MIGRATION AND RECRUITNIENT T O T H E COMMERCIAL STOCK O F GREENLAND HALIBUT,
REINHARDTIUS HIOPPOGLOSSOIDES (WALBAUI\/I), I N T H E SVALBARD AREA
O I . ~ \ , RUSE GOD@
Institute of Marine Research, Bergen, Norway
TORE H.AUG
Department of ltlarine Biology, Trolnscl Museurn, Uni\sersity ofTroms0, Trornsn, Norway
ABSTRACT
G C I D ~ . 0. R. and H-\cc;. T. 1987. lligration anti r c c r ~ i i t ~ n c n t to thr c ~ m n ~ c r c i a l stock of Greenland halibut, R e i n l ~ n ~ d t i ~ ~ s hi,fJ,fJo,glossoi(les (\Yalbaum). in the Svalbartl area. 1:isl;l)ir. Skr. .Sir.
Ho~CSldelel-s., 18: 31 1-328.
Results of stratified bottom trawl surveys in the Svalbard area during the period 1983-1985 shotv that depth distribution of Greenland halibut is size dependent. Higher proportions of large fish are found in deeper strata. T h e species exhibits geographically heterogeneous length distributions with a relatively high proportion of s n ~ a l l fish north of 76°K. This intes-area licterogenrity could be attributed to a southwards migration of larger fish from nursery grounds in the north.
Recaptures of fish tagged in the northern area supports this view in that fish not recaptured in the release area ]lad moved south rd. During the pcriod of investigation a general decrease in the abundance of small Greenland halihut was ohser\red in the S\.arlbard area. This is interpreted as indicating reduced recruitment success. 4 possible connection bet~\recn the decreasing recruit- ment of Grce~iland halibut arid an increase of yong cod. Gndllus 111orhun (L.), abundance is discussed.
I N T R O D U C T I O N
Spawning of the Greerlland halibut is thought to occur a t depths between 400 and 800 m, a n d in the eastern Norwegian Sea the spawning areas are probably along the slope of the c o ~ l t i n e ~ l t a l shelf bctweetl 66ON and 75"N
' ) Authorship equal.
(Hoc;ri:s.~-,\I) 1969, NIZO\..I.SE\~ 1969, BREIBI. and ELI.\SSEI 1984). Tlic commer- cial trawl fishtries for Grrenland halibut, which started in 1964, arc concen- trated in this region ( A r o s . 1978b). T h e size and age ranges of the fish caught in the spawning areas are 40-90 cm and 5-15 years (Nrzo\..rsl:\. 1969, L.\IIN- JOHANSESSI.:K 1972). During the feccling season, large numbers of fish are also
fbund in the more eastcrn parts of the Barcnts Sea ( ~ I I L I S S K Y 1944).
I n the Barcnts Sea/Svalbard area, younger G r e e ~ ~ l a n c l halibut, i.e., indi\,i- duals ranging in size a n d age from 10-35 cm and 1-5 years, are abundant in coastal areas to the west and north of Spitsbergen. This indicates that these areas may serve as nursery grounds for the species (HOGSEST.-\D 1961, 1969, H ~ u c and GLIL.L.IKSI:S 1982). Tllc first known observations of pelagic 0-group Greenland halibut in the Norwegian Sea/Barents Sea area were reported from the areas west of Spitsbergcn ( K O E F O E ~ 1907). More recently, large nunlbers of pelagic Greenland halibut larvac have been f o u ~ l d to the west and north of Spitsbergcn, a n d larvae have also been taken I>et\veen Spitsbergcn a n d Bear Island, and more rarely in thc central parts of the Barents Sea (HOC;SI:S.~AII 1969, A r o s . 1970, 1972, 1973, 1974, 1975, 1976, 1977, 1978a, 1979, 1980, 1981, 1983, 1984a).
I t has been suggested that larvae and juvenile Greenland halihut may be carried from southern spa\vning areas to Spitsbergei~ coastal waters by the Spitsbcrgen Atlantic Current ( H O G ~ E S I - A D 1969, H.\uG and GULLIKSES 1982).
If this is correct, older fish would be expected to migrate southwards from the Spitsbergen nursery grounds towards the spawning areas. In the present paper the migration of Greenland halibut in the S \ ~ i l b a r d area is examined using
a ) trawl indices and length distributions of Greenland halibut caught during stratified bottom trawl surveys along the slope of the continc~ltal shelf in the Svalbard area north a n d south of 76'S, and
b) tagging and recapture ofjuvenile Greenland halibut in Spitsbcrgcn coastal waters.
MATERI;-\I,S AND kIETHOUS
TRAT'fZ IlYDIC:E,S d i Y D L B Y G T H DISTIIIUL'TIO.\:Y
T h e fish used to calculate abundance indices and study size distributions were obtained from the yearly stratified bottom trawl surveys in the Svalbard area. T h e surveys are carried out by the Institute of Marine Research, Bergen.
