ring 82-H

Deep-Sea Research, Vol. 39, Suppl. I. PI'. S203-S21 8. 19'J2. Printed in Great Britain.

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© 1992 Pergam on

Vertical distribution and species composition of midwater fishes in warm-core Gulf Stream meander/ring 82-H JAMES E. CRADDOCK, * RICHARD H . BACKUS* and MARYANN DAHER* (Received 14 April 1989; ill revised form 30 July 1990; accepted 29 August 1990) Abstract-The integrated abundance and vertical distribution of rnidwatcr fishes in the upper 1000 m of warm- core Gulf Streamring 82-H show that the fauna of the ring was very similar to that of the northern Sargasso Sea. The data. indicate that warm-core rings have a large impact on the fauna of the Slope Water even though only a small fraction of the volume of rings is mixed into the Slope Water and the fishes (mostly non-migratory) are located in the "unreactive" parts of the ring.

INTRODUCTION

THE " northern wall" of the Gulf Stream between Cape Hatteras and the Grand Banks, one of the most conspicuous chemical-physical discontinuities in the ocean (FUGLISTER, 1960; WORTHINGTON, 1976), is considered to be a major faunal boundary, separating the subtropical and temperate North Atlantrc in the western half of the ocean (BACKUS et al., 1977). But in fact, open ocean biogeographic boundaries are poorly.understood. Although their central positions are often depicted on maps and charts as lines (BACKUS, 1986), such boundaries are broad areas of tr ansition and are not perfect barriers to the movement of plants and animals (MCGOWAN, 1974). One mechanism for the movement of even planktonic animals across such boundaries is mesoscale eddies such as Gulf Stream rings (RING GROUP, 1981; JOYCE arid WIEBE, 1983) . Cold-core Gulf Stream rings transport cold-water species (including some subpolar ones) into the Northern Sargasso Sea. Their effect on the overall species composition of the Northern Sargasso Sea is minimal, however, because the volume of the rings is small when compared with the size of the Northern Sargasso Sea (BACKUS and CRADDOCK, 1982). Warm-core Gulf Stream rings, on the other hand, move tropical and subtropical species into the temperate North Atlantic. West of the Tail of the Grand Banks, that movement can greatly affect the Slope Water because the volume of the rings is a significant fraction of the volume of the Slope Water. Midwater fishes are the longest-lived animals being stud ied in association with warmcore rings , their lives exceeding the span ofeven the longest enduring ring. Although fishes are generally considered to be nektonic, most mid water fish species, too small and weak to sustain movement against currents, are planktonic. Their presence/absence in a ring is thus a strong indicator of the history of the ring . This paper documents the abundance, species composition and vertical distribution of mid water fishes in the central core of warm-core 'Biology Dep artment, Woods Hol e Oceanographic Inst itution, Woods Hole, MA 025·B, U.S .A. S203

S204

J. E.

CRADDOCK

et al.

Gulf Stream ring 82-H at the time of the ring's formation. The dominant species are discussed with respect to their depth in the ring and to their geographical distribution in the North Atlantic.

RING 82-H

The following history of meander/ring 82-H is summarized from the R.Y. Endeavor cruise 90 study of STALCUP et al. (1986). When first surveyed (24-29 September 1982), 82-H was a "typical" northward meander of the Gulf Stream, 150-190 km in diameter with a central core of Gulf Stream water extending to at least 3000 db. The presence of Sargasso Sea water in the core of meander 82-H was corroborated by CONTE et al. (1986) on the basis of a 12-kHz non-migratory sound scattering layer. Between 29 September and 6 October, 82-H had separated from the stream, in the process becoming somewhat more shallow (2000-2500 db) but wider (220 km in diameter); its water .was now very Sargasso-like. The thickness of the layer of 18° water had increased from 150 to 200 db in the core of the ring. By 13 October, in the final observations by both STALCUP et al. (1986) and ourselves, the ring had moved toward the WSW at a speed of about 10 km day-I. Ring 82-H interacted with a Gulf Stream meander on 18 October, was slightly deformed but moved on to the SW, and was last observed in satellite imagery in February 1983 at an age of 5 months (TYNAN and HOOKER, 1984).

FJSHING METHODS

Fishes were collected in the upper 1000 m in meander/ring 82-H during R.Y. Knorr cruise 98 with the MOCNESS-20 trawl system (WIEBE et al., 1985), a scaled-up successor to MOCNESS-l (WIEBE et al., 1976b). The system has a mouth area of 20 m2 when fishing with a mouth angle of 45°. The MOC-20 consists of a set of 3-mm mesh rectangular nets that can be opened and closed by command from the surface via a signal-conducting towing wire. An instrument package attached to the net frame measures and transmits to the ship's laboratory information about depth, temperature, conductivity, flow and netframe angle. Data for flow (net speed), vertical velocity and net-frame angle allow computation of the volume of water filtered. A set of six nets was used at each station. The first net (not used for quantitative purposes) was fished down to 1000 m, then closed as a second net was opened. All successive nets were closed and opened sequentially at intervals as the system was brought obliquely (average ascent angle 6.2°) back to the surface. A surface-to-surface cycle with the MOC-20 is referred to as a station, the contents of a single net as a collection, Eight stations were made in 82-H, seven (42 collections) in its central core; three of those were taken while 82-H was still a meander, four after it had become a ring fully separated from the Gulf Stream. The eighth station (MOC-20 55) was made at the edge of 82-H. A physical malfunction made volume computation and evaluation impossible at this station, but the specimens caught are included in the list of species. Some of the MOC-20 tow statistics have been given by WIEBE et al. (1985; table 2). Complete station and collection data have been given by CRADDOCK et al. (1987; tables 5 and 10).

