NAFO Sci. Cun. Studies, : 7- han Hlger H. Hansen Greenland Fisheries and Envirnmental Research Institute Tagensvej, Cpenhagen N, DK-00, Denmark Abstract Analysis f sampling data fr Atlantic cd (Gadus mrhua) frm tile ffshre catclles f trawlers ff West Greenland in the first and secnd quarters f 979-8 revealed significant reductins in the size f fish cmprising the majr age-grups frm 979 t 98. Mean length-at-age f age-grups -8 declined by -7% and mean weight-at-age declined by -7%. Hydrgraphic cnditins as well as the distributin and migratin patterns f cd have changed substantially during the 979-8 perid. The change in size-ai-age may be attributed in part t the effect f envirnmental cnditins n grwth and perhaps in larger part t the effect f changing distributinal patterns f the cd ppulatin. Intrductin The Atlantic cd (Gadus mrhua) ppulatin and fishery in West Greenland waters have experienced great changes since the peak years f the fishery (96-68) when nminal catches varied between 0,000 and 0,000 (metric) tns (ICNAF, 976). After 968, catches declined rapidly t a lw f,000 tns in 976. Slight imprvement t abut 60,000 tns in98 was fllwed by near-cllapse f the fishery with a nminal catch f nly,000 tns in 98 (NAFO, 98, MS 986). Substantial changes in the grwth f Atlantic cd (hereinafter called cd) and the structure f the ppulatin ff West Greenland have been bserved during the early 980's. These changes have been attributed t a cmbinatin f lw recruitment and substantial mig ratin f cd frm West Greenland waters (Crnus et a/., MS 98). Bth prcesses are believed, at least in part, t be influenced by water temperatures, which were unusually lw during the same perid (Rsenrn et al., MS 98; Buch, MS 986). The purpse f this paper is t dcument the cnsiderable decrease in mean size-at-age f cd in recent years and t discuss briefly the pssible causes fr this change. Materials and Methds Bilgical sampling f the cd fishery in West Greenland waters (Fig. ) has been cnducted fr many years. Hwever, nly the data fr 979-8 have been cmputerized, and analysis f this data set was prmpted by recent changes in apparent grwth f cd. During 979-8, there were few prblems with sample cverage f the ffshre trawl fishery, which is cnducted mainly by a fleet f state-wned trawlers. These vessels usually fished n the same grunds, but their catches were landed at varius prts in West Greenland, including Nuuk where mst f the sampling tk place because the fisheries research labratry is lcated there. Sampling cverage f the inshre fishery, n the ther hand, was very patchy because f the numerus small fishing cmmunities and the large distances between many f them. Because f differences in size cmpsitin f inshre and ffshre 7 r--~~~~~-~----r---... ---~ 70" 68" 66", A B OA I I) 7 L~.:::-::..J I ( I I Llile I Helleftske i C B,ank GREENLAND I I ~----..'.-:::.. 6" OB 0 -------- 6" I I E, ~----------~- '- I ~ " if ~, 60" ",~ ", G ", '- 60" " SO" " 0" " Fig.. Map f Greenland shwing the NAFO divisins and ther areas mentined in the text.
8 Sci. Cuncil Studies, N., 987 catches, nly the data frm the ffshre trawl fishery were used t reduce the effect f unbalanced gegraphical sampling cverage ver the years. Mst f the trawl catches were taken in Div. C, D and E (Fig. ), and this is reflected in the sampling cverage (Table ). In rder t reduce the pssible effects f grwth differences between the extreme parts f the West Greenland area, nly the data fr these divisins (88% f the age readings) were used. Seasnally, mst f the ffshre fishery, and hence sampling effrt, tk place in the first, secnd and furth quarters f the year. Hwever, nly the data fr the first and secnd quarters were used, because this is the perid when grwth is minimal. The resulting database cnsisted f length, weight and age data fr nearly,000 fish. Lengths were recrded as ttal lengths t the cm belw (i.e. 0.0-0.9 = 0 cm), weights were recrded in grams, and ages were deter- TABLE. Numbers f age determinatins fr cd in samples f trawl catches by divisin ff West Greenland and by quarter in Div. CDE, 979-8. 979 980 98 98 98 98 Ttal Tlal age determinatins by divisin B 8 990 0,97 C, 87 0 78,78 0,9 60 68,68, 8 7,7 E,,,7,708 6,80 F 8 99 Ttal,897,799,6,9,09, 9,8.. _-------_.. ----_._.----- --------------".-.