murrayi Coelorinch us occa Coryphaenoides guen theri

1250 m Agassiz Trawls (n = 1)

2.59 2.07 2.07

41.45 19.17 11.92 9.84 4.66 4.66

35.48 32.26 19.35 6.45 3.22 3.22

o

o

o

8

Coryphaenoides guentheri Antimora rostrata Coryphaenoides (Chalinura) mediterraneus Coryphaenoides (Chalinura) brevibarbis Alepocephalus agassizi

2000 m Single warp Trawls (n = 5)

TABLE 3. CONTINUED

3.28

6.01

60.11 16.39 7.65

Corphaenoides guentheri Synaphobranchus kaupi A n tim ora rostra ta Coryphaenoides (Chalinura) mediterraneus Polyacanthonotus rissoanus Halosauropsis macrochir Raja bigelowi Coryphaenoides (Chalinura) brevibarbis

2000 m Agassiz Trawls (n = 2)

2.25 2.25 1.50 1.50

36.09 33.08 13.53 6.77

:Z

0 Z

©

Ecology of deep-sea fish of the Rockall Trough

53

TABLE 4. THE TOP RANKING SPECIES AT EACH STATION IN THE 500 m BATHYMETRIC ZONE WITH THE NUMBER OF INDIVIDUALS EXPRESSED AS A PERCENTAGE OF THE TOTAL FOR EACH STATION Station 7/77/29

Station 5B/76/13 Micromesistius poutassou Argentina silus Helieolenus d. dactylopterus

97.2 1.6 0.4

Station 9/76/19 Helicolenus d. dactylopterus Chimaera monstrosa Etmopterus spinax Epigonus teleseopus Argentina silus

Chimaera monstrosa Argentina silus Phycis blennoides Helicolenus d. daetylopterus Aphanopus earbo

38.7 22.2 6.0 5.6 4.8

Station 16/76/23 19.5 18.2 11.6 9.9 7.9

Lepidion eques Chimaera monstrosa Phycis blennoides Etmopterus spinax

32.7 20.9 11.0 5.8

Station 16/76/25 Chimaera monstrosa Gadiculus argen teus thori Phyeis blennoides Etmop terus spinax Micromesistius poutassou

27.8 12.3 10.9 7.9 6.9

another possibility is that it may be a diurnal effect. Whenever possible during the survey all the Granton trawls were carried out during the hours of daylight but station 16/76/23 was an exception and when an opportunity presented itself later in the cruise this station was repeated during the day (station 16/76/25). The prominence of Gadiculus argenteus thori in the latter may be a diurnal effect, since it is well known from shelf studies that many small gadoid species such as Trisopterus esmarki and juvenile Merlangius merlangus are in mid water during the hours of darkness. Little is known of the biology of Lepidion eques to indicate whether it might be more abundant on or close to the b o t t o m during the night. Chimaera monstrosa, Phycis blennoides and Etmopterus spinax account for similar percentages of the total catch at the two stations. Between all the stations the generally low values for the percentage similarity coefficient are caused by the differing abundances of species such as Helicolenus dactylopterus dactylopterus and Argentina silus as well as those mentioned above. Although the values of the percentage similarity coefficients between the 11 Granton trawls at the 750 m bathymetric zone were generally at a low level (Table 2) there appeared to be some clustering when arranged in order of month of sampling irrespective of the year. For example the catches in the periods January to April and September to November tended to be clustered at a higher level of similarity than the intervening months. The number of individuals per 1 0 0 0 m 2 for some of the dominant species was calculated for grouped seasonal data and, despite the inaccuracies inherent in this calculation, certain seasonal trends were evident. Aphanopus carbo was relatively more abundant in the early months of the year, a fact which is probably related to the piscivorous nature of the diet and the seasonal abundance of prey species such as Micromesistius poutassou within the Rockall Trough. The macrourid, Coryphaenoides rupestris, was more abundant in the later months of the year and an examination of the length frequency composition showed that immature fish of less than 5 0 c m

54

J.D.M. GORDON and J. A. R. DUNCAN

total length comprised 5.0% of the population in January to April, 0.6% in May to July and 14.8% in September to November. Juveniles are most abundant at the 1000m bathymetric zone (GORDON, 1979) and this together with the fact that the only records of C rupestris from the 500 m bathymetric zone were in October is suggestive of an up-slope movement of this species in the autumn and winter. Such a vertical migration has been postulated by PECHENIK and TROYANOVSKII (1970) in the western North Atlantic to account for seasonal fluctuations in catch rate. The morid Halargyreus johnsonii and the alepocephalidAlepocephalus bairdii were virtually absent from the catches between January and July but became relatively abundant in the September to November period. Both these species have their centres of abundance at greater depths as do many of the rarer species, such as Etmopterus princeps, Trachyrincus murrayi and Antimora rostrata which were only recorded during the September to November period in this bathymetric zone. Their shallower distribution at this time of year supports the suggestion of an up-slope movement. The single warp trawl was deployed on three occasions at the 750 m bathymetric zone, but only two of the catches, both in May, contained large enough numbers of fish to be useful. The percentage similarity between these stations was only 40% and this was mainly due to the differences in the relative abundance of the morid Lepidion eques which accounted for 73.6% at one station and 23.9% at the other. The percentage similarity between the combined catches of all the Granton trawls and all the single warp trawls was 32.2 witha range from 14 to 63. A total of 47 species were represented in all the Granton trawls from this bathymetric zone but only 23 were obtained by the single warp trawl. There were 21 species common to both nets of which Deania calcea, Coryphaenoides rupestris and Aphanopus carbo were relatively more abundant in the Granton trawls and Brosme brosme and Lepidion eques were relatively more abundant in the single warp trawls. Two species were only taken by the single warp trawl but both were single individuals. The larger number of species which were exclusive to the Granton trawl is in part related to the number of trawls fished and the greater area covered at this zone (see p. 58). Six of the 17 species caught only by Granton trawl were sharks of the family Squalidae. Three species, of other families, Hydrolagus mirabilis, Alepocephalus bairdii and Halargyreus johnsonii, which were absent from the single warp trawl catches, were relatively abundant in the Granton trawl. The latter two species were absent from Granton trawl catches between January and July (see above). The 2 single warp trawls which yielded large catches were both in May and the trawl in October only yielded 17 fish, it is possible that a different result would have been obtained if the single warp trawl had been used during the autumn. A comparison of the length frequency composition of the dominant species caught by both the single warp trawl and the Granton trawl on the same cruises showed that they were similar. The Agassiz trawl was used on four occasions at this bathymetric zone and although 91 fish were caught, only 7 species were represented. The top ranking species were Coryphaenoides rupestris, Lepidion eques and Synaphobranchus kaupi. Four juvenile specimens of Alepocephalus bairdii (8-10 cm standard length) were present in an October Agassiz trawl sample. This was the only species which did not also occur in the single warp trawl at this bathymetric zone. The percentage similarities between the pooled data from all the Agassiz trawls and the pooled Granton trawl and pooled single warp trawl data were 54 and 40 respectively. When the percentage similarity coefficients for the 11 Granton trawls at the 1000 m bathymetric zone (Table 2) were rearranged in order of month of sampling there was no evidence of seasonality. Two stations 7/78/36 and 1/79/39 had lower similarities to all the others but were 77% similar to each other. A common feature of both these catches was the great abundance of Alepocephalus bairdii which in the former were in spawning condition. It is worth

