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The effect of invading harp seals (Phoca groenlandica) on local coastal fish stocks of North Norway
l'isheries Research, 13 (1992) 25-37 Elsevier Science Publishers B.V., Amsterdam
2s
The effect of invading harp seals (Phoca groenlundica) on local coastal fish stocks of North Norway Kjell T. Nilssen”, Per E. Grotne? and Tore Ha@ %stitufe ofMarine
Research, c/o Norwegian College of Fisheries Science, University oJTroms~. Breivika. N-9000 Tromse, Norway bNorwegian College ofFisheries Science, University of Tronw Breivika. N-9000 Tromse, Noway (Accepted I8 June 1991)
ABSTRACT Nilssen, K.T., Grotnes, P.E. and Hag, T., 1992. The effect of invading harp seals (Photo groenlandim) on local coastal fish stocksof North Norway. Fuh. Rex, 13: 2.5-37. Annual invasions of harp seals (Phoca groenlandica) into coastal ilreas and fjords of northern Norway during the last decade have created problems for fisheries in the area. Bottom-trawl survey data collected during 1986-1990 in Ullsfiord were used to evaluate possible interactionsbetween invading sealsand local fish stocks.The pre~enm in seal stomachs (sampled % Ullsfjord) of saithe (Polk&us virenr) and small cod (Gadus morhua), which were not frequent I” trawl catches, seems to indicate that seals may pursue prey in waters other than the trawl areas. Many of the speciesoccurring in the seal stomachs were. however, also encountered quite frequently III the trawl catches. Harp seals may cause considerable damage to gill-net catches by eating parts (usually the ventral soft pans) of fish entangled in the nets.
INTRODUCTION
In their general migration pattern, the harp seals (Phocu groenlundica) in the Barents Sea (Smimov, 1924; Chap&ii, 1938; Wiig, 1988) leave the White Sea area after breeding and moult, and follow the pack-ice belt northwards into the Barents Sea. Usually, they are distributed in open waters along the pack-ice belt in the Barents Sea during summer and autumn, and in OctoberDecember they return to coastal waters in the southeastern Barents Sea. Contrary to this habitual migration pattern, harp seals started to appear in large numbers along the coast of Finnmark, North Norway, in winter and spring from 1978 onwards (Bjsrge et al., 1981). This aberrant migratory pattern has prevailed throughout the 1980s with a dramatic increase in magnitude and extent in 1987 and 1988 (Hauget al., 1991). The invading seals have caused major problems for Norwegian fisheries. 0 1992 Elsevier Science Publishers B.V. All rights reserved 01657836/92/$05.00
K.T.NlLsSEN ET AL.
26
This applies, in particular, to fisheries in coastal areas and fjords where substantial numbers of seals have been captured and drowned in gill nets every winter (Haug et al., 199 1). In adWon to the damage to gill nets, fishermen claim that the presence of the seals leads to changes in fish behaviour such that tish become less available on the traditional fishing grounds. It is also said that the seals attack and eat fish entangled in the gill nets. During the years 1986-l 990, the Norwegian College of Fisheries Science carried out trawl surveys in several fjords in Trams, North Norway. These surveys were conducted twice a year in several typical ‘seal invasion areas’, first in January (i.e. before the seals arrived in these coastal areas) and then in February/March (after the arrival of seals). Furthermore, the survey years were either characterized by very heavy invasions (1987 and 1988) or invasions on a much more moderate scale ( 1986, 1989 and 1990; Haug et al., I99 1)_These trawl survey data have been analysed and standardized in order to investigate whether the claims about interactions between the invading seals and local fish stocks, in particular cod (Gadus morhua) can be substantiated. MATERIALS
AND METHODS
Harp seai occuv~ Ice in Urrsjord 1986-I 990 Detailed records of the extent and seasonal variation of harp seal catches in Ullstjord for the period in question are not available. Such information must, therefore, be drawn from the more general patterns of the harp seal invasions in North Norway (Haug et al., 199 1) and from personal communication with local fishermen and managers of fish-processing plants in the area. Harp seal stomach samples Stomach samples were collected from harp seals taken in gill-net fisheries
(using gill nets designed to select large cod) in Ullsljord, North Norway (Fig. 1) in January/February 1986 (nine seals) and in February 1988 (50 seals). The material was collected as part of a more thorough study of the diet of harp seals invading coastal areas of North Norway during winter (Haug et al., I99 I ) . A detailed description of both the sampling procedures and the methods applied in the stomach analyses (where both the numerical and biomass contribution of each prey species to the harp seal diets were evaluated) is given by Haug et al. ( 199 1).
Fig. I. Map showing the survey area, Ullsfjorden, with its innermost part, Serijorden. calilies of the lrawl sampling stations (with approximate depths in m) are given.