Abundance and distribution of \vCR fishes

S205

ot

At each the ring core stations (four night, three day) the water column was divided into five depth strata, usually 200-m thick. The collections filtered an average of 32,000 m3 of water (161,000 m3 at each station). An effort was made to keep the net moving at 2.0 knots at all times and to keep the vertical velocity constant within each depth interval, thus allowing integration of catch rates in the water column. Fishes were immediately preserved in 10% buffered formalin. Later they were separated from the invertebrates and transferred to 70% ethyl alcohol. Counts, measurements and displacement volumes were then made on a lot-by-Iot basis. The specimens are to be deposited at the Museum of Comparative Zoology, Harvard University. RESULTS AND DISCUSSION

The collections

The fishes collected in meander/ring 82-H are listed in Table 1, together with catch rates of the 25 top-ranked species. The five top-ranking species in certain depth intervals are listed in Table 2. In all, 21,254 specimens of at least 161 species are represented. All specimens from nets fished upward, except larvae and eel leptocephali, have been identified, a few only at the family level. Most speciose families are the Myctophidae (49 species), the Stomiidae (37 species), and the Gonostomatidae (11 species). Gonostomatids dominate the water column, comprising almost 79% of the specimens and 47% of the biomass. Myctophids rank second in both abundance and biomass (11.0 and 15.9%, respectively). Cyclothone braueri comprises an astonishing 60.8% of the fishes in the upper 1000 m. The 12 most numerous species (5 gonostomatids, 5 myctophids, 1 melamphaid and 1 sternoptychid) account for 87.5% of the specimens. . Cyclothone braueri is also the most voluminous fish, making up 26.2% of the upper 1000-m biomass. The 12 top-ranked species make up 71% of the total volume. Abundance and vertical distribution

Collection catch-rates and water-column abundance are expressed in the same units, specimens or biomass (ml wet weight displacement) per ten thousand cubic meters (W~ nr') of water filtered. (For the water column, this is equivalent to number or volume under 10 m2 of sea surface to a depth of 1000 m.) Thus, 1 specimen per io' m3 equals lOS specimens per krrr': similarly, 1 ml per rrr' equals 100 kg per krrr' (1 ml displacement = 1 g). Catch rates of midwater fishes at the seven ring-core stations averaged 188.09 fishes per 104 m3 with biomass of 14.24 ml per 104 m 3 (Table 1). These rates are very similar to published values for the extreme northern Sargasso Sea. At two stations made with the MOC-20 in the Northern Sargasso Sea, fishes averaged 126.9 specimens per 104 m3 with a biomass of 13.0 ml per 104 m 3 (CRADDOCK et al., 1987; BOYD et al., 1986). Northern Sargasso Sea biomass reported by WIEBE et al. (1976a) averaged 14.8 ml per 104 m3 (based on 12 oblique, 1000-m collections made with a 3-m IKMT). During the day, almost no fishes were at depths shallower than 400 m; almost 98% of both specimens and biomass is from depths between 400 and 1000 m (Table 3). The largest

Hr

S206

J. E.

CRADDOCK

et al.

Table). Fishes collected at eight stations in warm-core GIIlf Stream meander/ring 82-H during R. V. KnoTT cruise 98. Catch rates are averages of the integrated values for the upper 1000 m at sel"en stations in the ring core

Number of specimens

Spccirncns/Itf m 3 (s.) [Rank]

All fishes

21,254

188.09 (33.60)

Gonostornatids Photiehthyids Stcrnoptychids Storniids Myctophids Mclamphaids

15,835 364

147.97 2.64 7.50 2.27 20.77 4.73

729 284 3132 535

Eurypharynx pelecanoides"

6

Derichthys serpentinus"

I

Serrivomer beanii' Serrivomer brcvidentatus"

24

Nemichthys scolopaceus"

12

Batliylagus spp.·

10

MI1104 m3 (s.) [Rank) 14.24 (2.78)

(31.43) (0.86) (1.49) (0.54) (4.66) (1.25)

6.71 0.13 0.56 2.10 2.26 0.60

(1.55) (0.05) (0.22) (2.11) (0.81) (0.59)

3

Opisthoproctus grimaldii Rhynchohyalis natalensis

Bonapartia pedaliota Cyclothone acclinidens Cyclothone alba' Cyclothone braueri" Cyclothone microdon" Cyclothone pallida' Cyclothone pseudopallida" Gonostoma atlanticum Gonostoma bathyphilum Gonostoma elongatum Margrethia obtusirostra