Ecology of deep-sea fish of the Rockall Trough

55

noting that at 3 stations at this bathymetric zone on the Hebridean Terrace fished by the FFS 'Walther Herwig' in the months of January, June and October, only October yielded an exceptionally large catch of this species (S. EHRICH, pets. commun.). Patchiness in distribution rather than seasonal spawning aggregations may, therefore, account for the occasional large catches of this species and it is interesting that large catches are often associated with a large amount of mud and sponges in the codend, suggesting that the apparent patchiness may be due to a preference of this species for soft mud areas. The number of individuals per 1000 m 2 of some of the dominant species was calculated for seasonal groupings but there was little evidence for any seasonal trends except for a general decrease in abundance of all species in the period January to April and for a higher proportion of juvenile Alepocephalus bairdii in the September to November period. The single warp trawl was only deployed on 2 occasions at the 1000 m bathymetric zone in May and August, and the percentage similarity between the catches was 76. The percentage similarity between pooled data for all the Granton trawls and the combined catch of the two single warp trawls was 66% and the range of similarity coefficients between individual catches was from 44 to 80%, the lowest values being associated with Granton trawls which had the large numbers ofA. bairdii. The total number of species represented in the Granton trawl catches was 44 compared with only 18 for the single warp trawls. All the species obtained by the single warp trawl were also present in the Granton trawl catches and if the rare fish, on the basis of their occurrence in less than 4 of the 11 Granton trawls, were excluded, then the species not sampled by the single warp trawl were the squaloid shark Etmopterus princeps, the alepocephalid Xenodermichthys copei, the macrourids Coelorinchus occa and Trachyrincus murrayi and the gadoid Micromesistius poutassou. None of these accounted for more than 3.5% of the total of the pooled Granton trawl catches. Only two of the species common to both nets were markedly different in their relative abundance, Alepocephalus bairdii being much more abundant in Granton trawls and Lepidion eques in single warp trawls. The Agassiz trawl was only used on two occasions at this bathymetric zone, but yielded 84 fish belonging to 12 species. The percentage similarities of the pooled Agassiz data compared with the pooled Granton and single warp trawl data were 68 and 81 respectively. The percentage similarity between the 4 Granton trawls at the 1250 m bathymetric zone ranged from 77 to 86. The percentage similarity coefficient between two single warp trawls in May and November was 62. The similarity coefficient between the individual single warp trawls and the Granton trawls ranged from 56 to 72% and between the pooled data for each of the nets it was 70%. The similarity coefficient between the single Agassiz trawl and the pooled Granton trawl data was 46 with a range of 43 to 49 and between pooled single warp trawl data was 48 with a range of 37 to 47. Twenty-nine species were represented in the Granton trawl catches compared with 22 in the single warp trawls and of these 18 species were common to both nets. The four species which were only sampled by the single warp trawl were all rare as were 7 of the 11 species which were exclusive to the Granton trawl. The remaining four species not sampled by the single warp trawl were the squalid shark Centroscymnus crepidater, the notacanths Notacanthus bonapartei and Polyacanthanotus rissoanus and the black scabbard fish Aphanopus carbo. Alepocephalus bairdii was poorly represented in the single warp trawl catches at 5.4% of the total catch, compared with 24.9% in the Granton trawls. Only 6 species were present in the Agassiz trawl all of which were present in the other nets. Table 3 shows that Lepidon eques, which was of minor importance in the Granton and single warp trawl catches, was dominant in the Agassiz trawl. JPO 15:I-E

56

J.D.M. GORDON and J. A. R. DUNCAN

TABLE 5. THE PERCENTAGE SIMILARITY BETWEEN THE SINGLE WARP TRAWLS AND THE AGASSIZ TRAWLS AT THE 1500 m BATHYMETRIC ZONE

1 Single warp trawl 1 Single warp trawl 2 Single warp trawl 3 Single warp trawl 4 Single warp trawl 5 Agassiz trawl 6 Agassiz trawl 7

14A/78/SWT30 7/77/SWT2 7/78/SWT25 12/77/SWT9 16/77/SWT19 16/76/AT2 11/82/AT229

2

3

4

5

6

7

83

84 77

78 84 72

55 53 60 44

25 20 25 11 34

44 35 41 28 47 75

No. of fish 67 56 31 228 42 71 122

Table 5 shows the percentage similarity coefficients between the 5 single warp trawls and the Agassiz trawls at the 1500 m bathymetric zone. One of the single warp tram stations (16/77/ SWT19) had a lower similarity to all the others and the Agassiz trawls were also dissimilar to the single warp trams but 75% similar to each other. Coryphaenoides rupestris and C guentheri were the dominant species at station 16/77/SWT19 each accounting for 43% of the total catch while at the other stations C. rupestris was dominant accounting for 61 to 78% of the catch. C. guentheri was less abundant, 1 to 13% of the total catch, and in rank order of abundance it ranged from second to fifth. The Agassiz trawl catches at this bathymetric zone were relatively large, 71 and 122 fish, and Synaphobranchus kaupi was the dominant species at both stations. The lower abundance of Coryphaenoides rupestris is probably because the Agassiz trawl is selective for juveniles. The results from the single warp trawl indicate that juveniles apparently are not present at this depth. At the 750 and 1000 m bathymetric zones juvenile C. rupestris of 20 cm total length and less comprised 91 and 70% of the catches even although adult fish are abundant at these depths (GORDON, 1979). The Synaphobranchus kaupi are of a larger size at this depth (unpublished observation) and are unlikely to escape through the meshes of the Agassiz trawl. Their presence was indicated by the single warp trawl, but many would probably escape through the large meshes of this net before reaching the codend. A "bigger-deeper" trend has been reported for this species on the continental slope off New England (POLLONI, HAEDRICH, ROWE and CLIFFORD, 1979). The presence of 5 Alepocephalus bairdii and single specimens of the squalid sharks Centroscymnus coelolepis and Etmopterus princeps in the single warp trawl catches raises the question as to whether a large trawl fished on paired warps at this depth would have yielded a different result. The opportunity to use such a trawl at this depth and in the same area by the FFS Walther Herwig in October 1981 has helped to resolve this question. The percentage similarity between this catch and the combined single warp tram catches was 70%. A total of 28 species were represented in the Walther Herwig catch, but if single occurrences are excluded, then the number reduces to 18. Eighteen species were also sampled by the single warp trawls and of these 16 were common to the Walther Herwig catch. The two species not sampled by the single warp trawl were the squalid shark Centroscyllium fabricii and the notacanth Polyacanthonotus rissoanus. Of the species common to the two nets one of the most obvious differences was in the abundance of Alepocephalus bairdii which accounted for 24% of the WaltherHerwig catch but only 1% of the pooled single warp trawl catch and was reduced from second to seventh in rank order. Coryphaenoides rupestris accounted for 71% of the individuals in single warp trawls and only 47% in the WaltherHerwig catch but it was the dominant species for both nets.