The lo-
28
Trawlsurveys in VIlsjord/Serfiord
in 1986-1990
Bottom trawling using a shrimp trawl (REFA Gisund Super 1280 mesh shrimp trawl with rubber/steel bobbins, sweep wires of 40 m, fishing line length of 53.2 m, and the following mesh sizes: wings and square 65 mm, belly 44 mm, codend 10 mm) was carried out wi:? K/V ‘Johan Ruud’ in Ullstjorden/brljorden, North Norway (Fig. I ), during January and February/March in 1986- 1990 (Table 1). The towing speed was 2 knots. Trawl surveys were not specificially conducted for the purposes of the present investigation, and the sampling was not uniform with respect to the time of day, areas, depths and duration of the hauls. The latter varied between 9 and 60 min, with the majority (8 1%) lasting between 15 and 34 min. Also, information about the quantities of the various species taken in each haul is sometimes incomplete and, in some cases, includes only the number of individuais. Thus, data were reanalysed, transformed and extrapolated to a standard haul of 30 min duration. Information was then used to provide a crude estimate of the relative contribution, by weight, of each fish species to the total catch in each such standard haul. The final results per station are the calculated mean values per standard haul. Cod was a main target species during the trawl surveys and considerable TABLE
I
Samplinglocalions. depths and number of shrimp trawl hauls performed in inner (Stwijorden) and outer par&of Ullsfjorden, North Norway, in January (J) and February/March (F/M) 1986-1990. For map referencer, see Fig. I Location
Depth cm)
Numberofhauls 1986 -JFJFJMJ
1987 -
1988 -
1989 F/M
44
57
Stordal Njosken Skognes Olderbakken
125 61 I25
Eidstranddjupct West of Ameya
80 110 107 270 274 450
3
3
2 7
I
I 2
2
:
2
4
1 I
I
2
I I
I
I 3 I 3
I I I
1 I I
I
I
I
I
2
2 I
I 1
1990 J F
I I
information about the size composition of this species is the catches is available. The size composition of cod in seal stomachs was calculated from otolith sizes (Hark&ten, 1986), and results from trawl catches and stomach analyses compared in order to assess size selectivity in the predation by seals on fish.
On coastal banks off Troms county (Fig. 1 ), records were kept of possible seal damage on cod entangled in gill nets on board two commercial vessels during the period 13-27 January 1986, which was a year when considerable numbers of harp seals invaded these and other fishing grounds in North Norway (Haug et al., 1991). Similar records were kept-of two gill-net and two long-line vessels (all commercial) during the period 3 January-14 Febrnary 1991, when the winter invasions of harp seals to the coastal areas of Troms appeared to have ceased almost completely (L. Pettersen, Norges RBfisklag, Tromsa, Norway, personal communication, 1991). Damage to the fish was assessed by estimating the relative proportions (by weight) of fish that had been preyed upon by seals in relation to the total catch (both damaged and undamaged fish). All fish were gutted before weighing. Damaged fish were weighed with no correction being made for the pieces removed by seals, although it is acknowledged that this may lead to some underestimation of the proportion of damaged fish. RESULTS
Harp seal occurrence in Ullsjjorden in 1986-l
990
Haug et al. ( 1991) describe the seal invasion in North Norway as being moderate in 1986, very extensive in I987 and 1988, and at a very low level in 1989 and 1990. The patterns of harp seal invasions in Ullsflord appear to have been similar to the more general patterns in Northern Norway (B. Pedersen, fish processing plant manager, Ullsfjord, Norway, personal communication, 1990): bycatches from gill nets in the area were estimated to be 100-200 seals in 1986, in excess of 800 seals in 1987, approximately 500 seals in 1988 and less than ten seals in each of the years 1989 and 1990. In the years with the most extensive invasions, single animals were observed as early as around New Year, but the main bycatches which were, in some instances, as many as 35 seals per boat (40 gill nets) per day, were always made from the middle of January and in February/March (A. Hansen, fisherman, Ullsfjord, Norway, personal communication, 1990).
30
K.T. NlLssEN
ETAL.
TABLE 2 Relative composition (%) of stomach contents in harp seals (based on calculated fresh weight) and ofboltom-trawl catches (based an estimated round weights) as observed in Ullsljord. North Norway, in Januarv-March in 1986 and 1988. Prey item
1988
1986 Trawl
Seal
Trawl
SeaI
a
a
a
Mollusca Cephalopoda Todarodn sagimus Crustacea Decapoda
a
a a
I3
a
a a 34 36
a a 21 a
6 7 a a
44 20 a a
20 I2 a a
I9 21
I
27 I7
a Anarhichadidae
,~rwtrrctlus hprr
Scorpcnidae SPhaac.~sp. Plcuronectidae
a a
a
a
311
a a a a
a a a
a
31
Fig. 2. Relative contribution (by calculated fresh weight) of various prey organisms to the total biomass of various prey organisms in harp seal stomachs and of organisms captured in standardized bottom-trawl hauls in Ullsijorden in 1986 and 1988.
Species composition in seal stomachs and bottom-trawl catches In 1986, the number of prey species found in the nine seal stomachs examined was 12, whereas at least 16 taxa were taken in the trawl catches (all 1986 stations pooled) (Table 2). In 1988, when 50 seal stomachs were examined, 14 different prey species were identified and 19 taxa were captured in bottom-trawl catches (all 1988 stations pooled). Of the species that contributed most to the total biomass in the bottomtrawl catches, cod, haddock (Melanogrammus aeglefinus) and Norway pout (Trisopfenrs tsmarckii) were the major prey ofthe harp seals taken in the ared (Fig. 2, Table 2 ). Saithe (Pollachius virens), a species that contributed little
to the trawl catches, was present in large quantities in the seal stomachs. Size composition of codfrom seal stomachs and trawl hauls The 1986 samples of cod identified
in seal stomachs
(25 fish taken from
two stomachs) comprised only small fish and these were largely within size groups that were absent from the trawl samples (Fig. 3). The 1988 samples of cod from seal stomachs (99 fish from 25 stomachs) were generally characterized by fish which were smaller in size than those taken in a bottom trawl, although otoliths from some larger cod were also found in the seal stomachs (Fig. 3).