------------------ Age determinatins lr Div. CDE by quarter,6, 97,9,,70 8,0,67 6 669,09,80 6 8 00,869 8 78,7 6,69 Ttal,8,6,6,9,608,90 7, mined by micrscpic examinatin f tlith crsssectins shwing alternating paque and translucent znes. in an assessment cntext, the mst interesting variable is weight-at-age. Hwever, much f the reprductive cycle (maturing, spawning and refattening) ccurs in the first and secnd quarters, with cnsequent changes in cnditins f the fish. Fr this reasn, the analysis was perfrmed n length-at-age data, which were subsequently cnverted t weight-at-age values by using cnditin factrs fr the prespawning perid (i.e. January-February), the cnditin factrs (CF) being defined by CF = 00 W/L where W is weight (gm) and L is length (cm). The calculated weight-at-age values were cnsidered t be clsely representative f cnditins at the beginning f the year, because practically n grwth in length ccurs during the first and secnd quarters. Changes in length-at-age Results A substantial reductin in average length f cd was evident fr nearly all age-grups during 979-8 (Table ). The reductin is clearly illustrated in Fig. fr the mst numerus age-grups ( t 8). The trend fr age cd is less cnsistent than thse fr agegrups -8, mainly because the minimum landing size f 0 cm wuld bias the mean length f age cd upward when the ppulatin mean appraches the minimum landing size. The decrease in mean length-at-age is apprximately linear fr each f age-grups (Fig. ), with increasing negative slpe fr the lder fish. Linear TABLE. Number f cd examined and average length by age-grup in samples f trawl catches frm Div. CDE ff West Greenland during the first tw quarters f 979-8. Age Number f cd Average length (cm) (yr) 979 980 98 98 98 98 979 980 98 98 98 98.0 6.0 6 8 9 8.0.. 7.0 6. 0 0 7 9 7.6 7.6 8.0.9. 9. 76 86 7 9 86 9.0 7.7 8.9.6 0.8 9. 6 0 77 07 9 7 0 70.7 68. 6. 6.7 9.8 9.8 7 90 0 0 6 68 78.0 76. 7. 7. 70. 6.6 8 9 6 9 7 87. 8. 8. 8. 76. 7. 9 8 0 66 97 0 89. 86.6 80.8 86. 8.0 78. 0 6 6 9. 9.0 88.8 9. 79.9 80.6 7 0 97.8 9.0 9. 98.7 0. 8. 7.7 86..0 87. 0.6 0.0 08.9.0 78.0 0.0 86.0 0. 8..0.0 6 7 080
HANSEN: Changes in Cd Size-at-age ff West Greenland 9 90 8 TABLE. Parameters f the relatinship between mean fish length and year f sampling fr ages -8 cd ff West Greenland ver the 979-8 perid, and predicted lengths fr 979 and 98. 80 7 70 E ~ ;;;; en 6 c...j '" 60 Age 0 Predicted Percent Age Regressin parameters' length (cm) length (yr) Intercept (a) Slpe (b) 979 98 decrease 6. -.0 60. 9.8 7. 6 0.8 -.8 70. 9.0 6. 7 6.6 -. 78.7 67.0.9 8 0. -.9 87.9 7. 6.7 Linear mdel is L = a + by, where L is mean length (cm) and Y is year. TABLE. Parameters and test statistics f the relatinship between cnditin factr and fish length fr cd ff West Greenland, 979-8. Regressin parameters' Sign. test Year Intercept(a) Slpe(b) P{b=O) 979 0.99-0.000 0. 980. -0.00 <0.00 98 0.9 0.000 0.6 98 0.98 0.000 <0.00 98.00-0.000 <0.00 98 0.9-0.000 0.8 Linear mdel is CF = a + bl, where CF is cnditin factr and L is length (cm). 0 Fig.. 979 980 98 98 98 98 Trends in mean length f cd f age-grups t08, frm samples f ffshre catches in Div. CDE during 979-8. regressins were fitted fr each age-grup, and predicted lengths were calculated fr 979 and 98 (Table ). Fr age-grups -8, the reductins in predicted length were between.9 and 7.%. Cnditin factrs Cnditin factrs fr cd in January and February f each year were calculated and regressed against length t check fr any relatinship between these parameters (Table ). N significant effect was fund in the data fr 979,98 and 98 (p>0.0), but significant deviatins f the slpes f the regressin lines frm zer were fund fr 980, 98 and 98. The pattern was nt cnsistent, because cnditin factr increased with length (psitive slpe) in 98 and decreased with length (negative slpe) in 980 and 98. The simplest interpretatin f these smewhat ambiguus results was t assume that fish cnditin is independent f length, in which case the significant effects in sme years may be attributed t unbalanced sampling r t errrs in the initial recrding f fish lengths and weights. The largest abslute deviatin f the slpe frm zer ccurred in the 980 data (Table ). T illustrate the pssible invlvement f errrs (utlying values) when applying a cnstant cnversin factr, the frequency distributin f bservatins n cnditin factr and fish length fr 980 are shwn in Fig.. The high utlying pints pertain predminantly t small cd, and these cntributed t making the slpe f the