T h e main objectives of these surveys are to provide data for the n ~ a n a g e m e n t and monitoring of demersal fish stocks in the Svalbarcl area (R.IxD.\ and Sni~sr.-\u 1982, 1983, Go11n et (11. 1984, Goucl 1985, G o r ~ n and NEDRF..\.As 1986).
O u r material was obtained from surveys carried out in September-October,
1983, 1984 and 1985. T h e gear used was a Campelet~ 1800 mesh shrimp trawl with rubber bobbins, a cod end mesh sizr of 35 nim, ancl sweep wlres of 80 113.
A standard haul consisted of the trawl being towed for threr nautical miles at a speed of three knots.
T h c stratified random trawl survey comprised 22 strata north of 76"N (the northcrn area) and 23 strata south of this latitucle (the southcrtl area) (Fig. 1 ) . Hauls wcre made at the following depth intervals: 0-100 ni, 100-200 m, 200- 300 m , 300-400 m and greater than 400 m. T h e catch clata were used to calculate abu~ldailce indices which were defizlcd as the stratified mcan catch in number pcr haul:
- k
X,,= ( I I A ) C a;x, i= 1
Fig. 1. T h e indic
78'
76'
74O
area investigated during the stratifiecl t r a ~ v l surveys \\.it11 tllr diffcrc :ated.
311t strata
w e r e x s , = stratified mean catch pcr haul. il = total area of all strata, a, = area of stratum i, %, = sample mean catch in stratum i, and k number of strata. T h e presented values represent swept area indices I =
x,
. A.
10." / SA, where SA is t h r area swept by the trawl. Since the main objectix e of the study was to study north-south differences in fish abundance, pooliilg of data from a number of strata was performed. T h e southern area is collsiderable larger (44600 square nautical miles) than the northern area (9700 square nautical miles). hlfethodo- logy is more fully described by R ~ Y D A and ShnAs? ,in ( 1982, 1983).For each haul the total number of Greenland halibut was recorded. Total lengths were measured for all, or, in cases of large catches, for random samples consisting of a t least I00 fish.
T H E T A GGIl\J\TG E,YPLRLIIEIYT
Tagging of Greenland halibut was carried out during routine rcsearch cruises in Spitzbergen coastal waters performed by the University of T r o m s ~ . A total of 2437 juvenile Greenland halibut were tagged during August in 1983,
1984 and 1985 (Table 3). Fish were caught in four areas: Svensksundcljupet a t the entrance to Isfjorden, Isfjordrenna and R e k e s ~ y l a just outside Isfjorden, and Kongsfjorddjupet, a trough leading into Kongsfjorden (Fig. 2). Hydro- graphical d a t a were take11 in all areas using a CTD-sonde connected to a Nord- 10 computer.
Fish were capturrd at depths between 260 and 370 m, using a shrimp trawl.
T h e standard haul lasted for 60 minutes, and towing speed was 2.5 knots. Two of the 1984 hauls lasted 180 instead of 60 minutes (Table 2 ) . T h e trawl was raised very slowly to the surface and as soon as the cod end of the trawl arrived on deck all living Greenland halibut were immediately transferred to a large tank with running sea water. Total fish lengths were measured, and a hydrostatic LEA-tag (see D.-\SYE\,IG 1953) was sewn 011 to the dorsal, anterior part of the eye side of the fish. Tagged fish were held in a flowing sea water tank until released. Only exceptionally were fish less than 20 cm tagged, and larger fish showing injury were also excluded from the tagging experiment.
When tagged fish were released, dead small fishes from the catches were thrown overboard in an attempt to draw the attention of glaucous gulls, L a u ~ ~ s hyperboreus, away from the tagged fish. Despite these measures some attacks on tagged fish were observed.
RESULTS
T R A I I 'L ILYDICES
North of 76°K the highest trawl indices were obtained at 200-300 m depth during 1983 and 1984, but in 1985 depths greater thall 400 m had by far the
Fig. 2. Results from the Greenlancl Ilali!,itt tagging experiments: O p r n circlrs inclicatr the sour areas of tagging in Spitzhcrgen coastal walers. Gllrcl circles indicate tllr long distance migrants 1,-Ilel-c recapture positioll outside the area of relcasr \\.as gi\.rn. 'l'hc dottrcl line indicates tlrc approsimate localization of the rclge of the continrnta! shelf ;Is given on a bathymrtrical chart by HzLI..\sI)-E-I.\ssEs ancl S.\SSES (1909).
highest index (Table 1). During the 1983-1985 period there was a general decrease in fish abundance in all depth intervals, except those greater than 400 m where trawl indices clearly had increased.
I n the southern area Greenland halibut were always most numerous in the deeper strata, with maximum trawl indices a t depths greater than 400 m (Table 1). A general increase in fish abundance was observed in the southern area a t 200-300 m depth during the 1983-1985 period.
T h e total stratified trawl illdices (all depths poolecl) wcre higher north of 76"N than south of this latitude in 1983, but a marked ilicreasc in fish abundance in the southern arca from 1983 to 1984 resultccl in higher illdices being recorded in the south than in the northern arca during the 1984 ancl 1985 surveys (Table 1 ).