Bottom-trawl surveys in Ullsjorden 1986-1990 In Sortjorden in 1986, the mean total catch weight per standard haul tended to be larger in the February/March surveys than in the January surveys (Fig.
32
K-T. NltsSEN
ET AL.
1988
TOTAL LENGTH @cm groups) Fig. 3. Size composition catches (white columns)
of cod found in harp seal stomacL& in Ullstjorden in 1986 and 1988.
(black
columns)
and taken
in trawl
In 1987 and 1988, the situation in this inner part of Ullsfjorden was reversed in that the January surveys tended to give very large mean catches, whilst only small catches were obtained in February/March. Also in 1989, the largest mean catches were obtained in January, but the difference between the two periods tended to be less than in the two previous years. The distribution of fish in Sortjorden is apparently quite patchy, thus allowing for the possibilities of obtaining both small and large catches in most periods (cf. the usutrlly large standard deviations). The low values in both means and standard deviations in the February/March catches in Serfjorden in 1987 and 4).
33
Fis. 4. Mean total catch weight per standard (30 min) trawl haul taken in 1986-1990 in Sarfiorden and wtcr Uiisfiord (above and below 200 m depth) in January (white columns) and in February/March (hatchedcolumns). Standard deviationsare indicated.
1988 seem to indicate, however, that the chances of obtaining large catches have been very small in all parts of the fJord during this period in these 2 years. Trawl survey data from the outer parts of Ullstjorden are incomplete (Fig. 4). This applies in particular to January data which are not available for 19S1, 1989 and 1990 The relatively large catches obtained in Sorfjorden in January and February/March in 1986 and 1989, and In January in 1987 and 1988, were mainly comprised of cod and haddock (Fig. 5). In the February/March catches in 1987 and 1988, cod and haddock were almost absent. These catches contained mainly herring (Clupeti harengus), flat&h and other miscellaneous species. must
Harp seal damage on cod in gill nels Observations made on board the two gill-net vessels during 2 weeks of fisheries on coastal banks off Troms, North Norway, in January 1986 suggest that
SORFJORO
OUTER “LLSFJORO
Fig. 5. Relative contribution (by calculated weight) of the various organisms to the total biomass obtained in average standard hauls made in 1986-1989 in Swtjord and outer Ullstjord (above and below 200 m depth) in January (J), February (F) and March (M).
2.2-26% of the daily catches (varying between 200 and 1400 kg) of cod might have been damaged by seals (Table 3). The damaged fish were often large specimens weighing 3-4 kg. Usually, the ventral soft parts, including the liver and intestines, were taken. During similar observations made in I99 I, when the four vessels engaged in the study obtained daily catches of 400-4000 kg, no fish were observed to have been damaged by seals. Apparently, this also seems to have been the case during the winter cod fisheries in these areas in the years 1989 and 1990 (H. Robertson, fisherman, Tromvik, Norway, personal communication, 1991).
EFFECTS OF INYADINC
“*RP
SEALS
IN
35
N NORWAI
TABLE 3 Relativeproponionsoffish damaged by seals in totalgtll-“etcalchesmadeoncoastal banksoffTroms in January 1986. The undamaged catch isgivcn as total gutted weight and the damaged catch as round weight with no corrections being made for the p~cccsrcmovcd by seals. The total catch is the sum of Ihe damaged and undamaged catch. Data are drawn from cashes of two commercial vcsscls.with nets being hauled cvcry second day Date
Jan. I3 Jan. 15 Jan. I7 Jan. I8 Jan. 20 Jan. 22 Jan. 24 Jan. 26 Jan. 27 Jan. 27 Mea”
Undamaged catch
Damaged catch
(kg)
(kg)
Relative proportion damaged (sb)
505 350 1330 378 468 452 400 I71 385 674
50 40 30 I5 50 26 IO 60 40 40
9.0 10.3 2.2 3.8 9.7 5.4 2.4 26.0 9.4 5.6 8.4
DFXUSSION There appears to be a considerable degree of overlap between prey species occurring in the stomachs of the opportunistic (Bowen, 1985; Kapel and Angantyr, 1989; Haug et al., 199 1) harp seals and organisms captured in shrimp trawl within the same major area and period of time. This applies in panicular to species such as cod, haddock and Norway pout. There are, however, some apparent differences between the contents of seal stomachs and trawl catches, and some of these differences may give some indication of the harp seal feeding habitats in the area. The saithe is a shoaling species which occurs mainly in the littoral zone as O-group (Godo et al., 1989), in upper and mid-water layers when small and of medium size, and in deeper water when large (Pethon, 1985). Thus, the rare occurrence of saithe in the Ulls~ord bottom-trawl catches is not surprising. The species was, however, frequently encountered in seal stomachs. This may suggest that the harp seals feed mainly in shallow and/or pelagic waters. Feeding in more shallow areas is also inferred from the observed size compositions of cod in the seal stomachs. Cod in seal stomachs were generally smaller than those in the trawl catches. The smallest cod are known to occur in shallow sheltered areas and they generally avoid more exposed locations and deeper waters where larger cod are usually found (Hawkins et al., 1985; Gode et al., 1989). Additional factors contributing to the observed difference in size compo-
K.T. NllsSEN ET AL.