176 23 263 11,718 J388 763 312

4 2 148 38

Ichthyococcus O\'atus Pollichthys mauli " Vinciguerria attenuata Vinciguerria nimbaria Vinciguerria poweriae

95 108 122

Argyropelecus aculeatus Argyropelecus hemlgymnus Sternoptyx diaphana Sternoptyx pseudobscura Valenciennellus tripunctulatus

20 160 500

Aristostomias grimaldii Aristostomlas tittmanni Astronesthes macropogon Astronesthes micropogon Astronesthes niger Astronesthes simi/is Bathophilus longipinnis Bathophilus pawned Bathophilus vaillanti

1.21 (1.06) (14) 2.41 114.44 18.61 7.26 2.87

(0.98) (23.80) (11.57) (1.80) (0.58)

[8)

(I] [2) [3) [7)

0.49 (0.29) (25)

0.14 (0.12) (17) 0.08 3.73 0.44 0.73 0.16

(0.03) (1.02) (0.28) (0.44) (0.03)

[25) [I)

(7) (4) [16)

1.34 (1.08) [2)

0.60 (0.37) [22] 0.81 (0.60) [18J 0.76 (0.53) [19J

11 28 1.27 (0.81) [13) 3.34 (1.41) [6]

0.11 (0.08) [21) 0.34 (0.21) [I0J

2 47

2 1 15 1

2 3 3 7 2

0.41 (1.09) [8]

S207

Abundance and distribution of \vCR fishes

Table 1.

Number of specimens Chauliodus danae' Chauliodus sloani"

Echiostoma barbatum Eustomias bibulbosus Eustomias schmidti Flagellostomias boureei Grammatostomias circularis Grammatostomias dentatus Grammatostomias jfagellibarba Idlacanthus faseiola Leptostomias sp. Malacosteus niger Melanostomias bartonbeani

Melanostomiasbiseriatus Melanostomiasmelanopogon Mclanostomias sp. nay.

Melanostomiastentaculatus Neonesthes capensis Pachystomias microdon Photonectes braueri Photonectes margarita Photonectes mirabilis

Photonectesparvimanus

37 98 13 1 2 3 1 _1 1 11 1 9 6 11 1 1 3 1 2 1 2 5 1

Photostomias guernei

is

Photostomias megistus

Stomias longibarbatus"

t'3 3 3 2

Xenodermichthys copei

1

Stomias affinis' Stomias brevibarbatus"

Unidentified searsiids

15

Alepisaurus spp.

15

Omosudis lowei

2 39 1

Coccorella atlantica Evermannella indica

32

Benthosema glaeiale Benthosema suborbitale Bolinichthys indicus Bolinichthys supralateralis Centrobranchus nigroocellatus

Ceratoscopelus maderensis

0.97 (0.28) [15J

Ml/lO~ m 3 (s.) [Rank]

0.13 (0.18) [19J 0.53 (0.99) [6J

0.20 (0.23) [15J 0.38 (1.01) [9J

7

Scopclarchusanalis Scopelarchusmichaelsarsi

Sudis hyalina unidentified paralepidids

Spccimens/Ifr' m 3 (s.) [RankJ

5

Scopelosaurus argenteus Scopelosaurusmauli

Sudis atrox

Continued

7 2 2 94 9 57 375 54 5 86

2.20 (1.19) [10J

0.49 (0.43) [25J

0.13 (0.06) [18J

5208

J. E.

CRADDOCK

Table 1.

Number of specimens Ceratoscopelus warmingii Diaphus brachycephalus Diaphus dumerilii Diaphus effulgens Diaphus fragi/is Diaphus lucidus Diaphus leutkeni Diaphus metopoclampus Diaphus mollis Diaphus perspicillatus Diaphus problematicus Diaphus rafincsquii Diaphus splendidus Diogenichthys atlanticus Gonichthys cocco Hygophum benoiti

Hygophum hygomii Hygophum reinhardtii Hygophum taaningi Lampadena luminosa Lampadena speculigera Lampadena urophaos Lampanyctus alatus Lampanyctus ater Lampanyctus crocodilus Lampanyctus cuprarius Lampanyctus [estivus Lampanyctus intricarius Lampanyctus lineatus Lampanyctus photonotus Lampanyctus pusillus Lepidophanes gaussi Lepidophancs guentheri Lobianchia dofleini Lobianchia gemellarii Loweina rara Myctophum affine Myctopllllmnitidululll Myctophum selcnops Notolychnus valdiviae Symbolophorus rufinus Symbolophorus ,'erallyi Taaningichthys bathyphilus

383 5 21 14 4 1 3 57 6S 22 1 2 62 310 7 206 1 13 54 4 1 13 12 8

5 143 16 3 15 lOS 207 102 85 5 1 5 12 12 5 540 1 4 8

Neoscopclus microchir Chaunax sp. Caulophryne sp. Melanocetus spp.