Table I. Number of hauls, the catches with respect to
1985 survcys.
stratified trawl indices and mean lengths of Greenlalid halibut in depth intervals and areas as recorded d~irilig the 1983, 1984 and
Nulliber Stratified Mean fish
Depth intervals (111) of hauls trawl illdices length (cm)
North of 76"N
0 - 100 12 14 13 0.02 0.00 0.00 26.4 - -
100 - 200 16 17 21 1.91 0.24 0.13 28.2 40.8 38.3 200 - 300 19 16 18 6.48 5.52 2.37 18.1 28.6 3 5 2 300 - 400 1 1 12 12 2.25 2.12 0.69 32.2 34.1 41.8
> 400 12 13 13 3.71 4.80 10.74 40.8 43.2 42.2
All dcpths pooled 70 72 77 14.36 12.68 13.93 27.5 35.3 41.0 South of 76"N
All depths pooled 117 122 125 12.33 23.96 25.52 50.3 44.1 42.7
LE>YGTH DISTRIBL.TIO,Y
In all surveys there was a general trend for tllc proportion of larger fish to increase with increasing depth, I ~ u t the few fish caught between 0-200 n~ were larger specimens, 40-60 cm in length (Figs 3-5).
During the 1983 survey, the northern samples were characterized by a high proportion of small fish (Fig. 3 ) . Generally, Greenlalid halibut caught in the southern arca were larger than those foullcl in the northern arca. Mean fish length (all depths pooled) in the n o r t h c r ~ i arca was 27.5 cm ancl in tlie southerll area 50.3 em (Table 1). Also, the lcllgtll distributions of fish in the t w o areas wcrc highly significantly different (Fig. 3. all dcpths.
x',.;
= 14032.p<0.005).
20 40 60 80 100 20 40 60 80 100 LENGTH (5 cm groups) LENGTH (5cm groups)
Fig. 3. Length composition of Greenlancl Iialibut cauglit at various depths 11ort11 (left) and soutll (right) of 76°K in 1983. P e r c e n t a ~ c s as? haseci on stratified tra\vl indices per 5 c111 length grouj] of lish.
20 40 60 80 100 20 40 60 80 100 LENGTH ( 5 cm groups) LENGTH (5cm groups)
Fig. 4. Length composition of Greenland l~alihut caught at \~arious depths north (left) and south (right) of 76"N in 1984. Percentages are based 011 stratified tra~vl indices per 5 cm length group of fish.
I
All dephtsI i
All dephts ILENGTH ( 5 crn groups) LENGTH ( 5 cm groups)
Fig. 5. Length composition of Greenland halibut caught at various depths north (left) and south (right) of 7 6 9 i n 1985. Percentages are based on stratified trawl indices per 3 crn length group offish.
Although thc 1984 niaterial taken in thc southcrn arca contained more large fish (mean fish Irngth, all drpths poolecl, 44.1 cm) than thc northern area (mean fish length, all dcpths poolcd, 35.3 c m ) , the difference in size was lcss than that found in 1983 (Table 1). T h c length distriubtions of fish caught in t h r northern a n d southcrn arcas wcrc significantly clifferent (Fig. 4, all depths, x ' , 3 = 5495, p<O.O05).
Results from thc 1985 survey show a marked change in length distriubtions iri the two areas as comparcd with 1983 a n d 1984 (Fig. 5 ) . In the northern arca thc number of small fish had dccrcascd, whilst in the south the proporti- ons of small fish had increased. T h u s , thc mcan fish lengths (all depths pooled) were now 41.0 cm a n d 42.7 cm in the northern a n d southern areas, rcspcctivcly (Table 1). Despite the small diffcrcnccs in mcan lcngths observed in 1985, the Icngth distributions of fish caught in the southern and northern area were still stignifianctly cliffcrcnt (Fig. 5, all depths, x L L 4 = 4376, p<0.005).
AB C:\'DAi\'CE O F .S,\I"lLL FISH
A general impression gained from inspection of the length distributions is that there was a change in relative abundance of small fish during the three years of study. This impression is strengthened by the results presented in Tablc 2, which show that there \,vas a marked decrease in the tralz.1 indiccs (both areas pooled) both for fish lcss than 25 c111 and 20 cm in Icngth. Fish lcss than 25 cnl in length are one or two years old, lvhilc most of those less than 20 cm in lcngth can bc assumed to one year old ( H a m a n d GL.I.I.IKSI:X 1982).
Table 2. Stratificd trawl indices of srllall Greenland halibut (all depths and areas pooled) recorded during the 1983, 1984 and 1985 surveys.
Stratified trawl indices Year
Fish < 20 cm Fish < 25 clll
TAGGI'YG A,\%) REC4PTCRES
O f a total catch of 772 Greenland halibut takcn in I 1 hauls in 1983, 648 individuals were tagged (Table 3 ) . These tagged fish, which were all captured with bottom temperatures betwecn 0.7 and 2.7'C, ranged in size from 20 to 55 cm (Fig. 6 ) .