Ecology of deep-sea fish of the Rockall Trough

57

TABLE 6. THE PERCENTAGE SIMILARITY BETWEEN THE SINGLE WARP TRAWLS AT THE 1750m BATHYMETRIC ZONE

1 1 2 3 4

7/77/SWT3 16/77/SWT18 7/78/SWT24 14A/78/SWT31

2

3

4

45

56 28

75 42 69

No. of Fish 73 16 23 100

The low percentage similarities between the 4 single warp trawls at the 1750 m bathymetric zone (Table 6) reflects to a large extent the variation in relative abundance of the two macrourids Coryphaenoides rupestris (0-31%) and C. guentheri (6-70%) and to a lesser extent that of the morid Antimora rostrata (0-14%). Small sample size at two of the stations may also be a contributory factor. The single warp trawls at this bathymetric zone indicate the presence of the squaloid shark, Etmopterus princeps and the alepocephalid Alepocephalus bairdii and since both these species are likely to be inadequately sampled by this net it is of interest to compare the catches with that of a Walther Herwig trawl at this depth. The percentage similarity between the pooled single warp trawl catches and the Walther Herwig catch was only 32%. A total of 22 species were represented in the combined catches of both nets, but only 10 species were shared. Six species were exclusive to each of the nets and most of these were single occurrences. The only species which accounted for I% or more of the total catches were Coryphaenoides (Chalinura) brevibarbis in the single warp trawl (2.4%) and Polyacanthonotus rissoanus (1.0%) in the Walther Herwig trawl. Etmopterus princeps and Alepocephalus bairdii were common to both nets but in terms of percentage abundance (Table 3) the differences were not large. There were, however~ considerable differences in the relative abundances of Coryphaenoides rupestris and C. guentheri between the nets. A possible explanation could be that 6'. rupestris is at the lower limit of its depth range and may be exhibiting patchiness in its distribution. The percentage similarity coefficients between the 5 single warp trawls at the 2 0 0 0 m bathymetric zone are given in Table 7 and show with the exception of station 7/77/SWT4 a high degree of similarity. Coryphaenoides guentheri was the dominant species comprising 48.0 to 71.4% of the individual catches. The second most dominant species at four of the stations was Antimora rostrata (11.9 to 23.7%) while at one station (7/77/SWT4) only one specimen was captured resulting in the lower similarity between this station and the others. The total number of species represented in the catches was 16 but if single occurrences are excluded this was reduced to 6 of which only 3, namely C. guentheri, A. rostrata and C. (Chalinura) mediterraneus, were common to all stations. The other 3 species were Alepocephalus agassizi and C. (Chalinura) brevibarbis which each occurred at 4 stations and Halosauropsis macrochir which occurred at two stations. Table 7 shows that there was a high similarity between the 3 Agassiz trawls, even though one was at a sounding of 2 1 9 0 m , but less similarity between the individual Agassiz trawls and the single warp trawls. The percentage similarity between the pooled data for the two nets was 61%. The dominant species in the Agassiz trawl was Coryphaenoides guentheri (36%) followed by Synaphobranchus kaupi (33%). S. kaupi, which at this bathymetric zone were large in size, were not caught by single warp trawls, which accounts for most of the dissimilarity between the two nets. Only 2 other species, Polyacanthonotus rissoanus and Lycodes sp., of the

58

J.D.M. GORDON and J. A. R. DUNCAN

TABLE 7. THE PERCENTAGE SIMILARITY BETWEEN THE SINGLE WARP TRAWLS AND AGASSIZ TRAWLS AT THE 2000 m BATHYMETRIC ZONE

1 Single warp trawl 1 Single warp trawl 2 Single warp trawl 3 Single warp trawl 4 Single warp trawl 5 Agassiz trawl 6 Agassiz trawl 7 AgassiZ trawl 8

7/77/SWT4 12/77/SWT10 16/77/SWT17 7/78/SWT23 14A/78/SWT32

12B/81/AT191 11/82/AT219 11/82/AT228

2

3

4

5

6

7

8

59

64 87

67 79 78

68 66 70 74

52 63 58 59 55

44 53 52 59 63 71

52 49 53 47 54 71 71

No. of Fish 25 28 42 59 29 27 40 66

total of 12 caught by the Agassiz trawl were not present in the single warp trawl catches and neither was abundant. The lack of any evidence for the occurrence of squalid sharks and of the alepocephalid, Alepocephalus bairdii, in the single warp trawl catches makes it probable that a large trawl fished at this depth would have yielded a similar catch. From the above account it is obvious that when different nets are used to sample the same bathymetric zone there can be considerable differences in the samples of the fish fauna of that zone. Many of these differences are undoubtedly due to the physical characteristics of each net and the speed at which it is towed. In some instances the failure of the small nets to catch large sharks or alepocephalids is probably due to the slow towing speed whereas the larger catches of the synaphobranchid eel by the Agassiz trawl is because of its small mesh size. That there is apparently greater similarity between the catches of the various nets at the deeper bathymetric zones is probably related to the absence of the larger more mobile species from the lower slopes and continental rise. A decrease in the number of fish species with increasing depth, at least from the upper slope to about 2000m, had been reported from other areas (HAEDRICH, ROWE and POLLONI, 1980; PEARCY, STEIN and CARNEY, 1982). PEARCY et al. (1982) found that the diversity of beam trawl catches was not significantly different between the continental shelf and slope stations while earlier work with large trawls had shown a decline in the number of species with depth (ALTON, 1972; and ALVERSON, PRUTER and RONHOLT, 1964) and several times as many species for a given depth when compared with beam trawls. That small trawls are ineffective samplers of large fish has also been demonstrated by MERRETT and MARSHALL (1981) who found that many species, at upper and mid slope depths, were only sampled by baited gear. Table 8 shows the number of species taken by each net in each bathymetric zone in this study. It suggests that there is a trend for a decrease in the number of species in the Granton trawls and perhaps also in the single warp trawls but no such trend in the Agassiz trawls. The number of species recorded from a bathymetric zone will depend on the fishing effort and it is obvious that this has been inadequate especially for the smaller nets. Figure 3 shows the cumulative number of species collected by the Granton trams taken in chronological order at the 1000 m bathymetric zone. The number of additional species only begins to decrease after about 6 trawls and the histogram shows that of the 44 species 18 are represented by 5 or less individuals in the total pooled catch. Considering that both the single warp trawl and the Agassiz trawl were only used twice at this bathymetric zone and that the area of sea bed swept