36
sition of cod found in seal stomachs and trawl catches may be that all larger cod either leave the area (however, recent tagging data suggest that cod are rather sedentary, T. Pedersen, Norwegian College of Fisheries Science, Tromse, Norway, personal communication, I99 I ) , or move to deeper strata or strata unfit for surveying with trawl gear. The results from the trawl surveys may support this suggestion. In Sorfjorden (the shallowest innermost part of Ullsljorden), the trawl catches taken in the ‘seal-years’ 1987 and 1988 in February/March (i.e. after the seasonal arrival of the seals) were very small compared with those obtained in January before the arrival of the seals. In the years with only moderate invasion ( I986 and 1989; Haug et al., I99 I ), no such changes occurred. It appears that cod and, to a certain extent, haddock disappeared from the catches when seals were present in the area. Tke data presented do not allow any firm conclusions to be made regarding the emigration of cod after the arrival of seals. The survey data from the outer parts of Ullsljord are scarce and inconclusive, but it may be noted that the catches obtained in the deepest strata were particularly large in both I987 and 1988, and included considerable amounts of cod in the latter year. Whatever the emigration mechanisms may be, it appears that in the presence of large numbers of seals, certain commercial fish species, such as cod, may change their behaviour and disappear from the traditional fishing grounds. Observations made in commercial gill-net fisheries reveal that the harp seals
may cause considerable damage to the catches (2-26% by weight) by tearing and eating pieces (usually the ventral soft parts) of large (3-4 kg) fish entangled in the nets. However, whether the seals display this feeding habit on freeswimming large fish is not known. If it did occur, such predation would go undetected in studies where otoliths were used as the primary aid for identifying fish prey in stomach contents. ACKNOWLEDGEMENTS
Thanks are due to J.-E. Eliassen, field assistants and the crew of R/V ‘Joban Ruud’ who surveyed Ullsfjord during 1986-l 990 and gave us access to the data. H. Robertson helped to organize the registrations of seal damage on commercially captured fish, whereas K.A. Fagerheim, C. Lydersen, N.H. Markussen and J. dos Santos collected the harp seal stomachs, and M. Jobling provided valuable comments on the manuscript. The ecological studies on harp seals are supported by funding from the Norwegian Council of Fisheries Research (NFFR), project nos. I 500.024 and I 701.260. REFERENCES B~orge. A.. Cbriuen5en.
1. and Orilsland, T., 1981. Current problems and research related 10 mteraclions between marine mammals and fisheries in Norwegian coastal and adjacent wavn. ICESCM. 198l/N:lS: IO pp. (Mimcogr.).
EFFECTSOF “WADING
HARPSEALS
INN NORWAY
37
Bowen. W.D., 198.5. Harp seal feeding and interactions with commercial iisheries in the northwest Atlantic. In: J.R. Beddington. R.J.H. Bevenan and D.M. Lavigne (Editors), Marine Mammals and Fisheries. George Allen and Unwin. London, pp. I35- 152. Chapskii, K.K., 1938. Noveishie dennye o raspredelenskii Belmorskoi rasy Grenlandskogo tyulenya we Behnorskogo (New data on distribution of the White Sea race of harp seal outside the White Sea basin). Probl. Arktiki, 4: 105-131. Code, OX., Gjowter, J.. Sunnan& K. and Dragesund, 0.. 1989. Spatial distribution ofO-group gadoidsoffmid-Norway. Rapp. P.-V. R&m. Cons. lot. Explor. Mer. 191: 273-280. Hsrkiinen. T., 1986. Guide to Otoliths of the Bony Fishes of the Northeast Atlantic. Danibu ApS, Hellerup. Denmark. 256 pp. Haug. T. Krqer, A.B.. Nilssen, K.T., Ugland. K.I. and.Aspholm. P.E., 1991. Harpseal (Pl,occ grocnlandica) invasions in North Norwegian coastal waters: age composition and feeding habits. ICES J. Mar. Sci.. 48: in press. Hawkins, A.D.. Soutiani. N.M. and Smith. G.W., 1985. Growth and feeding ofcod, Gadm wmhua L. J. Cons. Int. Explor. Mer. 42: I l-32. Kapel. H. and Angantyr, L.A., 1989. Feeding patterns of harp seals (F’hocu groenlandica) in coastal waters of West Greenland, with a note on of&hare feeding. ICES CM, 1989/N% 28 pp. (Mimeogr. ). Pethon, P.. 1985. Aschehougs store tiskebok. H. Aschehoug & Co. (W. Nygaard) A/S, 447 pp. Smirnov, N.. 1924. On the eastern harp seal Phoca (Pa~ophhor.a),ro~nla~,dicavar. oceanica Lepechin. Tmmse Mus. Arsh.. 47: I-I 1. Wiig. 0.. 1988. Gronlandssel og relinvasjon Hva vet vi-hva tror vi? Naturen, l988( 2): 35-41.