1985 KONGSFJORDDJUPET N=H2 KONGSFJORDDJUPET ISFJORDRENNA (13 REKESOYLA w 2 3 SVENSKESUNDDJUPET SVENSKESUNDDJUPET ?. x z.
-
N $ 15 20 25 30 35 40 45 5 55 6 15 20 25 30 35 40 45 50 55 60 15 20 25 30 35 40 45 50 55 60- ..
TOTAL LENGTH (cm1 TOTAL LENGTH (cm1 TOTAL LENGTH (cm) 2-
Table 3. 'Tra\v1 hauls fiom \vhich C;rernIancl llalibut \\ere taggrti ancl t-elcasctl in Spitzhrr~r~l coastal lvaters during August in 1983, 198.1- and 1983.
Position' Bottom No. No. of fish
Year Depth temp. of
N E ( m ) ("C) Hauls caught tagged
330
1983 79'01 ' 10°57' (Kongsfjorddjupet) .i2 357 273
7X014' 1 1'00' (Isfjorclrcnna) 370 1.7 3 153 146 78"08' 13"30' (Svc~~sksunddjupet) 360 2.3 3 260 219 1984 79'01 ' 10'57' (Kongsfjorddjupet) 330 2.2 2' 260 110 78" 14' 1 1°00' (Isfjordrenna) 370 3.2 3 146 145
7X018' 12'22' (RekcsQyla) 260 2.7 2 112 103
78"08' 13"30' (Svenskundcljupet) 360 2.2 5' 598 448 1985 79'01 ' 10'57' (Kongsfjorddjupet) 330 0.9 2 155 112 78"14' 1 l000' (Isfjordrenna) 370 1.5 2 26 26
78'18' 1222' (RekcsQyla) 260 2.0 12 792 745
78'08' 1 3"30' (Svensks~~ndcljupet) 360 0.8 5 115 115
' Approxinlate position of release is given although trawli~ig was performed in various directions around this position.
One ofthcse hauls wcre pel-formed in ICrossfjorden (N79"111 -El 1°.19'), 370 m clrpth.
bottom tetlipcrature 0.7"C, a side arm oCKongsfjordc~~; fish were released in Kongsfjordcljupet.
' One of these hauls lastccl in 3 instead of the usual 1 hour.
From the 12 hauls perfbrmecl in 1984, 1 1 15 fish Ivcrc capturecl (Table 3 ) . Of these, 799 i~ldi\.icIuals, \\,hich ra~lgccl in size bct~vccn 13 anel 61 cm (Fig. 6 ) , were taggccl. During the 1984 taggillg sur\,cy, the hottom tcmperaturcs rangcct
~ C ~ T V C C I I 2.2 and 3.2"C.
Tlle bottom temperatures recorded cluring tllc 1983 sur\.c). \vrrc consicle- rably lo\\ler (0.8 - 2.0°C) than those rrcordccl during the prc\.ious t\vo ).ears (Table 3). T h e 21 trarvl hauls pcrSormed in 1983 ~~irlclecl 1088 Crccnland halibut, of which 998 wcrc tagged (Tahle 3 ) . Thcsc rallgccl in size b c ~ \ \ - c e n 22 and 6 1 CIII (Fig. 6 ) .
By 26 February 1987, a total o f 2 8 tagged fish had hcen rcportccl rccapturccl ('I'ablc 4 ) . Fi1.c of the recapturccl fish \verr from tllc 1983 taggings. 19 from tllc 1984 taggings, and 4 from the fish taggccl in 1985.
Eleven of the taggccl fish \\.ere rccapturccl close to tllc arca of rrleasc 131' commercial Norlvcgian slrrinlp tralvlcrs (Table 4 ) . O n e it~di\~iclual appears to ha1.c mo\:ccl northwards to the arca north of Spitsbcrgcn ( c . 8 0 ° S ) , but informatio~i gi\.en about this recapture ma!- ha\.? I)ccn inexact. Xnotllcr tag fbu~lcl north of Spitsbergcn \vas obscr\.cd among the remailis of a ringccl seal.
Pusa Izispida, Sound on the beach in \\'i,jcclcfjorclcn. Of the remaining ~ , c c a p t u - rccl fish, most \\,ere reported by So\.iet and East Ckrman tra\vlcrs. Eiglit fish were c a u g l l ~ in clepths ranging bctc\vcen 470 ancl 800 m along tllc cclgc of the
Tablc 4. Details of sitc of rclcase. rccapturc and sizc of the rccapturcd taggcd Grccnlanci hnlibut.