Ecology of deep-sea fish of the Rockall Trough

59

TABLE 8. THE NUMBER OF SPECIES SAMPLED BY THE GRANTON TRAWL (GT), WALTHER HERWIG TRAWL (WHT), SINGLE WARP TRAWL (SWT) AND AGASSIZ TRAWL (AT) AT DIFFERENT BATHYMETRIC ZONES. THE NUMBER OF HAULS BY EACH NET IS GIVEN IN PARENTHESIS Bathymetric zone (m)

GT

500 750 1000 1250 1500 1750 2000

42(5) 47(11) 44(11) 29(4)

WHT

28(1) 16(1)

~, 50[

(a) I000 rn GT

SWT

AT

23(3) 18(2) 22(3) 18(5) 16(4) 16(5)

7(4) 12(2) 6(1) 12(2) 12(3)

[~

1 2-5

6-50

L._J >50

~o c

.~ 3o

0

(b) 1500m

WH

~

1 2-5 6-50 >5O

2-5 6-50 >50

FIG. 3. The cumulative number of species collected at selected bathymetric zones by different nets, (a) the Granton trawls at the 1000m bathymetric zone in chronological order together with a histogram showing the number of species whose abundance in terms of the total pooled catch falls within the limit of 1, 2-5, 6-50 and 50 specimens, (b) similar histograms for the 5 single warp trawls and the single WaltherHerwig trawl at the 1500 m bathymetric zone. was considerably smaller, it is not surprising that only 18 and 7 species respectively were sampled by these nets. A similar histogram for the 5 single warp trawl catches at 1 5 0 0 m suggests that almost all the species present at this bathymetric zone have been sampled yet the single catch of the German trawl yielded a further I 0 species.

3.2. Bathymetric changes in biomass using different trawls That there are differences in the relative importance of different species in the catches of the different nets and that large species such as the sharks, Alepocephalus bairdii and Aphanopus carbo are more prominent in the Granton trawl catches (Table 3) has important implications for estimates of fish biomass in the deep sea. Table 9 shows the relative importance by weight of the top ten species by net and depth horizon. The influence of fish size is clearly shown by the sharks at the 7 5 0 m bathymetric zone. One shark, Deania calcea, was placed eighth in the rank order of abundance (Table 3) in the Granton trawl but was second by weight and another 3 sharks were ranked in the top ten. Sharks are not ranked in the top ten by either number or weight by the other nets at this bathymetric zone. The discussion of biomass can be simplified when it is realised that a limited number of families or sub-orders contribute to the total biomass at any given bathymetric zone and Table 10 shows in percentage terms the relative biomass for some of the dominant families for

Alepocephalus bairdii Coryphaenoides rupestris Cen troscymnus coelolepis Chimaera monstrosa Cen troscymnus crepidater Lepidorhinus squamosus Deania calcea Lepidion eques Mora moro Aphanopus carbo

Granton Trawls - 1000 m

Coryphaenoides rupestris Deania calcea Aphanopus carbo Lepidorhinus squamosus Chimaera monstrosa Centroscymnus coelolepis Molva d. dypterygia Centroscymnus crepidater Lepidion eques Hy drolagus mirab ilis

Granton Trawls - 750 m

Micromesistius pou tassou Chimaera monstrosa Argentina silus Deania calcea Phycis blennoides Merluccius merluccius Molva molva Lepidorhinus squamosus Etmopterus spinax Aphanopus carbo

G ra nto n Trawls - 500 m

2.44 1.86 1.70 1.05 0.87

2.72

4.48

8.91

34.26

37.17

7.08 5.99 4.66 3.47 1.58 1.47

7.58

26.02 21.36 8.84

1.20 1.00

1.32

4.55 2.31 1.46

4.56

55.23 15.63 5.04

Coryphaenoides rupestris Lepidion eques Alepoeephalus bairdii Molva d. dypterygia Raja nidarosiensis Mora moro Chimaera monstrosa Centroscymnus coelolepis Deania calcea Centroscymnus crepidater

SWT Trawls

1000 m

Brosme brosme Lepidion eques Mora moro Coryphaenoides rupestris Molva d. dypterygia Chimaera monstrosa Phycis blennoides Helicolenus d. dactylopterus Synaphobranchus kaupi Lophius piseatorius

SWT Trawls - 750 m

53.01 9.10 8.71 7.30 7.10 5.90 2.77 1.35 1.02 0.30

28.86 20.92 8.14 5.25 5.15 5.04 4.91 3.10 2.71 2.23

Lepidion eques Coryphaenoides rupestris Alepocephalus bairdii Trachyrincus murrayi Hoplos te th us a tlan ticus Apristurus spp. Cottunculus thomsoni Antimora rostrata Synaphobranchus kaupi Chimaera monstrosa

Agassiz Trawls

Coryphaenoides rupestris Lepidion eques Nezumia aequalis Synaphobranchus kaupi Alepocephalus bairdii Glyptocephalus cynoglossus Coelorinchus occa

1000 m

Agassiz Trawls - 750 m

TABLE 9. THE TEN TOP RANKING SPECIES BY WEIGHT IN EACH OF THE BATHYMETRIC ZONES AND BY D I F F E R E N T TRAWLS

39.41 34.27 13.87 3.90 2.93 2.57 1.10 0.91 0.80 0.09

52.17 40.37 6.09 0.66 0.63 0.06 0.02

~Z ¢'1 > :Z

0

0

50.58 30.70 6.91 3.05 1.81 1.70 0.82 0.75 0.65 0.64 0.58

46.2 33.1 5.8 3.0 2.9 2.2 1.4 0.9 0.8 0.7

Coryphaenoides rupes tris Alepocephalus bairdii Cataetyx laticeps Hydrolagus affinis Etmopterus princeps Coryphaenoides guentheri Coryphaenoides (Chalinura ) mediterraneus Synaphobranchus kaupi Coelorinchus occa

O.9

1.1

2.1 I. 3 1.1

7.6 3.9 2.5

78.5

140 f oo t B o t t o m Trawl - 1 7 5 0 m

Coryphaenoides rupestris Alepocephalus bairdii Centroscymnus coelolepis Harriotta ralieghana Trachyrincus murrayi Hydrolagus affinis Cataetyx laticeps Cen troscylliu m fabricii Apristurus spp. Coelorinchus occa

140 f oot B o t t o m Trawl - 1500 m

Alepocephalus bairdii Coryphaenoides rupestris Centroscymnus coelolepis Trachyrincus murrayi Aphanopus carbo Chimaera monstrosa Hoplostethus atlanticus Centroscymnus crepidater Apristurus spp. Centroscyllium fabricii Harriotta raleighana