Himantolophus albinares'l

3

et al.

Continued

Specimens/Itt' m 3 (s.) [Rank)

Ml/lO~ m3 (s.) [Rank)

2.36 (0.98) [9)

0.24 (0.16) [13)

0.53 (0.28) [23]

0.09 (0.19) [23]

0.09 (0.14) [23) 2.04 (1.36) [11) 0.91 (0.66) [16)

0.22 (0.54) [14) 0.87. (0.63) [17]

0.77 1.44 0.63 0.51

(0.39) (0.41) (0.58) (0.60)

[20] (12) [21] [24)

3.46 (1.69) [5]

0.60 (0.48) [5)

0.12 (0.06) (19)

0.11 (O.OS) [21]

5209

Abundance and distribution of \vCR fishes Table 1.

Number of specimens Chaenophryne longiceps Danaphryne nigrifilis Dolopichthys karsteni Dolopichthys sp . Lophodolus acanthognathus Oneirodes sp. Spiniphryne gladisfenae Ceratias holboelli Cryptopsaras couesi

Gigantactis sp.

Continued

Spccimcns/Itr' m 3 (s.) [Rank)

~1lJ10~

m3

(s.) [Rank)

1 1 1

3 2 2 1 1

17 2

Neoceratiasspinifer Haplophryne mollis Linophryne spp. Bregmaceros spp. '

Melamphaes longivelis" Melamphaes pumilus" Poromitra capito"

Scopeloberyx opisthoptcrus" . Scopeloberyx robust/IS'

Scopelogadus beanii" Scopclogadus mizolepis"

1 4

30 1 472 4 28 19 9

3.97 (1.31) [4)

0.27 (0.10)[11) 0.27 (0.61) [11)

2

Diretmus argenteus Anoplogaster cornuta

6

Howella brodei

9

Caristius sp.

2

1.07 (1.82) [3)

Chiasmodon nigcr

Dysalotus alcocki Pseudoscopelus sp.

'Specimens from up nets only.

catch rate is in the 40Q-600-m depth interval, a stratum which contains 47.5% of the population in the upper 1000 m. The next two deeper strata contain progressively fewer specimens, 37.4 and 12.7% of the water-column population, respectively. The three deepest depth strata, on the other hand, contain 16.6, 25.5 and 55.8% of the 0-1000 m biomass, respectively, indicating an increase in fish size with increasing depth. At night there are changes in vertical structure, but the differences are not as great as the literature suggests. In fact, only 9.5% of the fishes are in the upper 200 m, but 13.2% in the upper 400 m. Thus the net upward migration (into the upper 400 m) is only 10.7% of the 1000 m population. Night-time catch in the 40G-600-m interval is over four times that of the upper 200 m, and even the catch from 800 to 1000 m is larger than that from the upper200 m. Diel shifts in biomass show a similarpattcrn. At night about 16% of the water column biomass is in the upper 400 m.

S210

J. E.

Table 2.

CRADDOCK

et al.

Rail king fishes in certain depth intervals ill the core of meanderlring 82-H, together with the percentages ofall fishes caught in that depth interval

Depth interval

Rank

Species

% of interval

Night

0-200m

1 2 3 4 5

Notolychnus valdiviae Bolinichthys indicus Diogenichthys atlanticus Melamphaes pumilus Ceratoscopelus warmillgii

17.0 12.1 11.4 11.3 3.5

Night

2()()....4oom

1 2 3 4

Melamphaes pumilus Bonapartia pedaliota Notolychnus valdiviae

5

Bolinichthys supralateralis

25.6 20.4 6.2 5.7 2.3

1 2 3 4 5

Cyclothone braueri Argyropelecus hemigymnus Cyclothone alba

1 2 3 4 5

Cyclothone braueri Cyclothone microdon Cyclothone pallida Sternoptyx diaphana Cyclothone pseudopallida

62.3 9.5 7.8 3.8 3.0

1 2 3 4 5

Cyclothone microdon Cyclothone pallida Cyclothone braueri Cyclothone pseudopallida Sternoptyx diaphana

63.5 6.5 3.9 3.8 3.1

Night

Night

Night

40Q-6oom

6OQ-800 rii

800-1000m

Vinciguerria attenuata

Vinciguerria attenuata Ichthyococcus

o~'atus

91.2 2.0 0.6 0.5 0.5

Day

40Q-600m

1 2 3 4 5

Cyclothone braueri Notolychnus valdiviae Pollichthys mauli Cyclothone alba Vinciguerria attenuata

89.1 2.4 0.9 0.9 0.7

Day

6OG-8oom

1 2 3 4 5

Cyclothone braueri Cyclothone pallida Cyclothone microdon Sternoptyx diaphana Melamphaes pumilus

58.5 6.3 3.8 3.4 3.4

Day

800-1000 m

1 2 3 4 5

Cyclothone microdon Cyclothone pallida

36.1 17.4 7.2 6.0 4.7

Sternoptyx diaphana Melamphaes pumilus Ceratoscopelus warmingii

S211

Abundance and distribution of \vCR fishes

Table 3.