2s
The effect of invading harp seals (Phoca groenlundica) on local coastal fish stocks of North Norway Kjell T. Nilssen”, Per E. Grotne? and Tore Ha@ %stitufe ofMarine
Research, c/o Norwegian College of Fisheries Science, University oJTroms~. Breivika. N-9000 Tromse, Norway bNorwegian College ofFisheries Science, University of Tronw Breivika. N-9000 Tromse, Noway (Accepted I8 June 1991)
ABSTRACT Nilssen, K.T., Grotnes, P.E. and Hag, T., 1992. The effect of invading harp seals (Photo groenlandim) on local coastal fish stocksof North Norway. Fuh. Rex, 13: 2.5-37. Annual invasions of harp seals (Phoca groenlandica) into coastal ilreas and fjords of northern Norway during the last decade have created problems for fisheries in the area. Bottom-trawl survey data collected during 1986-1990 in Ullsfiord were used to evaluate possible interactionsbetween invading sealsand local fish stocks.The pre~enm in seal stomachs (sampled % Ullsfjord) of saithe (Polk&us virenr) and small cod (Gadus morhua), which were not frequent I” trawl catches, seems to indicate that seals may pursue prey in waters other than the trawl areas. Many of the speciesoccurring in the seal stomachs were. however, also encountered quite frequently III the trawl catches. Harp seals may cause considerable damage to gill-net catches by eating parts (usually the ventral soft pans) of fish entangled in the nets.
INTRODUCTION
In their general migration pattern, the harp seals (Phocu groenlundica) in the Barents Sea (Smimov, 1924; Chap&ii, 1938; Wiig, 1988) leave the White Sea area after breeding and moult, and follow the pack-ice belt northwards into the Barents Sea. Usually, they are distributed in open waters along the pack-ice belt in the Barents Sea during summer and autumn, and in OctoberDecember they return to coastal waters in the southeastern Barents Sea. Contrary to this habitual migration pattern, harp seals started to appear in large numbers along the coast of Finnmark, North Norway, in winter and spring from 1978 onwards (Bjsrge et al., 1981). This aberrant migratory pattern has prevailed throughout the 1980s with a dramatic increase in magnitude and extent in 1987 and 1988 (Hauget al., 1991). The invading seals have caused major problems for Norwegian fisheries. 0 1992 Elsevier Science Publishers B.V. All rights reserved 01657836/92/$05.00
K.T.NlLsSEN ET AL.
26
This applies, in particular, to fisheries in coastal areas and fjords where substantial numbers of seals have been captured and drowned in gill nets every winter (Haug et al., 199 1). In adWon to the damage to gill nets, fishermen claim that the presence of the seals leads to changes in fish behaviour such that tish become less available on the traditional fishing grounds. It is also said that the seals attack and eat fish entangled in the gill nets. During the years 1986-l 990, the Norwegian College of Fisheries Science carried out trawl surveys in several fjords in Trams, North Norway. These surveys were conducted twice a year in several typical ‘seal invasion areas’, first in January (i.e. before the seals arrived in these coastal areas) and then in February/March (after the arrival of seals). Furthermore, the survey years were either characterized by very heavy invasions (1987 and 1988) or invasions on a much more moderate scale ( 1986, 1989 and 1990; Haug et al., I99 1)_These trawl survey data have been analysed and standardized in order to investigate whether the claims about interactions between the invading seals and local fish stocks, in particular cod (Gadus morhua) can be substantiated. MATERIALS
AND METHODS
Harp seai occuv~ Ice in Urrsjord 1986-I 990 Detailed records of the extent and seasonal variation of harp seal catches in Ullstjord for the period in question are not available. Such information must, therefore, be drawn from the more general patterns of the harp seal invasions in North Norway (Haug et al., 199 1) and from personal communication with local fishermen and managers of fish-processing plants in the area. Harp seal stomach samples Stomach samples were collected from harp seals taken in gill-net fisheries
(using gill nets designed to select large cod) in Ullsljord, North Norway (Fig. 1) in January/February 1986 (nine seals) and in February 1988 (50 seals). The material was collected as part of a more thorough study of the diet of harp seals invading coastal areas of North Norway during winter (Haug et al., I99 I ) . A detailed description of both the sampling procedures and the methods applied in the stomach analyses (where both the numerical and biomass contribution of each prey species to the harp seal diets were evaluated) is given by Haug et al. ( 199 1).
Fig. I. Map showing the survey area, Ullsfjorden, with its innermost part, Serijorden. calilies of the lrawl sampling stations (with approximate depths in m) are given.