'TAGGING KECAlyI'UKt ~l'iinc
uncc
Datc Position Dcpth Lcngth date Position Dcpth Gcnr Lcngth rclcasc
N E ( m ) (cm) N E ( m ) (cm) (dayh)
Recapturcs within the rclcasc arca
84.15.08 78'08' 13"30' 360 42 84.21.09 78"14' 1OC53' 310 84.18.08 78'18' 12"22' 260 3 8 84.10.10 7X003' 12"IO'
84.15.08 78'08' 13'30' 360 37 84.13.10 78O08' 1 1°05' 260
84.17.08 78"18' 12"22' 260 49 84.14.10 78"05' 11°45' 245 84.15 .08. 78'08' 13O30' 360 38 84.16.1 0 78'07' 13"30' 320
84.15.08 78'08' 13'30' 360 30 84.25.11 78'10' 13'30' 325
84.15 .O8 78'08' 13'30' 360 44 84.10.12 78"07' 13'35'
84.15 .O8 78"08' 1 3"30' 360 41 85.03.03. Isfjordrenna 337 85.16.08 78'18' 12'22' 260 40 85.18.08 78"16' 12"19' 260 85.18.08 78'18' 12"22' 260 4 1 85.22.08 78'1 2' 13'25' 290
85.15.08 78'08' 13"30' 360 5 1 85.26.08 IsQordcn 300
Recapturcs outsidc thc rcleasc area
83.16.08 78"08' 13"301 360 33 83.08.10 76"54' 1 2"40' 520
83.17.08 79'01' 10°57' 330 47 83.12.11 Barcnts Sea 600
83.16.08 78'08' 13'30' 360 44 84.17.01 74'43' 15"42' 780
83.16.08 78'08' 13'30' 360 46 84.02.04 73'55' 15"50' 620
83.16.08 78"08' 13"30' 360 43 84.19.05 76"53' 12"43' 520 84.17.08 78"18' 12O22' 260 46 84.??. 10 80" ( N of Sp.bcrgc11) 84.15 .O8 78'08' 13'30' 360 36 84.28.1 1 73'44' 19"46' 305 84.15.08 78"08' 13O30' 360 32 84.18.12 76"50' 13"00f 620 84.17.08 78'18' 12°22' 260 36 84.22.12 7 7 2 4 ' 1 1°13' 580
84.15.08 78"14' 1 1°00' 370 43 84.27.12 Barcnts Sea 800
84.15.08 78'08' 13'30' 360 46 85.??.?? Sovict rccapt. No info. given 84.15.08 78"14' 1 1°00' 370 42 85.04.03 Barcnts Sea
84.17.08 78'18' 12'22' 260 42 85.16.06 74'25' 14'23' 500
84.17.08 78'18' 12"22' 260 45 85.18.07 77'33' 10°58' 600
84.14.08 78"08' 13O30' 360 36 85.04.08 Sovict recapt. 110 info. given 85.15.08 78"08' 13"30f 360 46 85.20.12 73'04' 16'28' 470
84.18.08 78'18' 12'22' 260 36 86.23.08 7X0.55' 16'30' 0'
'
'l'lic tas \\,as Courid amor15 tlic rcniaiiis ora ringccl sml, 1'1ua hi~/~zrln. on tlic I>rac.li i11 \\'~jrdcljordci~, riortlicril Spirzbcrgcil.. I rawl
.
Traw I TI-awl Truwl . I rawl
.
Traw I TI-aw l 'Traw l Trawl Trawl Trawl 'Trawl Trawl Trawl Trawl Trawl Trawl 'Trawl
Trawl Trawl
Trawl
coi~tiileiltnl slopc bct\vccn N77'33' and S73°01', ancl o n r fish ivas taltcn at 305 m depth southcast of Bear Islallci (Fig. 2 ) . Fi1.e fi~l.tllcr So\.irt rccapturcs \vcrc rcportccl of \vhicll three fish were taken ill tlle Barents Sea ( a t lrast t11.o of them along the slopc of tlle colltillclltal shelf): ancl t\vo lverc ~vitllout an!. informa- tion of recapture sitc.
Generally, fish recaptured in t h r release arca ~ v c r c takrn rclati\.cl>- shortl>.
after release wheras those taken at morr clistailt sites liacl bren Srrc for sc\.eral weeks o r months ( T a b l e 4 ) . This ma), rcflrct a trcllcl of racliatioil a\va!. form the sitc of releasc. T o date, 11ollr of the slliallrst fish ( 2 0 - 30 c m ) lras I~ccll recaptured.
DISCUSSIOS
Polis11 in\.cstigations on Grecnlailcl halibut c o ~ ~ c l ~ ~ c t c c l in the northeast Arctic in 1973 sho\vccl that fish fi-om thr \vestcrii sloprs of the Bear Island grouilcls \\>ere generally larger than tllosc from the areas \vrst of $ l ~ i t s l ~ c r g c n (KOSIOR 1975). A similar trend \vas obscr\~rcl in the results of grouiiclfish surveys co~tductccl ill the S\.albarcI arca cluring 1982-83 (R.\xI).\ ancl SAII:IS,I,.\II
1982, 1983, Govm e l (11. 1984). These authors also found that thr proportions of larger Grccnlancl halibut illcreasecl \\:it11 ilicrcasiilg clcpth. Results of tllc present study support these earlier finclings. T h e ~lortll-south l~ctcrogrneit).
was clearly evident in tlic 1983 material, hut in 1984 and 1983. ho\vc\.cr, the size differences hctrveen the llortherll ancl southcrn arcas 11-crc lcss pro- nouilced than ill previous years.