Depth distribution ofmidwaterfishes in the corea/meander/ring 82-H. Percentagesa/the upper 1000-m ... population are in parentheses All fishes (ml per 104 rrr')

All fishes (specimens per 104 m 3) Depth (m) Night

Night

Night

Day

Day

Day

%

Rate

Rate

%

%

Rate

Rate

%

0-200 200-400 400-600 600-800 800-1000

0-300 300-400 400-600 600-800 800-1000

(9.5) (3.7) (42.7) (29.5) (14.6)

95.10 37.07 426.00 294.00 145.33

9.93 11.77 411.78 324.30 1l0.47

(1.1) (1.4) (47.5) (37.4) (12.7)

(11.3) (6.7) (26.0) (27.3) (28.6)

7.33 4.38 16.90 17.73 18.60

0.50 1.93 13.30 20.50 44.83

(0.9) (1.2) (16.6) (25.5) (55.8)

0-1000

0-1000

199.50

173.47

12.99

16.07

Dominant species

The following, in order of abundance, are the 12 most abundant species in the core of 82-H. Their vertical distributions are given in Tables 4 and 5. (1) Cyclothone braueri occurred in the core of 82-H between 400 and 800 m and showed no evidence of vertical migration. Two thirds of its population was from the 400-600-m interval, one third from the 60G-800-m interval. As stated previously, C. braueri comprised 60.8% of all fishes in the upper 1000 m of 82-H; it made up about 90% of the fishes in the 400-600-m interval and about 60% of those in the 60O-S00-m interval. Interestingly, at two stations made in the Northern Sargasso Sea with the MOC-20 during R.V. Oceanus 125 in August 1982, C. braueri comprised,?2.8 and 66.6% ofthe fishes in the upper 1000 m. In the Atlantic Ocean, C. braueri occurs from 600N to almost 600S (MUKHACHEVA, 1974) but is far more abundant in the subtropics than in either the tropics or temperate waters (BADCOCK and MERRElT, 1977). It does not reproduce north ofAooN in the eastern Atlantic (BADCOCK and MERRElT, personal communication) but does reproduce in the Eastern and Western Mediterranean Sea (GOODYEAR et al., 1972), provinces of the temperate North Atlantic. (2) Cyclothone microdon (9.9% of the fishes in the upper 1000 m), likewise a nonmigrator, occurred in 82-H at depths greater than 600 m. Three-fourths of its 0-1000-m population was in the 80G-1000-m interval, where it was the top-ranked species. It is unlikely, however, that we sampled throughout its entire depth range. At Ocean Acre off Bermuda, BOND (1974) reported C. microdon to depths of 1550 m (the limit of his sampling); between 10 and 28% of microdon's population was in the upper 1000 m. BADCOCK and MERRElT (personal communication) collected C. microdon to 2500 m at 42 and 44°N in the eastern Atlantic. Like C. braueri, C. microdon is widespread in the Atlantic (MUKHACHEVA, 1974) but is abundant in both the subtropical and temperate North Atlantic (BADCOCK and MERRElT, 1977). (3) The non-migratory Cyclothone pallida (3.9% of all fishes in the upper 1000 m) occurred in 82-H between 600 and 1000 m. It was slightly more abundant in the 600-800-m interval than in the 800-1000-m interval. At Ocean Acre, C. pallida occurred to 1100 m, but 95% of its population was in the upper 1000 m (BOND, 1974). In the Atlantic, C. pallida occurs throughout both the tropics and subtropics; temperate records are few (MUKHACHEVA, 1974).

0-300 300-400 400-<,{)I1

RlKl- looo

0-1000

11-200 200-400 4(10-(,()() 1I0()....R(KI 8011-1000

0-1 000

(-.(lO-R(1!l

Day

Night

Day

Il 0 366.97 (65.6) 189.63 (33.9) 2.57 (0.5)

11504 2 111.83

0 0 (67.3) 388043 (31.7) 183.03 ( 1.0) 5.(,]

% Rat e Rate %

Night

Cycloth one braucri

Day

24.29

0 0 (0.9) 1.1l5 (23.1) 28m (76.1) 92.35 10.47

0 0 0.23 (004) 12.20 (23.3) 39.93 (76.3)

% Rate Rate %

Night

Cyclothonc microdo n

6 . ~)

0 0 (2.1) 1l.611 (62. 1) 22.fi0 (28.8) 9.50

% Rate

Night

/

%

8.or,

0 0 O.W (1.5) 20.50 (50.9) 19.20 (47.6)

Rate

Day

Cycl otho ne iJallida

Day

2.R9

(0.7) 0.10 (61.5) 8.88 (37.8) 5045

0

0

2.86

0 0 0.53 (3.7) 9.50 (61104) 4.27 (29.9)

% Rat e Rate %

Night

Cyclothon e pscudopallida

2.111

0 0 (21.1) 2.75 (112.2) 8.10 (16.7) 2. 18

2. \1

0 .63

0 0 3.53 (>.37

(33.5) «.0 .5) (6.0)

%

Day

% Rate Rate

Night

Cycloth on c alba

3.24

0 (0 .9) 0.15 (2.7) 0043 «,8.5) 11.10 (27.9) 4.53

%

Rat e

Night

%

3.%

0 0 0. 73 (3.7) 11.17 (511.4 ) 7.90 (39.9)

Rate

D ay

Stern optyx diaphan a

Depth distribution oftile principal non-migratory fishes in rile core of mcanderlring Sl- H , Th ese six sp ecies rank 1,2,3,6,7 and 8 in abundance, Catch rates arc . specim ens per 104 m 3 ; percentages of eacli species' upp er 1000'111 po pulation arc in paretltheses

Depth (m )

Table 4 .