Gr an to n Trawls - 1 2 5 0 m

1750 m

Antimora rostrata Coryphaenoides rupestris Coryphaenoides guentheri Alepocephalus bairdii Coryphaenoides (Chalinura) mediterraneus Notacanthus chemnitzii Coelorinchus occa Etmopterus princeps Hoplostethus atlan ticus Alepocephalus agassizi

SWT Trawls

Coryphaenoides rupestris Hydrolagus affinis Centroscymnus eoelolepis Trachyrincus murrayi Coryphaenoides guentheri Alepocephalus bairdii Cataetyx laticeps Alepocephalus sp. Coelorinchus occa Harriotta raleighana

SWT Trawls - 1500 m

Coryphaenoides rupestris Alepocephalus bairdii Trachyrincus murrayi Centroscymnus coelolepis Chimaera monstrosa Cataetyx laticeps Hoplostethus atlanticus Molva d. dyptergia Rhinochimaera atlantica Centroscyllium fabricii

SWT Trawls - 1 2 5 0 m

TABLE 9. CONTINUED

1.31 1.29

1.96 1.39

2.57

34.39 23.81 17.20 9.42 4.02

69.87 4.11 4.06 3.98 3.96 3.41 3.13 2.16 1.91 1.16

29.61 19.31 10.94 7.33 6.61 4.50 4.36 4.02 2.88 2.75

Lepidion eques Trachyrincus murrayi Coryphaenoides rupestris Synaphobranchus kaupi Coryphaenoides guentheri Coelorinchus occa

Agassiz Trawls - 1250 m 74.03 13.76 9.54 2.16 0.33 0.18

,..]

O ,<

2000m

Antimora rostrata Coryphaenoides guentheri Hydrolagus affinis Coryphaenoides (Chalinura) mediterraneus Alepocephalus agassizi Ra]a fyllae Halosauropsis macrochir Spectrunculus grandis Narcetes stomias Harriotta raleighana

SWTTrawls

TABLE 9. CONTINUED

4.88 1.21 1.17 1.09 0.69 0.61

47.15 27.00 9.51 5.57

Antimora rostrata Coryphaneoides guentheri Synaphobranchus kaupi Raja bigelowi Spectrunculus grandis Cataetyx laticeps Halosauropsis macrochir Polyacan thonotus rissoanus Coryphaenoides (Chalinura) mediterraneus Coryphaenoides (Chalinura) brevibarbis

Agassiz Trawls - 2000 m

0.34

58.39 20.02 11.13 2.69 2.07 2.02 1.67 0.73 0.39

:Z

Z

0

0

Z7

Ecology of deep-sea fish of the Rockall Trough

63

TABLE 10. THE P E R C E N T A G E BY WEIGHT OF THE DOMINANT FAMILIES OR S U B O R D E R S OF FISH FOR THE COMBINED HAULS FROM EACH OF THE BATHYMETRIC ZONES AND FOR EACH OF THE NETS

Paired warp trawls 500 750 1000 1250 1500 1750

7.6 39.7 16.0 8.7 7.6 2.1

0.1 1.4 0.1 0 + 0

15.6 8.8 5.3 1.7 5.2 2.5

+ 0.8 37.2 50.5 33.2 8.0

+ + + 0.1 0.4 1.1

1.4 27.0 35.1 34.3 50.7 81.9

62.8 7.4 1.1 0 0 0

0.6 2.4 3.1 0.5 + +

1.0 8.8 0.9 1.8 + 0

0 0 0 0 1.5 4.0

2.0 2.7 10.1 4.6 1.4 0

1.4 7.1 0 + 0 1.2

5.0 2.8 10.3 5.3 0 10.1

0 8.7 19.5 5.9 10.7 5.6

+ + + 0.3 0.3 0

6.2 53.3 44.6 80.2 48.1 33.1

40.0 7.3 4.0 0 0 0

29.1 15.6 1.3 0.1 34.4 47.1

0.5 0.2 0 0 0 0

0 0 4.5 3.1 0 1.2

0 0 0 0 0

0 0 0 1.7 2.7

0 0.1 0 0 0

0.6 13.9 0 6.5 0.3

0.7 0.8 2.1 22.7 11.1

58.3 38.3 23.8 62.1 20.7

0 0 0 0 0

40.4 40.4 74.0 3.8 58.4

0 0 0 0 0

0 0 0 0 4.1

Single warp trawls 750 1000 1250 1500 1750 2000 Agassiz trawls 750 1000 1250 1500 2000

each of the nets and at each of the bathymetric zones. Considering first the Granton trawls it is evident that the family Gadidae make the largest contribution to the biomass at the 500 m bathymetric zone but if the large seasonal catch of Micromesistius pou tassou in April is excluded then the chimaerids become the dominant component of the biomass accounting for 36.4% of the total. The gadids and sharks would then contribute 18.9 and 17.8% respectively. At the 7 5 0 m bathymetric zone the squalid sharks become the dominant family followed by the macrourids. The principal component of the macrourid biomass is due to Coryphaenoides rupestris at this and deeper bathymetric zones sampled by the Granton trawl. At the 1000 m bathymetric zone the alepocephalids become the dominant family closely followed by the macrourids and this trend continues into the 1 2 5 0 m zone. Most of the alepocephalid biomass is attributable to the single species Alepocephalius bairdii. The 1500 and 1750 m bathymetric zones sampled by the Walther Herwig show that the dominant family becomes the Macrouridae and that the relative importance of alepocephalids diminishes. The increasing importance of the family Synaphobranchidae simply reflects their increasing size with depth. The single warp trawl data at the 750 m bathymetric zone were based on 3 trawls but depend mostly on 2 large catches in one of which the large gadid Brosme brosme accounted for 50% of

64

J.D.M. GORDONand J. A. R. DUNCAN

the number of this species caught during the survey. Nevertheless, the contribution to the biomass by the squalid sharks and the chimaerids is considerably reduced, while that of the morids is increased in comparison to the Granton trawls. A similar trend exists at the I000 m bathymetric zone but it should be noted that the 7.1% attributable to the Rajidae is in fact due to a single large specimen of Raja ? nidarosiensis. The greatest difference between the single warp trawls and the paired warp trawls at the 1250 and 1500 m bathymetric zones is the reduced importance of the alepocephalids in the former. The relative decrease in importance of the macrourids at the 1750 m depth zone is due to the contribution to the morid biomass made by the large species Antimora rostrata. At the 2000m bathymetric zone the biomass is dominated by the macrourids and morids. Coryphaenoides guentheri dominates the macrourid biomass and the morid biomass is attributable to a single species Antimora