T h e abunclance of Greenland Ilalihut in \vatcrs shallo\~.cr than 200 In xvas low both north and south of 7F0N, and saml>lcs fi-om thcsc clcptlis Iverc generally clomiilatccl by large fish. Grcrnland halibut is knoll-n to undrrtakc vertical movements (CI-ILAI.\I<O\ 1969, S\IIII.I. 1969), ancl thr fc\v largr fisli taken a t shallo\v dcpths c o ~ ~ l c l have been participating in kedillg migrations.
T h e largest concciltrations of Grcelllalicl halibut occurrccl lxlo\\- 200 m depth. Gc~lcrally, t h r proportions of larger Greenlaiicl Ilalibnt incrcasccl \vith increasillg clcpth. T h e tra\\-l iiiclices \.ariccl .rvitll clrpth. but thrre ~vcl-c differences het\vccn the two areas. I n 1983 ancl 1984 tllc i~icliccs inci-eased cot~tinuously with clcpth in the southern area. Since the proportion of large fish in the catcllcs also iilcrcasccl ~ v i t h increasing depth: tllc 1983 a n d 1984 catches in t h r southern arca \ucrc clominatccl by large fish (larger than 40 c m ) . By contrast, in the ~ l o r t l ~ c r n area the maxiinurn fish abundancr in 1983 ancl
1984 was founcl in the 200-300 m depth intel,\.al, and c;itcllcs \\.err clominatccl by small fish (less than 40 cm) ivith a coilsiclei-able proportion I~cillg sm:rllcr than 20 cm.
r 7
I hc clilTcrcilces bcr\vrcil tllc northern and southern arc;ts ol,srr\.cci in 1983
and 1984 \\.rrc much lrss pronoumcccl in 1983. T h u s . the 19113 - 1083 cllangcs in tralvl incliccs/size clistributions can l ~ r irlterpretecl as reflecting a n o\,rrall decrease in the abundance of small fish and a n incrcasc in n u m l ~ c r of largc fish in the north, acconipanieci by a n incrcasc in thc n u m h r r of small fish ill tllc south.
I n addition, a gcncral incrcasc in t h r pooled t r a ~ v l indices occurretl in tllc southern arca fro111 1983 to 198411983, If thr rcsults fi-om tlic 1981 and 1982 sur\.eys (S.\rl:l,s.r.\~) a n d K.\SI).\ 1983) arc includccl. t h r inclcs has incrcnscd
fL.0111 6.49 in 1981 to 25.32 in 1985. Tlie indiccs in the northern arca ha\.?
renlainccl alnlost unchangctl (12.68 - 1~1-.3ii) during the same pcriotl (see Germ P / 01. 1984). T h c s r changes ma!, be a result of changcs in rrcruitmcnt. ;\
sc\.crc recluction in ahunclance of Greci~lancl h a l i l ~ u t occnrrcl in the Sor\vtgian -Barrnts Sea stock clue to hca\.!. lishillg in the 1970's. 'l'his lccl to increascd incIi\.idual gro\vth rate ancl rccluccd age at mat~irit!. of the sl~ccirs (Iio\..l.so\..\
ancl N ~ z o \ . - r s ~ : \ . 1985). Quotas inlposecl on tllc fisllcr!. in 1978 ( L A s o s 1978b) ha1.e rcsultccl in a stead!, incrcase in stock sizc (.-\sox. 1983). 'I'llc incrrasccl abundance indices in tllc southern arca may be attributal~lc to increasing stock size. 'The lack of s n ~ a l l fish ill tllc norhcrn a r r a in 1983 could. ho\vc\.cr, indicate l o ~ v rccruitnlcnt succcss. T h e increase in tllc pl,oportictn oi' s~llall fish in the soutllern nrca in 1983 \vas insufficient to compensate for tllc r c c ~ ~ n i t m c n t reduction ol,scr\~rd in the north. O n e can only spcculatc a l ~ o u t possiblc reasons fbr this recruitment failure. Tllc 0-group indescs for Greenland halibut in 1982, 1983 ancl 1984 lvcrc 17.0. 13.8 ancl 40.4 rcspccti\.cly (;\so\- 198411). Normall)., t h r large 1984-\.aluc ~ \ ~ o u l d bc taken to indicate g:-oocl recruitment. T h e r e seems to he no correlation, I~o\\.c\.cr, ljct\vccn tllc 0-group inclcs and the ahundancc i~lclcs of fish shorter than 20 em. i.c., onc-).car-old fish (see H.\r~c: and G L ~ I . I . I I ; ~ E X 1982), tllc fbllolvillg !-car. Factors snch as thr hea\,y tra~vling \zit11 pra\vn tra\vl \vhich no\v takes place in SljitzIjc1~gcn coastal areas ( i l s o s 1986), a n d l o r the incrcasc in t h r stock of cod \vliicli has occurrccl in the Svalbard arca in rcccnt yrars ( G o l ) ~ and NI:I)KI:\.\s 1986), ma). ha\.c resulted in largr nlortalitics ofslllall fish ancl thcrel~y 11a1.c contril)utccl to the lo\v indices of young fisli. High abundanccs of rcsprcti\.cl!. LAtlantic, C:crdrrs
~ n o l h z l n . ancl I'acific,
G.