-

~

~

0

"" r.

('l

~

I~

N

N

CJ)

11-300 300-401l 4IMH',(MI 600..SIIO SiMI-1000

1l-1I100

11-200 21J()....4110

61MI-RIMI SIMI-1000

11-1000

~IMH',(MI

Day

Night

Depth (m)

Day

4.22

0

n

(5l.1) 1O.7S (45.0) 9049 (3.9) 1l.83

3.68

1l.63 (3.4) 1l.11l (60.3) 6.67 (36.3)

n

Il

% Rate Rate °It)

Night

Mclamphaespumilus Day

4.26

2044

Il Il 9.8K (Kl.1) 2.30 (18.9) 0

Rate Rate °It)

(76.1l) 16.18 (10.8) 2.31l (1.6) 1l.33 (11.6) 2.4K 0

o/«)

Night

(2704) (5.7) (1l.7) (47.5) (18.8)

%

2043

3.33 0.69 0.1l8 5.78 2.2S 2.29

Il Il 0.2K 6.IMI 5.17 (~5.2)

(204) (5204)

%

/'" Day

Rate Rate

Night

Day

2.52

(91.5) 11.53 (3.9) 0049 (104) II.1S (3.2) IlAIl 0 1.81

0 0 1.30 (14.4) 7.63 (K4.5) 0./11 (l.1)

% Rate Rate %

Night

2.6K

(80.8) ](I.S0 (304) 004(, 0 (S.5) I.B (7.3) O.9S

l.13

Il Il 1l.75 4.63 0.27

(13.3) (KI.9) (4.8)

°It)

Day

% Rate Rate

Night

(21.5) (3.8) (2.1l) «(16.5) (6.1)


1.30

lAO 0.25 0.13 4.33 0040

1.62

0 0.95 5.50 1.67

o (11.7) (67.7) (20.6)

%

Day Rate Rate

Night

Notolychnus valdiviae Ccratoscopelus warmingii Bolinichthys indicus Diogenicluhysatlanticus Lampanyctus pusillus

Table 5. Depth distribution of/he principalmigratoryfishes ill/he core ofmcandcrlring 82-H.These six speciesrank 4, 5,9, 10, 11 and 12 ill abundance. Catch ratesarc specimens per I o~ m); percentages of each species' upper IOOO-m population arc ill parentheses

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(4) Melamphaes pumilus (2.1% of the upper 1000 m fishes) was the most abundant migratory species in 82-H. By day, the bulk (60.3%) of its population was in the 600-800-m interval, the remainder (36.3%) in the 800-1000-m interval. At night, M. pumilus migrated into the upper 400 m, and was about equally abundant in the 0-200- and 200-400-m intervals. It was the most abundant species in the 0-200-m stratum, and ranked second in the 200-400-m stratum. A small percentage of its population may have been missed, as KEENE et al. (1987) reported it from as deep as 1300 m off Bermuda. M. pumilus is almost wholly restricted to the Sargasso Sea and its spillage (EBELING, 1962). (5) Notolychnus valdiviae (1.8% of the fishes) was likewise migratory, and was the most abundant myctophid in 82-H. By day most of its population (81.1 %) was in the 400-600-m interval (where it was second-ranked), the rest in the 600-800-m interval. At night, 76% of its population migrated into the upper 200 m where it was the most abundant midwater fish. About 13% of its population failed to migrate. At Bermuda, KARNELLA (1987) reported Notolychnus to depths of 700-850 m. Notolychnus is a tropical-subtropical species in the Atlantic (Bxcxus et al., 1977); records in the temperate North Atlantic, however, are numerous (NAFPAKTITIS et al., 1977). (6) Sternoptyx diapliana (1.8% of all fishes) is, like the cyclothones, non-migratory. About 60% of its population was from the 600-800-m interval, about 35% from the 8001000-m interval. It was among the five top-ranked species in those two intervals both during the day and at night. We presumably sampled S. diapltana from throughout its entire depths range. At Bermuda, most specimens of S. diapliana were from depths shallower than 1000 m (HOWELL and KRUEGER;1987). S. diapltana lives throughout both the tropics and subtropics in the Atlantic, but occurs seldomly north of 400N (BADCOCK and BAIRD,1979). (7) Cyclothone pseudopallida (1·.5% of all fishes) was non-migratory and occurred in 82-H between 600 and 1000 m. Its vertical distribution was very similar to that of C. pallida, being more abundant in the 600-800-m interval than in the 800-1000-m interval. At Ocean Acre, C. pseudopallida likewise occurred between 600 and 1000 m (BOND, 1974). C. pseudopallida occurs throughout the tropics and subtropics, but is more abundant in the former than in the latter (MUKHACHEVA, 1974). (8) Cyclotlione alba (1.3% of the fishes) occurred in 82-H between 400 and 1000 m, with the major part of its population between 600 and 800 m (62% of its upper 1000-m population). At Ocean Acre, C. alba occurred between 550 and 850 m with its maximum abundance in the 700-7S0-m interval (BOND, 1974). C. alba is principally a tropical species, but occurs throughout the subtropics (MUKHACHEVA, 1974). (9) Ceratoscopelus warmingii (or C. townsendi, see BADCOCK and ARAUJO, 1988) (1.3% of the fishes) was the second most abundant myctophid and was a vertical migrator. By day, it occurred mostly deeper than 600 m (about equally abundant in the two deepest depth intervals). At night, it was spread throughout the entire upper 1000 m, but only 27% of the population was shallower than 200 m; 66% of the population remained deeper than 600 m. At Bermuda, KARNELLA (1974) recorded warmingii to depths of 1500 m; its maximum catch rates were at depths greater than 1000 m in two of the three seasons that collections were made. BADCOCK and MERRETT (1976) likewise collected warmingii far deeper than 1000 m in the eastern Atlantic. C. warmingii occurs through the tropics and subtropics in the Atlantic (NAFPAKTITIS et al., 1977). (10) Bolinichthys indicus (1.2% of all fishes), like the other abundant myctophids, was strongly migratory. By day, most of its population (84.5%) was in the 600-800-m interval.