rostrata. A problem with the Agassiz trawl was that because the catches were small a single large specimen could bias the results. One obvious feature of the Agassiz trawl results is the enhanced importance of the synaphobranchid eels which are more readily caught by the small mesh of the trawl. The dominant families were the macrourids and morids and their relative importance is somewhat similar to those observed for the single warp trawl catches. It must be emphasised that the above analysis refers to the relative biomass for each of the bathymetric zones and does not yield any information on whether the actual biomass of a given family changes with depth. Reference has already been made to total biomass of individual trawl catches expressed as kg/1000m 2 for Granton trawl catches (see Fig. 2). Table 11 shows the mean biomass expressed as kg/1000m z for the dominant families sampled by the Granton trawl at the different depth horizons. The calculations take into account the differing path widths of the two rigs of the net but assume a towing speed of 3.75 knots for all the stations and therefore the results can only be considered as showing trends of biomass change with depth. Likewise the inclusion of data calculated from the single stations at 1500 and 1750 m fished by a 140 foot Walther Herwig trawl may not be strictly comparable because the net used is almost certain to have different fishing characteristics. The family Squalidae reach a peak biomass at the 750 m bathymetric zone and thereafter decline in importance. The sub-order Chimaerea has its greatest biomass at shallower depths which is attributable mainly to Chimaera monstrosa. The increase at 1500 m, if significant, is attributable to Harriotta raleighana. The alepocephalid biomass is greatest over the depth range I000 to 1500 m. The macrourid biomass, which is mainly due to Coryphaenoides rupestris, changes little between depths of 750 and 1750 m. The gadid biomass is greatest at the 500 m bathymetric zone and declines rapidly thereafter. The contribution to the biomass by the Moridae is small and a curious feature is the absence ofAntimora rostrata in the 1750 m Walther Herwig trawl when in single warp trawls at the same bathymetric zone this species accounted for 34.4% of the biomass. The Trichiuridae, represented by Aphanopus carbo, was only significant at the 750 m bathymetric zone while the Bythitidae, represented by Cataetyx laticeps, contributed little to the total biomass. Similar calculations of total biomass have been made for individual single warp trawls and these are plotted in Fig. 4. They must, however, be treated with caution since with such lightweight trawls it was difficult to determine when the net reached the bottom and therefore the duration of the trawls could only be approximated. In view of this difficulty and also the small size of the catches it was considered inappropriate to attempt to calculate the biomass of the families as was done for the Granton trawls. It was impossible to estimate the biomass of the Agassiz trawl catches because the duration on the bottom was not known.

500 m 750 m 1000m 1250m 1500 m 1750m

0.336 1.998 0.905 0.449 0.100 0.043

Squalidae

0.642 0.452 0.283 0.134 0.235 0.049

Chimaerea 0.001 0.031 2.752 2.729 1.497 0.160

Alepocephalidae 0.059 1.367 1.822 1.731 2.270 1.621

Macrouridae 1.320 0.400 0.055 0 0 0

Gadidae 0.024 0.126 0.169 0.025 + +

Moridae

0.044 0.511 0.038 0.088 0.004 0

Trichiuridae

0 0 0 0 0.068 0.079

Bythitidae

2.779 5.166 6.079 5.251 4.517 2.007

All fish

TABLE 11. THE BIOMASS (kg/1000m 2) OF THE DOMINANT FAMILIES OR SUB-ORDERS SAMPLES BY THE PAIRED WARP TRAWLS FOR THE DIFFERENT BATHYMETRIC ZONES

7~

P~

66

J.D.M. GORDON and J. A. R. DUNCAN

I I

kg. I 0 0 0 m £

0

5

500

:

:

:

tO

15

•

I;.<"

28.3

2000~ FIG. 4. The estimated biomass of the total fish catch from individual single warp trawls (small symbols) on different cruises. The large circles give the mean biomass and its range for Granton trawls with the 50 m bridles, while the large squares give the biomass for the single Walther Herwig catches at 1500 and 1750 m. • Cruise 7/77 × Cruise 12/77 = Cruise 7/78 o Cruise 14A/78

At the 750 and 1 0 0 0 m bathymetric zones there is a considerable variation between the estimates of biomass for each o f the two single warp trawls. Until further trawling can be done at these depths one can only speculate on the reasons for this discrepancy. The gadid Brosme brosme at the 7 5 0 m bathymetric zone was the dominant species by weight in the combined hauls (Table 9) but only occurred at one station where the number of fish caught equalled the total collected by all the Granton trawls at all the bathymetric zones. The 1000 m station on the same cruise yielded an excellent catch o f Coryphaenoides rupestris even by Granton trawl standards. At 1250 m and deeper there is a greater degree of consistency between the estimates of biomass from individual trawls and there appears to be little change in biomass with depth below 1500 m. Also shown in Fig. 4 is the mean and range of biomass for the Granton and Walther Herwig trawls. The single warp trawl is poor at catching the squalid sharks, the alepocephalid Alepocephalus bairdii and the scabbard fish Aphanopus carbo, all of which make significant contributions to the biomass of the upper and mid slope in the Granton trawl catches. It is to be expected that this net will underestimate the biomass at these depths as the data in Fig. 4 suggest. By 1 7 5 0 m however the differences, albeit based on only one Walther Herwig catch, are small. If none of the species mentioned above occur at depths greater than this in the Rockall Trough it is probable that were it possible to fish a Granton type trawl at this depth the estimate o f biomass would be of the same order o f magnitude as for the single warp trawl. FORSTER (1964, 1968) caught squalid sharks by line fishing at depths down to 2 0 0 0 m in the Bay of Biscay and MERRETT and MARSHALL (1981) caught a single Centroscymnus coelolepis from 2375 m off West Africa. It is not known whether sharks occur at these depths further north in the Rockall Trough but many species appear to exist at shallower depths with increasing latitude.