nzact-ue~lrhalus, cocl lla\.c I~cen sllo\vn to coincide ~ v i t l i declining r c c r ~ ~ i t m c n t of Crccnlantl h a l i l ~ u t in thc \vcstcrn X\'ol.th Atlantic (S.\irr).r 1969) and it1 the Kcririg Sca ( K I ( . I I . \ K I ) B.\E;I;.\I..\. ~ c r s . c o m n ~ n ) . Further in\.rstigatioll is ncccssary. ho\vc\.c.r. bcforc firm col~cl~lsions can I,r clra\\rn.I 7 he rcccnt changcs in stock structure and tllc 1o\v reel-liitnlrnt SLICCCSS i l l the 1985 season clo not mask the gcncral trend of' the northern samplrs of' Grccnlxld h a l i l ~ n t c s h i l ~ i t i n g Icnght distril~utions \\.it11 man!. sm;tll fish ;tnd small proportions of largc fish, \vllilst tlic soutlltrn sanrj~lcs contain rclati\.cl>.
lo\v n u m l ~ c r s of slllall fish. O n r csplitnation f i ~ r this co~ilcl be t1i;lt tllc ~ i o r t h c r ~ l
arca serves as a nursery ground, a n d t l n t a iiligration of largrr fish takrs placc towards the southern areas.
T h e results from the tagging experiment tend to support this explanation, althougll few fish have been recaptured. With thr exception of one spcciliien, which had moved northwards to the placvli trawl arras llortll of Spitzbcrgen.
all fish not recaptured in the area of release had moved southwards. I\'e suggest that this is a reflection of a real soutl~~varcl migration ratller than an effect of commercial fishery pressure, silice extensive commercial fishcrirs take place both to the liorth a n d to the south of the release arca. Some of the fish had migrated south of Bear Islancl. Only larger fish (30 - 46 cm) Ivere observccl to 1eal.c the Spitsbcrgen arca. These results are, ho~vc\~ri-, not ullequivocal since none of the tagged Grcciilalid halibut released as sliiall fish (20-30 cm) has 1 et bten recaptured.
T h e presence of some small Greclllalid h a l i l ~ u t in the soutlicrn arca, especially in 1984 a n d 1985, a n d the lack of corresponclcncr in the change in trawl indices in north and south suggest that not all recruits to the southern area collie from the north. T h e occurrence, in solile years, of colisiclcrable numbers of pelagic 0-groups Greenlalld halibut south of 76"N in the Bear Islaild - Hopen area (ASOX. 1981, 1983, 1984a) points to~vards possiblc recruitme~lt also from these sites. Spa\\rtling of Greeliland lialibut may occur as far south as the T r z n a hank, 66" - 67OS (BKCIBY a n d E1.1.-ISSES 1984), and as far north as 75"N (Hocs~s-I-.ID 1969). Such large north-soutli exteilsio~~s of the spawning area opens the possibility of considerable north-south \,ariation ill the settlemelit of larvae, which n o u l d be dependent upoil factors such as velocity of larvae-carrying currents ancl \vhetller the bulk of spalvning occur- red in the northern or southern parts of the spawning arca.
Migration of Greellland halibut from the areas soutliwest of Bear Island.
i.e., where the most lollgdistaiicc migrants from the present tagging expcri- ment were recaptured, were studied by SOROI<IS (1967). His recaptures showed that longdistance migrants pelletratccl far into the Barelits Sea in a n easterly direction as well as south- and south~vcst~varcls towards the areas off the !vest coast of north Nor~vay. S o nortll~vard migration, c.g., to the areas to the \vest of Spitzbergcn, was, l i o w c \ ~ r , ohser\.cd.
ACRKOIYLEL)GEI\lEX'~S
Thanks are d u e to crews a n d field assistants o n boarcl R / V ccEldjarn)), M / T r ccB0tr;l IVD, M / T r ccstallon ancl M / T r ctRaiti), during the stratified trawl surveys a n d on board R/V ccJohan Ruudn c l u r i ~ ~ g the tagging experi- ment. T h e technical assistance of A. iVI. SKOKPCS and G. I\'. PE.I..T.I:RSCS is gratefully ackno~vlcdged, ancl M. JORI.ISC; is tl~alikcd for criticising the hIS a11d correcting the English text.