The lo-
28
Trawlsurveys in VIlsjord/Serfiord
in 1986-1990
Bottom trawling using a shrimp trawl (REFA Gisund Super 1280 mesh shrimp trawl with rubber/steel bobbins, sweep wires of 40 m, fishing line length of 53.2 m, and the following mesh sizes: wings and square 65 mm, belly 44 mm, codend 10 mm) was carried out wi:? K/V ‘Johan Ruud’ in Ullstjorden/brljorden, North Norway (Fig. I ), during January and February/March in 1986- 1990 (Table 1). The towing speed was 2 knots. Trawl surveys were not specificially conducted for the purposes of the present investigation, and the sampling was not uniform with respect to the time of day, areas, depths and duration of the hauls. The latter varied between 9 and 60 min, with the majority (8 1%) lasting between 15 and 34 min. Also, information about the quantities of the various species taken in each haul is sometimes incomplete and, in some cases, includes only the number of individuais. Thus, data were reanalysed, transformed and extrapolated to a standard haul of 30 min duration. Information was then used to provide a crude estimate of the relative contribution, by weight, of each fish species to the total catch in each such standard haul. The final results per station are the calculated mean values per standard haul. Cod was a main target species during the trawl surveys and considerable TABLE
I
Samplinglocalions. depths and number of shrimp trawl hauls performed in inner (Stwijorden) and outer par&of Ullsfjorden, North Norway, in January (J) and February/March (F/M) 1986-1990. For map referencer, see Fig. I Location
Depth cm)
Numberofhauls 1986 -JFJFJMJ
1987 -
1988 -
1989 F/M
44
57
Stordal Njosken Skognes Olderbakken
125 61 I25
Eidstranddjupct West of Ameya
80 110 107 270 274 450
3
3
2 7
I
I 2
2
:
2
4
1 I
I
2
I I
I
I 3 I 3
I I I
1 I I
I
I
I
I
2
2 I
I 1
1990 J F
I I
information about the size composition of this species is the catches is available. The size composition of cod in seal stomachs was calculated from otolith sizes (Hark&ten, 1986), and results from trawl catches and stomach analyses compared in order to assess size selectivity in the predation by seals on fish.
On coastal banks off Troms county (Fig. 1 ), records were kept of possible seal damage on cod entangled in gill nets on board two commercial vessels during the period 13-27 January 1986, which was a year when considerable numbers of harp seals invaded these and other fishing grounds in North Norway (Haug et al., 1991). Similar records were kept-of two gill-net and two long-line vessels (all commercial) during the period 3 January-14 Febrnary 1991, when the winter invasions of harp seals to the coastal areas of Troms appeared to have ceased almost completely (L. Pettersen, Norges RBfisklag, Tromsa, Norway, personal communication, 1991). Damage to the fish was assessed by estimating the relative proportions (by weight) of fish that had been preyed upon by seals in relation to the total catch (both damaged and undamaged fish). All fish were gutted before weighing. Damaged fish were weighed with no correction being made for the pieces removed by seals, although it is acknowledged that this may lead to some underestimation of the proportion of damaged fish. RESULTS
Harp seal occurrence in Ullsjjorden in 1986-l
990
Haug et al. ( 1991) describe the seal invasion in North Norway as being moderate in 1986, very extensive in I987 and 1988, and at a very low level in 1989 and 1990. The patterns of harp seal invasions in Ullsflord appear to have been similar to the more general patterns in Northern Norway (B. Pedersen, fish processing plant manager, Ullsfjord, Norway, personal communication, 1990): bycatches from gill nets in the area were estimated to be 100-200 seals in 1986, in excess of 800 seals in 1987, approximately 500 seals in 1988 and less than ten seals in each of the years 1989 and 1990. In the years with the most extensive invasions, single animals were observed as early as around New Year, but the main bycatches which were, in some instances, as many as 35 seals per boat (40 gill nets) per day, were always made from the middle of January and in February/March (A. Hansen, fisherman, Ullsfjord, Norway, personal communication, 1990).
30
K.T. NlLssEN
ETAL.
TABLE 2 Relative composition (%) of stomach contents in harp seals (based on calculated fresh weight) and ofboltom-trawl catches (based an estimated round weights) as observed in Ullsljord. North Norway, in Januarv-March in 1986 and 1988. Prey item
1988
1986 Trawl
Seal
Trawl
SeaI
a
a
a
Mollusca Cephalopoda Todarodn sagimus Crustacea Decapoda
a
a a
I3
a
a a 34 36
a a 21 a
6 7 a a
44 20 a a
20 I2 a a
I9 21
I
27 I7
a Anarhichadidae
,~rwtrrctlus hprr
Scorpcnidae SPhaac.~sp. Plcuronectidae
a a
a
a
311
a a a a
a a a
a
31
Fig. 2. Relative contribution (by calculated fresh weight) of various prey organisms to the total biomass of various prey organisms in harp seal stomachs and of organisms captured in standardized bottom-trawl hauls in Ullsijorden in 1986 and 1988.
Species composition in seal stomachs and bottom-trawl catches In 1986, the number of prey species found in the nine seal stomachs examined was 12, whereas at least 16 taxa were taken in the trawl catches (all 1986 stations pooled) (Table 2). In 1988, when 50 seal stomachs were examined, 14 different prey species were identified and 19 taxa were captured in bottom-trawl catches (all 1988 stations pooled). Of the species that contributed most to the total biomass in the bottomtrawl catches, cod, haddock (Melanogrammus aeglefinus) and Norway pout (Trisopfenrs tsmarckii) were the major prey ofthe harp seals taken in the ared (Fig. 2, Table 2 ). Saithe (Pollachius virens), a species that contributed little
to the trawl catches, was present in large quantities in the seal stomachs. Size composition of codfrom seal stomachs and trawl hauls The 1986 samples of cod identified
in seal stomachs
(25 fish taken from
two stomachs) comprised only small fish and these were largely within size groups that were absent from the trawl samples (Fig. 3). The 1988 samples of cod from seal stomachs (99 fish from 25 stomachs) were generally characterized by fish which were smaller in size than those taken in a bottom trawl, although otoliths from some larger cod were also found in the seal stomachs (Fig. 3).