Ecology of deep-sea fish of the Rockall Trough

67

The conclusion that small trawls underestimate the biomass of fish at least on the upper slope has been demonstrated by PEARCY, STEIN and CARNEY (1982) off Oregon. The catches of large commercial trawls were several times that of beam trawls for a given area of sea bed. As in this study, the differences between the nets diminished as the species composition changed with depth. In the waters off Oregon the biomass decreased almost exponentially with depth but in the Rockall Trough there was a peak of biomass at the 750 and 1000 m bathymetric zones. This peak was most evident in the Granton trawl but was not shown as conclusively with the small single warp trawl due to the lack of samples at 500 m and the small number of samples at mid and lower slope depths. EHRICH (1983) who sampled exclusively with commercial paired warp trawls in the Rockall Trough also showed a peak of biomass at about 800 m on some cruises but on other cruises there was a peak at 400 m. Further south in the area off southwest Ireland and the Bay of Biscay the biomass was less but tended to show a peak at 800 to 1000m. High abundances of benthopelagic fish at mid slope depths have been reported by MARSHALL and MERRETT (1977) and these authors concluded that the success of benthopelagic fish at these depths was due to their ability to utilise the mesopelagic fauna as a food source. Feeding studies (MAUCHLINE and GORDON, 1983a, b, and in preparation) on the Rockall Trough fish support this view. Studies on the fish biomass of the western North Atlantic slope off New England have given somewhat different results. In an early study using a 16 foot semiballoon trawl, HAEDRICH, ROWE and POLLONI (1975) found very little difference in fish biomass between 141 and 1928 m. In a later study which also used a larger 41 foot trawl HAEDRICH, ROWE and POLLONI (1980) found that the fish biomass was high on the shelf, relatively low between 283 and 1290 m but was more than twice the shelf biomass between 1380 and 2481 m. Thereafter there was a steady decline in biomass with depth. This peak at a depth greater than would be associated with the mesopelagic fauna suggests that food availability is not the primary cause. HAEDRICH, ROWE and POLLONI (1980) have also shown that fish abundance decreases rapidly from the shelf to the slope with a steady decline thereafter. It follows therefore that the peak of biomass must be due to an increase in the size of individual fish rather than to an overall increase in numbers. POLLONI, HAEDRICH, ROWE and CLIFFORD (1979) investigated the s i z e - d e p t h relationship of several deep-sea taxa and concluded that fishes showed a "bigger deeper" trend. While not ruling out the fact that in some species the size increases with depth PEARCY, STEIN and CARNEY (1982) suggest that the "bigger deeper" trend is in part caused by larger shelf and upper slope species being able to avoid small, slow moving trawls. Whether such a large active fish fauna exists on the mid slope off New England remains to be proved but it is interesting that the peak of fish biomass at greater depths is to a large extent due to two species the morid Antimora rostrata and the macrourid Coryphaenoides armatus. Both these species dominate the fish fauna of the Rockall Trough at depths greater than 2 0 0 0 m (unpublished observations). Acknowledgements-The authors would like to thank the officers and crew, especially Captain Maw and Fishing Skipper Dunning, of R.R.S. Challenger for their help in obtaining the samples and all the scientists who assisted in sorting and processing the catches. We are indebted to D. L. Burkel (Glasgow Museum and Art Galleries) and J. R. Badcock and N. R. Merrett (Institute of Oceanographic Sciences) for their assistance with the identification of some of the specimens. J. D. Gage and Mrs. M. Pearson kindly made available all the fish material from their benthic stations in the Rockall Trough. S. Ehrich and M. Stehmann (Institut ffir Seefischerei, Hamburg) invited one of us (J.D.M.G.) to participate in a cruise of the F.F.S. Walther Herwig to the Rockall Trough and have allowed us to use some of the data collected. J. P. Bridger of the MAFF Fisheries Laboratory, Lowestoft gave valuable advice in the planning of the project and A. Corrigal of the DAFS Marine Laboratory, Aberdeen provided information on the fishing gear. R. S. Batty assisted one of us (J.A.R.D.) with the data handling.

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J. Mauchline (zooplankton), J. D. Gage (benthos) and D. J. Ellett (hydrography) have offered much helpful advice and stimulating discussion. The Dunstaffnage Marine Research Laboratory is grant aided by the Natural Environment Research Council and part of the funding for the project was provided by a MAFF Commission.

REFERENCES ALTON, M. S. (1972) Characteristics of the demersal fish fauna inhabiting the outer continental shelf and slope off the northern Oregon coast. In: The Columbia River estuary and adjacent ocean waters, A. T. PRUTER and D. L. ALVERSON, editors, University of Washington Press, Seattle, pp. 583-634. ALVERSON, D. L., A. T. PRUTER and L. L. RONHOLT (1964) A study of demersal fishes and fisheries of the northeastern Pacific Ocean. H.R. MacMillan Lectures in Fisheries, Institute of Fisheries, University of British Columbia, Vancouver, 190 pp. BAILEY, R. S. (1982) The population biology of the blue whiting in the North Atlantic. Advances in Marine Biology, 19, 257-355. BLACKER, R. W. (1962) Rare fishes from the Atlantic slope fishing grounds. Annals of the Magazine of Natural History 13th series, 5,261-271. BOESCH, D. F. (1977) Application of numerical classification in ecological investigations of water pollution. Special Scientific Report No. 77, Virginia Institute of Marine Science. 115 pp. BRIDGER, J. P. (1978) New deep-water trawling grounds to the west of Britain. Laboratory leaflets, MAFF Directorate of Fisheries Research, (41) 40 pp. DAY, D. S. and W. G. PEARCY (1968) Species associations of benthic fishes on the continental shelf and slope off Oregon. Journal of the Fisheries Research Board of Canada, 25 (12), 2665-2675. EHRICH, S. (1980) Ergebnisse der Bodenfischerei am westeurop~iischen Schelfabhang im Zeitraum Januar bis M/irz 1980. lnformationen fur die Fischwirtschaft des Auslandes, 27 (3), 90-93. EHRICH, S. (1983) On the occurrence of some fish species on the slopes of the Rockall Trough. Archly fiir Fischereiwissenschaft, 33, 105-150. EHRICH, S. and H. P. CORNUS (1979) Ergebnisse weiterer Untersuchungen des Fischbestande des westeurop/iischen Schelfabhangs. Informationen fur die Fischwirtschaft des Auslandes, 26 (5), 125-129. ELLETT, D. J. (1978) Sub-surface temperatures at the Scottish continental slope. Scottish Marine Biological Association, Marine Physics Group Report, 2, 19 pp. ELLETT, D. J. and J. H. A. MARTIN (1973) The physical and chemical oceanography of the Rockall Channel. Deep-Sea Research, 20, 585-625. FARRAN, G. P. (1924) Seventh report of the fishes of the Irish Atlantic slope. Royal Irish Academy Proceedings, 36 B, 91-148. FORSTER, G. R. (1964) Line-fishing on the continental slope. Journal of the Marine Biological Association of the United Kingdom, 44, 277-284. FORSTER, G. R. (1968) Line-fishing on the continental slope: II. Journal of the Marine Biological Association of the United Kingdom, 48, 479-483. FREYTAG, G. and H. MOHR (1974) Erschliepungneuer Fanggebiete und Nutzfischbestande im NO-Atlantik. lnformationen fur die Fischwirtschaft des Auslandes, 21 (3), 88-90. GORDON, J. D. M. (1979) Lifestyle and phenology in deep-sea ancanthine teleosts. Symposia of the Zoological Society of London. P. J. MILLER, editor, Fish Phenology." anabolic adaptiveness in teleosts. Academic Press, London, 44, pp. 327-359. GORDON, J. D. M. and J. A. R. DUNCAN (1983) The deep-sea demersal fish collected by 'RRS Challenger" in the RockaU Trough: station position and catch data for the years 1973 to 1982. Scottish Marine Biological Association Internal Report No. 89, 63 pp. (unpublished manuscript). GUNTHER, A. (1887) Report on the deep-sea fishes collected by H.M.S. Challenger during the years 18731876. Report on the scientific results of the exploring voyage of H.M.S. Challenger 1873-1876, 22, 1-269. HAEDRICH, R. L., G. T. ROWE and P. T. POLLONI (1975) Zonation and faunal composition of epibenthic populations on the continental slope south of New England. Journal of Marine Research, 33, 191212. HAEDRICH, R. L., G. T. ROWE and P. T. POLLONI (1980) The megabenthic fauna in the deep sea south of New England, U.S.A. Marine Biology, 57, 165-179. HICKLING, C. F. (1928) The exploratory voyages of the 'Florence Brierley'. Notes on the fish recorded. Annals of the Magazine of Natural History, 10,2,(8),33,196-209. HOLT, E. W. L. and L. W. BYRNE (1906) First report on the fishes of the Irish Atlantic slope. Fisheries, lreland Scientific h~vestigations, 1905 II, 1-28. HOLT, E. W. L. and L. W. BYRNE (1908) Second report on the fishes of the Irish Atlantic slope. Fisheries, Ireland Scientific Investigations, 1906 V, 1-63.