R E F E R E K C E S
ASOS 1970. Rcl~ol-t to the 0-groilp lish survey in tlic R;ircnts S r a ant1 aclj;lcent Ivatcrs in .Ailgust- Septemhrr lS(ik1. d r i ~ i l s hiol.. (;b/~ciih.. 6 : 287-300.
ASOX. 1972. l'rrliminary report ofloint Soviet-Sor\\.cgiali 0-group lisli survey in tlir Uarrnts Sca alitl acljacent xvaters in . A L I ~ L L S ~ allti S~11tc1iiI1cr 1970. .-lriiil\ bi01.. C.'o/~~~iili.. 27: 21(i-222 ASOX. 1973. Report of tllc international 0-group liali sur\-c! in the Uarc~its Sen ; ~ n t l ;tcljacent
waters in .-\ugust-Scptcml~cr 1971. .4iiiil~ biol.. C:o/ri~rih.. AS: 230-246.
.ASOX 1974. Preliminary rrport of tllc intcrn;~tionill 0-grorip fish surve!. ill tlic B;~rcnts Sr;l i ~ n t l adjacent \I-atcrs in .August-Srptcrii1,cl. 1972. .41inlr biol.. (;operi/i.. %(i: 205-210.
.ASOX 1975. Pr'liminary report on tllc intcrn;irional 0 - ~ S O L I I ) Iisli s~lrvcy ill t11r Bat-vnts Sc;l ; ~ n t l adjaccnt \vatel-s in Xugust-Septcn1l>r1. 1973. .iiiiiIr bicil.. C b / ~ ~ ~ i r I i . . 30: 234-240.
. A S ~ S 1976. Prr1imin;u.y report o n tlic internationill 0-grou11 lisli sil~-\.c!- in tlic Barct~cs Sea nlltl acljace~it jvatcrs in : \ i l g i l s t - S e ~ ~ t e ~ i ~ l ~ r r 1974. .inti/r hiol.. Coprirh.. 31: !20%-'.0.
.IS()\. 1977. Rcport on tlir internatioiial (I-gronl~ lisli surve!- in tllc Ral-cnts S r i ~ ant1 ac!j;lccnt Ivaters in ,August-Scl~tcmlm 1973. .-lirrrI\ biol.. C o / ~ ~ i i h . . 32: 199-205.
.ASOX. 1978a. Report on the intci-n;ltional 0 - g r o i ~ l ~ lisli sut-\-ey in tllc Uarcllts Sc;l ant1 aclj;tcc.~it Tvatcrs in ;\~tgust-srptrmlm 1976. . ~ I I ~ I / J biol.. C:op~rih.. .I?: 2 13-2 1:).
ASOX. 1978b. Ressursovcrsikt fol- 1978. 1;i.rL-eri Ifoz,.. 1978 (S;vriir. 2 ) : 1-97.
ASUS 1979. Report to tlie international 0-group fish sur\.ry in tlic Rarcnts Sc;l ant1 ac1j;ircnt Tvaters in . A i ~ ~ ~ ~ s t / S c p t c i i i t ~ c r 1977. .-Innis biol.. (,'o/~errh.. 34: 231-257.
.AS()\. 1980. Report to the international O - ~ I Y I U ~ I lisli survey in the B;II-cnts Sea ant1 ;~cljaccnt
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XSOS. 1981. Rcport on the international 0-group fish survey in the Barcnts Se;l and acljaccnt
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~ \ S O S . 1983. I'rc1imina1-) report on the intrl-nationill 0 - g r o u l ~ iisli sur\.ey in tile B;trcnts Sea a n d adjacclit Lvaters in . A ~ ~ g i ~ s t / S c p t c n ~ l , c ~ - 1980. .-1irrrI.r biol. (.h/~tiih.. 3'7: 25!1-26(i.
.ASOX. 198'4a. Rcport of the international 0-group fish silsvcy in tlrc Uarcnts Sc;l ; ~ n t l atljncent
\vatcrs in hugust/SeptrmI~cr. 1981. d i i r l l ~ hrol.. (;o/oprir/i.. .?8: 228.236.
ASUS. 198411. Preliminary report 011 tlir international 0-gruu11 !is11 sur\c! ill tlic Uarrnts Sea and acljaccnt \vatel-s in .I~igust-Scl1tc1i111cr 15184. C,~oti~i. .\fee/. iiii. (.'orin. /:'\plor-. .SIY~. /!I84 [ H : 3 6 ) :
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C I ~ V \ I . \ K O \ - . :\. K. 1969. l'hc Grccnl:untl halil~ut li(~iirhmic//iii~ h i / ~ / ~ o , ~ l o r t o i c / ~ i i \ Y ; ~ l l ~ ; ~ i ~ m ) in rhc Iceland a r r a - the 1i;llibut lishel-irs antl ta~ging.,]. Icht/!~,ol.. !I: 11011-1112.
I).\SSI:\-I(;. G. 193.1. Tagging c s p e ~ - i n l r ~ i t ( i l l cod. I,oli~tcn I!147-ll)52: Sirmr ~~rclimin;lry results. ,/.
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