Bottom-trawl surveys in Ullsjorden 1986-1990 In Sortjorden in 1986, the mean total catch weight per standard haul tended to be larger in the February/March surveys than in the January surveys (Fig.
32
K-T. NltsSEN
ET AL.
1988
TOTAL LENGTH @cm groups) Fig. 3. Size composition catches (white columns)
of cod found in harp seal stomacL& in Ullstjorden in 1986 and 1988.
(black
columns)
and taken
in trawl
In 1987 and 1988, the situation in this inner part of Ullsfjorden was reversed in that the January surveys tended to give very large mean catches, whilst only small catches were obtained in February/March. Also in 1989, the largest mean catches were obtained in January, but the difference between the two periods tended to be less than in the two previous years. The distribution of fish in Sortjorden is apparently quite patchy, thus allowing for the possibilities of obtaining both small and large catches in most periods (cf. the usutrlly large standard deviations). The low values in both means and standard deviations in the February/March catches in Serfjorden in 1987 and 4).
33
Fis. 4. Mean total catch weight per standard (30 min) trawl haul taken in 1986-1990 in Sarfiorden and wtcr Uiisfiord (above and below 200 m depth) in January (white columns) and in February/March (hatchedcolumns). Standard deviationsare indicated.
1988 seem to indicate, however, that the chances of obtaining large catches have been very small in all parts of the fJord during this period in these 2 years. Trawl survey data from the outer parts of Ullstjorden are incomplete (Fig. 4). This applies in particular to January data which are not available for 19S1, 1989 and 1990 The relatively large catches obtained in Sorfjorden in January and February/March in 1986 and 1989, and In January in 1987 and 1988, were mainly comprised of cod and haddock (Fig. 5). In the February/March catches in 1987 and 1988, cod and haddock were almost absent. These catches contained mainly herring (Clupeti harengus), flat&h and other miscellaneous species. must
Harp seal damage on cod in gill nels Observations made on board the two gill-net vessels during 2 weeks of fisheries on coastal banks off Troms, North Norway, in January 1986 suggest that
SORFJORO
OUTER “LLSFJORO
Fig. 5. Relative contribution (by calculated weight) of the various organisms to the total biomass obtained in average standard hauls made in 1986-1989 in Swtjord and outer Ullstjord (above and below 200 m depth) in January (J), February (F) and March (M).
2.2-26% of the daily catches (varying between 200 and 1400 kg) of cod might have been damaged by seals (Table 3). The damaged fish were often large specimens weighing 3-4 kg. Usually, the ventral soft parts, including the liver and intestines, were taken. During similar observations made in I99 I, when the four vessels engaged in the study obtained daily catches of 400-4000 kg, no fish were observed to have been damaged by seals. Apparently, this also seems to have been the case during the winter cod fisheries in these areas in the years 1989 and 1990 (H. Robertson, fisherman, Tromvik, Norway, personal communication, 1991).
EFFECTS OF INYADINC
“*RP
SEALS
IN
35
N NORWAI
TABLE 3 Relativeproponionsoffish damaged by seals in totalgtll-“etcalchesmadeoncoastal banksoffTroms in January 1986. The undamaged catch isgivcn as total gutted weight and the damaged catch as round weight with no corrections being made for the p~cccsrcmovcd by seals. The total catch is the sum of Ihe damaged and undamaged catch. Data are drawn from cashes of two commercial vcsscls.with nets being hauled cvcry second day Date
Jan. I3 Jan. 15 Jan. I7 Jan. I8 Jan. 20 Jan. 22 Jan. 24 Jan. 26 Jan. 27 Jan. 27 Mea”
Undamaged catch
Damaged catch
(kg)
(kg)
Relative proportion damaged (sb)
505 350 1330 378 468 452 400 I71 385 674
50 40 30 I5 50 26 IO 60 40 40
9.0 10.3 2.2 3.8 9.7 5.4 2.4 26.0 9.4 5.6 8.4
DFXUSSION There appears to be a considerable degree of overlap between prey species occurring in the stomachs of the opportunistic (Bowen, 1985; Kapel and Angantyr, 1989; Haug et al., 199 1) harp seals and organisms captured in shrimp trawl within the same major area and period of time. This applies in panicular to species such as cod, haddock and Norway pout. There are, however, some apparent differences between the contents of seal stomachs and trawl catches, and some of these differences may give some indication of the harp seal feeding habitats in the area. The saithe is a shoaling species which occurs mainly in the littoral zone as O-group (Godo et al., 1989), in upper and mid-water layers when small and of medium size, and in deeper water when large (Pethon, 1985). Thus, the rare occurrence of saithe in the Ulls~ord bottom-trawl catches is not surprising. The species was, however, frequently encountered in seal stomachs. This may suggest that the harp seals feed mainly in shallow and/or pelagic waters. Feeding in more shallow areas is also inferred from the observed size compositions of cod in the seal stomachs. Cod in seal stomachs were generally smaller than those in the trawl catches. The smallest cod are known to occur in shallow sheltered areas and they generally avoid more exposed locations and deeper waters where larger cod are usually found (Hawkins et al., 1985; Gode et al., 1989). Additional factors contributing to the observed difference in size compo-
K.T. NllsSEN ET AL.