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69

HOLT, E. W. L. and L. W. BYRNE (1910a) Third report on the fishes of the Irish Atlantic slope. Fisheries, Ireland Scientific Investigations, 1908 IV, 1-26. HOLT, E. W. L. and L. W. BYRNE (1910b) Fourth report on the fishes of the Irish Atlantic slope. Fisheries, Ireland Scientific Investigations, 1908 V, 1-7. HOLT, E. W. L. and L. W. BYRNE (1911) Fifth report on the fishes of the Irish Atlantic slope. Fisheries, Ireland Scientific Investigations, 1910 Vl, 1-33. HOLT, E. W. L. and L. W. BYRNE (1913) Sixth report on the fishes of the Irish Atlantic slope. Fisheries, Ireland Scientific Investigations, 1912, 1-28. HOLT, E. W. L. and W. L. CALDERWOOD (1895) Survey of fishing-grounds, west coast of Ireland, 18901891. Report on the rarer fishes. Transactions of the Royal Dublin Society, 5, 361-512. HUREAU, J. C. and Th. MONOD, editors (1979) Check-list of the fishes of the north-eastern Atlantic and of the Mediterranean. UNESCO, Paris, 1,683 pp. 2, 394 pp. IWAMOTO, T. and D. L. STEIN (1974) A systematic review of the rattail fishes (Macrouridae:Gadiformes) from Oregon and adjacent waters. Occasional Papers of the California Academy of Sciences, No 111, 79 pp. KOEFOED, E. (1927) Fishes from the sea bottom. Report of the Scientific Results of the Michael Sars North Atlantic Deep-Sea Expedition, 4 (1), 148 pp. LONSDALE, P. and C. D. HOLLISTER (1979) A near-bottom traverse of the Rockall Trough: hydrographic and geological inferences. Oceanologica Acta, 2 (1), 91-105. MARKLE, D. F. and J. A. MUSICK (1974) Benthic-slope fishes found at 900 m depth along a transect in the Western N. Atlantic Ocean. Marine Biology, 26,225-233. MARKLE, D. F. and C. A. WENNER (1979) Evidence of demersal spawning in the mesopelagic zoarcid fish Melanostigma atlanticum with comments on the demersal spawning in the alepocephalid fish Xenodemichthys copei. Copeia 1979 (2), 363-366. MARSHALL, N. B. and N. R. MERRETT (1977) The existence of a benthopelagic fauna in the deep sea. Deep-sea Research (Supplement), 483-497. MAUCHLINE, J. and J. D. M. GORDON (1983a) Diets of the sharks and chimaeroids of the Rockall Trough, northeastern Atlantic Ocean. Marine Biology, 75,269-278. MAUCHLINE, J. and J. D. M. GORDON (1983b) Diets of clupeoid stomiatoid and salmonid fishes of the Rockall Trough, northeastern Atlantic Ocean. Marine Biology, 77, 67-78. MAUCHLINE, J. and J. D. M. GORDON (1984a) Feeding and bathymetric distribution of the gadoid and morid fish of the Rockall Trough. Journal of the Marine Biological Association of the United Kingdom, 64,657-665. MAUCHLINE, J. and J. D. M. GORDON (1984b) Diets and bathymetric distribution of the macrourid fish of the Rockall Trough, northeastern Atlantic Ocean. Marine Biology, 81,107-121. MAUCHLINE, J. and J. D. M. GORDON (in press) Occurrence and feeding of berycomorphid and percomorphid teleost fish in the Rockall Trough. Journal de Conseil. MERRETT, N. R. and N. B. MARSHALL (1981) Observations on the ecology of deep-sea bottom-living fishes collected off northwest Africa (08 °-27°N). Progress in Oceanography, 9, 185-244. MOHR, H. and G. FREYTAG (1975) Weitere Untersuchungen zur ErschlieBurg neure Nutzfischbestiinde am westeuropaischen Schelfrand. InJbrmationen fur die Fischwirtschaft des Auslandes, 22 (3/4), 97-100. MURRAY, J. (1886) The physical and biological conditions of the sea and estuaries about North Britain. Proceedings of the Philosophical Society of Glasgow, 17, 306-333. MURRAY, J. and J. HJORT (1912) The depths of the ocean. MacMillan, 821 pp. PEARCE, G. J. (1980) Benthonic foraminifera from the continental slope and their fossil counterparts. Ph.D. Thesis, Aberystwyth: University College of Wales 2 volumes, 524 pp + plates. PEARCY, W. G., D. L. STEIN and R. S. CARNEY (1982) The deep-sea benthic fish fauna of the northeastern Pacific Ocean on Cascadia and Tufts Abyssal Plains and adjoining continental slopes. Biological Oceanography, 1,375-428, PECHENICK, L. N. and F. M. TROYANOVSKII (1970) Trawling resources of the North-Atlantic continental slope. Murmansk. Translated from Russian. Israel Program for Scientific Translations, Jerusalem 1971. POLLONI, P., R. HAEDRICH, G. ROWE and C. H. CLIFFORD (1979) The size-depth relationships in deep ocean animals, lnternationale Revue der Gesamten Hydrobiologie, 64, 39-46. ROBERTS, D. G. (1975) Marine Geology of the Rockall Plateau and Trough. Philosophical Transactions of the Royal Society of London, Series A, 278, 447-509. ROBERTS, D. G., P. A. HUNTER and A. S. LAUGHTON (1979) Bathymetry of the north-east Atlantic: continental margin around the British Isles. Deep-Sea Research, 26A, 417-428. THOMSON, C. W. (1874) The depths of the sea. MacMillan, London, 527 pp. TRESCHEV, A. I. (1978) Application of the fished volume method for measuring fishing effort. 1CES Cooperative Report No. 79, 54 pp. WAGNER, G. and M. STEHMANN (1975) M6gliche neue Nutzfische und daren Fangplatze im NO-Atlantik. Informationen fur die Fischwirtschaft des Auslandes, 22 (1), 8-12.