Abundance and distribution of \vCR fishes

S215

At night, most of the population (91.5%) migrated into the upper 200 m. At Bermuda, its maximum abundance was always shallower than 1000 m, although a few specimens were collected deeper (KARNELLA, 1987). B. indicus is a subtropical species (BACKUS et al., 1977). Both temperate and tropical records are few (NAFPAKTlTlS et al., 1977). (11) Diogenichtliys atlanticus (1.1 % of all fishes) was likewise migratory in 82-H. During the day it occurred from 600 to 1000 m, but most of the population (81.9%) lay between 600 and 800 m. At night, 80% of the population migrated into the upper 200 m. Maximum catch rates at Ocean Acre were from depths between 600 and 800 m both day and night; depending on season, 40-83% of the population were non-migrants (KARNELLA, 1987). D. atlanticus is abundant throughout the Atlantic tropics and subtropics (NAFPAJ...llTIS et al., 1977). (12) Lampanyctus pusillus (0.8% of the upper 1000-m fishes) was a vertical migrator in 82-H, but only about 20% of its population moved into the upper 200 m at night. Most of the population remained at 600-8PO m (66.5% of the population at night, 67.7% during the day). At Ocean Acre, its population was centered between 600 and 700 m and no specimens occurred deeper than 100o-m (KARNELLA, 1987); in winter, virtually the entire population migrated into the upper 200 m at night, but 22 and 37% of its population were non-migratory in spring and summer, respectively. L. pusillus is a bipolar species in the Atlantic, occurring in both the poleward halves of the subtropical gyres and in temperate waters (BACKUS et al., 1977). Filial thoughts

This paper makes available, in a form useful to other workers, data on the numbers and relative abundance of midwater fish species in the core of what turned out to be a "typical" warm-core Gulf Stream ring. Similar' data sets for other par!s of theA tlantic Ocean, based on all fishes from a substantial portion of the water column, do not exist. BADCOCK (1970) and BADCOCK and MERRETT (1976), who included all species of fishes in their studies in the eastern North Atlantic, steadfastly refused to quantify their data, and thus it is difficult to prepare water-column integrations using their data. The majority of other publications, whether or not quantitative, limit their scope either by setting narrow depth limits (BACKUS et al., 1977) or by considering only certain of the species (or families) present (BACKUS and CRADDOCK, 1982; HOWELL and KRUEGER, 1987; KARNELLA, 1987; KEENE et al.,1987). Although it was not the intention here to make detailed comparisons between the fish fauna of 82-H and those of the Northern Sargasso Sea and Slope Water, Table 6 is nevertheless included in order to show the similarities (and dissimilarities) of the respective species compositions of the three areas. The upper 1000 m of the ring and the extreme northern Sargasso Sea are quite similar to each other in that they are dominated by the same species, either subtropical or tropical-subtropical, none of which occurs abundantly in the temperate North Atlantic. Of the 10 top-ranked species in these two areas, only Cyclotlione microdon occurs abundantly in temperate waters. (The 12th most abundant species in 82-H, Lampanyctus pusillus, is a temperate-semisubtropical species.) On the other hand, the Slope Water fauna is different from the ring and Northern Sargasso Sea because of the abundance of temperate species-Co microdon, Ceratoscopelus maderensis, Benthosema glaciale, Hygophum benoiti and Argyropelecus hemigymnus rank 1,2,3,7 and 10, respectively. Of these five species, however, all but B. glaciale and C.