36
sition of cod found in seal stomachs and trawl catches may be that all larger cod either leave the area (however, recent tagging data suggest that cod are rather sedentary, T. Pedersen, Norwegian College of Fisheries Science, Tromse, Norway, personal communication, I99 I ) , or move to deeper strata or strata unfit for surveying with trawl gear. The results from the trawl surveys may support this suggestion. In Sorfjorden (the shallowest innermost part of Ullsljorden), the trawl catches taken in the ‘seal-years’ 1987 and 1988 in February/March (i.e. after the seasonal arrival of the seals) were very small compared with those obtained in January before the arrival of the seals. In the years with only moderate invasion ( I986 and 1989; Haug et al., I99 I ), no such changes occurred. It appears that cod and, to a certain extent, haddock disappeared from the catches when seals were present in the area. Tke data presented do not allow any firm conclusions to be made regarding the emigration of cod after the arrival of seals. The survey data from the outer parts of Ullsljord are scarce and inconclusive, but it may be noted that the catches obtained in the deepest strata were particularly large in both I987 and 1988, and included considerable amounts of cod in the latter year. Whatever the emigration mechanisms may be, it appears that in the presence of large numbers of seals, certain commercial fish species, such as cod, may change their behaviour and disappear from the traditional fishing grounds. Observations made in commercial gill-net fisheries reveal that the harp seals
may cause considerable damage to the catches (2-26% by weight) by tearing and eating pieces (usually the ventral soft parts) of large (3-4 kg) fish entangled in the nets. However, whether the seals display this feeding habit on freeswimming large fish is not known. If it did occur, such predation would go undetected in studies where otoliths were used as the primary aid for identifying fish prey in stomach contents. ACKNOWLEDGEMENTS
Thanks are due to J.-E. Eliassen, field assistants and the crew of R/V ‘Joban Ruud’ who surveyed Ullsfjord during 1986-l 990 and gave us access to the data. H. Robertson helped to organize the registrations of seal damage on commercially captured fish, whereas K.A. Fagerheim, C. Lydersen, N.H. Markussen and J. dos Santos collected the harp seal stomachs, and M. Jobling provided valuable comments on the manuscript. The ecological studies on harp seals are supported by funding from the Norwegian Council of Fisheries Research (NFFR), project nos. I 500.024 and I 701.260. REFERENCES B~orge. A.. Cbriuen5en.
1. and Orilsland, T., 1981. Current problems and research related 10 mteraclions between marine mammals and fisheries in Norwegian coastal and adjacent wavn. ICESCM. 198l/N:lS: IO pp. (Mimcogr.).
EFFECTSOF “WADING
HARPSEALS
INN NORWAY
37
Bowen. W.D., 198.5. Harp seal feeding and interactions with commercial iisheries in the northwest Atlantic. In: J.R. Beddington. R.J.H. Bevenan and D.M. Lavigne (Editors), Marine Mammals and Fisheries. George Allen and Unwin. London, pp. I35- 152. Chapskii, K.K., 1938. Noveishie dennye o raspredelenskii Belmorskoi rasy Grenlandskogo tyulenya we Behnorskogo (New data on distribution of the White Sea race of harp seal outside the White Sea basin). Probl. Arktiki, 4: 105-131. Code, OX., Gjowter, J.. Sunnan& K. and Dragesund, 0.. 1989. Spatial distribution ofO-group gadoidsoffmid-Norway. Rapp. P.-V. R&m. Cons. lot. Explor. Mer. 191: 273-280. Hsrkiinen. T., 1986. Guide to Otoliths of the Bony Fishes of the Northeast Atlantic. Danibu ApS, Hellerup. Denmark. 256 pp. Haug. T. Krqer, A.B.. Nilssen, K.T., Ugland. K.I. and.Aspholm. P.E., 1991. Harpseal (Pl,occ grocnlandica) invasions in North Norwegian coastal waters: age composition and feeding habits. ICES J. Mar. Sci.. 48: in press. Hawkins, A.D.. Soutiani. N.M. and Smith. G.W., 1985. Growth and feeding ofcod, Gadm wmhua L. J. Cons. Int. Explor. Mer. 42: I l-32. Kapel. H. and Angantyr, L.A., 1989. Feeding patterns of harp seals (F’hocu groenlandica) in coastal waters of West Greenland, with a note on of&hare feeding. ICES CM, 1989/N% 28 pp. (Mimeogr. ). Pethon, P.. 1985. Aschehougs store tiskebok. H. Aschehoug & Co. (W. Nygaard) A/S, 447 pp. Smirnov, N.. 1924. On the eastern harp seal Phoca (Pa~ophhor.a),ro~nla~,dicavar. oceanica Lepechin. Tmmse Mus. Arsh.. 47: I-I 1. Wiig. 0.. 1988. Gronlandssel og relinvasjon Hva vet vi-hva tror vi? Naturen, l988( 2): 35-41.