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et al.

Table 6. Species composition, as per cent ofall fishes caught with the MOCNESS-20 in the upper 1000 m ofthe core ofthe meander/ring 82-H, the Northern Sargasso Sea and the Slope Water. The ten top-ranking species for each location are given (ranks are in parentheses). The stations in the core of82-H (7) and ill the Slope Water were made during R. V. Knorr 98 in September and October 1982. The Sargasso Sea stations (2) were made during R. V. Oceanus 125 in August 1982

Species Cyclothone braueri Cyclothone microdon Cyclothone pallida Melamphaes pumilus Notolychnus valdiviae Sternoptyx diaphana Cyclothone pseudopallida Cyclothone alba

Ceratoscopelus warmingii Bolinichthys indicus Diogenichthys atlanticus

Ceratoscopelus maderensis Benthosema glaciale Hygophum benoiti

Argyropelecus liemigymnus

Core 82-H 60.8(1) 9.9(2) 3.9(3) 2.1(4) 1.8(5) 1.8(6) 1.5(7) 1.3(8) 1.3(9) 1.2(10) 1.1(11) 0.3(25) 0.1(25t) 0.5(16) 0.7(13)

Sargasso Sea

Slope Water

64.7(1) 5.4(2) 3.8(4) 2.7(5) 2.0(8) 4.6(3) 2.4(6) 1.1(10) 1.4(9) 0.4(25+) 2.0(7)

5.9(4) 55.2(1) 2.4(5) 0.4(19) 0.8(12) 0.9(9) 1.3(6) 1.0(8) 0.5(17) 0.3(23) 0.4(21)

0.2(25+) 0 (25+) 0.1(25+) 0.7(13)

11.5(2) 6.3(3) 1.2(7) 0.8(10)

maderensis, can also be found in the subtropics, at least in the poleward half of the subtropics. The subtropical and tropical-subtropical species that rank high in the Slope Water are moderately abundant there because of transport by.warm-core rings; all, with the possible exception of Cyclothone braueri, are relatively much more abundant in the Slope Water than in the eastern temperate Atlantic (see ~ADCOCK and MERRETI, 1977). Warm-core Gulf Stream ring 82-H was typical in size. About 200 km in diameter, it transported into the Slope Water in the upper 1000 m about 3 X 1013 rrr' of Sargasso Sea/Gulf Stream water. Thus, about half the volume of the Slope Water, whose total volume in the upper 1000 m is about 0.5 x 10 15 m', would be replaced each year if eight such warm-core rings were completely mixed away (together with their fishes) into the Slope Water (BACKUS and CRADDOCK, 1982). We now know, of course, that rings are not mixed completely into the Slope Water; although the average lifetime of a warm-core ring is approximately 4.5 months, within 1 or 2 months about a third of all rings interact with and are partially or wholly reabsorbed by the Gulf Stream (JOYCE and WIEBE, 1983; JOYCE et al., 1984). On the other hand, longer lived rings may mix almost entirely into the Slope Water. The well-studied ring 82-B (EVANS et al., 1985; JOYCE and KENNELLY, 1985; Scm-UTI and OLSON, 1985) lost 12, 43 and 93% of its volume after 2, 4 and 6 months, respectively; only the last 7% of its original volume was reabsorbed by the Gulf Stream. Mixing of warm-core rings into the Slope Water is slow for the most part and takes place primarily in the upper few hundred meters of the water column; however, three kinds of events--encounters with Gulf Stream meanders, large storms and interactions with the slope/shelf topography-can greatly speed up the process, especially when two of the three coincide (EVANS et al., 1985). The upper few hundred meters can even be completely removed (overwashed), in effect moving the core waters of the ring upward (JOYCE and

Abundance and distribution of WCR fishes

S217

KENNELLY, 1985). Mixing processes likewise seem to speed up further to the west where rings are squeezed between the Gulf Stream to the south and the continental slope to the northwest. Here, however, more ring water is exchanged with the Gulf Stream than is mixed into the Slope Water. Finally, it also should be noted that the vertical distribution of fishes in a warm-core ring slows their transfer into the Slope Water. Less than 10% of the fishes in 82-H were shallower than 200 m, and then only during the night. The majority of the fishes in 82-H (72% at night, 86% during the day) were in the most unreactive part of the ring, between 400 and 800 m. Exchanges of fishes at depths deeper than 800 m occur either when a ring is translating rapidly and overriding untrapped water or when a ring is pinched against the bottom.

Acknowledgements-We sincerely thank Valerie Barber, Don Bourne, Steve Boyd, Karsten Hartel, Charles Karnella, AI Morton, John Pirie, Peter Wiebe and 10e Wroblewski for all manner of help both at home and at sea. This work was supported by National Science Foundation grants OCE-I7270 and OCE-20-102. Contribution no. 7339 from the Woods Hole Oceanographic}nstitution.

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