CF-Iength relatinship negative in that year. It is clear that a great majrity f the bservatins fall within a narrw range f CF values (0.8-.) and that the pints seem t be randmly distributed arund the mean CF, which was.0 in that year (Table ). Changes in weight-at-age Mean weight-at-age values, which are cnsidered t be representative f cd at the beginning f each year, were calculated frm the mean cnditin factrs (Table ). The decreases in weight frm 979 t 98 fr the mst imprtant grups (ages -8) were in the range f -7%. Discussin The change in size-at-age f cd ff West Greenland can be interpreted as a change in grwth but it can als result frm a selective migratin f lder fish. In the fllwing discussin, emphasis has been placed n these tw aspects f the changes. Changes in grwth f cd The mst imprtant factrs which regulate the
0 Sci. Cuncil Studies, N., 987.6 " l. tl ~ c: ~ "0 c:.0 0.8 6 6 j d ~ ~ <I 6 ' I : 0.6 0 60 6 70 7 80 8 90 9 Fish length (em) Fig.. Frequency distributin f bservatins n cnditin factr and length f cd frm ffshre catches in January-February 980. TABLE. Calculated average weights (using mean cnditin factrs) f West Greenland cd by age-grup and year, 979-8. (See Table fr numbers f cd sampled.) Age Calculated average weight (kg) % decrease (yr) 979 980 98 98 98 98 979-98.07..06 0.8 0.8 0.7 6.7.0.00.9.77.7..6 6...6.87.9.98.6 7.77.6.96.09.0.6. 8 6. 6...78.9.6 7. 9 7.00 6.68.09 7.0.79.7 6. 0 8.9 7.66 6.86 9..69.9.8 9. 8.0 8.0 0. 9.8.. CF' 0.97.0 0.9.0 0.88 0.9, Mean cnditin factr. grwth f fish are temperature and fd availability, with the latter factr als being largely dependent n the frmer ne. Knwledge f fd availability fr cd ff West Greenland is scarce. N studies n fd and feeding f cd have been carried ut in recent years, and nne f the rgan isms (fish and invertebrates) which are believed t be the majr prey f cd are presently being studied with regard t thei r abundance and bimass. The verall temperature regime in West Greenland waters has undergne substantial changes during the past decade, with prnunced cling f the water masses frm 98 t 98 (Rsenrn et a/., MS 98). This decline in temperature may be respnsiblefrthe bserved reductin in size-at-age f cd, but it might be misleading t use verall temperature indices as majr cntributing factrs, because cd are quite capable f seeki ng water strata with su itable temperatures. The cd ppulatn ff West Greenland undertakes fairly regular annual migratins frm the deep ffshre areas where they stay frm autumn t early summer t the shallw castal waters where they are fished du r ing late summer and early autumn. T btain sme indicatin f the changes in temperature cnditins during these migratins, data fr three hydrgraphic regimes in the Fylla Bank area were summarized, tw fr the ffshre area (Statin at 6 'N, 'W) in February and Nvember, and ne fr the castal area (6 0'N, 0'W) in July, enabling the cmparisn f average temperatures ver the 97-8 perid (Fig. ). In general, the temperature cnditins did nt vary much and there was n evidence f a marked decline in temperatu re during 979-8. A significant questin in relatin t the effect f temperature n the grwth f fish is: hw much temperature change will be necessary t accunt fr the reductin in size-at-age as large as that which has been bserved fr cd ff West Green land? As a starting pint, ne can assume that grwth rate is prprtinal t temperature, i.e. the degree-day mdel Time x Temperature = Cnstant where "Time" refers t the time necessary fr the
HANSEN: Changes in Cd Size-at-age ff West Greenland 7r-~--~-'--'-~--'--'--.--.--'--'---r-. 6 - - A. Fylla statin (February) -r-~ ~~ ~~ ~~ ~~ ~~ ~~ 7 G 6 ::... ~ ' Q) ~ ~ - - B. Fylla castal statin (July) -r-~--~~--~-l--~~ ~~ ~~~~~ 7 6 - - C. Fylla statin (Nvember)... - -... - '," -... - ~ ~~~ ~-~ ~ ~... ~....",.-... /... ~ ',...............-.., / '''' ;'... e---r,,' /,... ~... ~/.-~/ ~...... 0-0 m _ 0-00 m 00-00 m J. -~~~~~~~-=~~~~~~~_~~~~~ n M ~ M n M r 00 ~ ~ ~ M Fig.. Year Trends in mean water temperature f three depth znes in the vicinity f Fylla Bank, 97-8: A, Fylla statin in February; B, Fylla castal statin in July; and C, Fylla statin in Nvember. animal t grw t a certain size. Fr cmparisn f tw different grwth situatins Time()/Time() = Temp.()/Temp.() Weight-at-age data fr 979 and 98 were used t calculate the Time()/Time() ratis relevant t unit increase in fish weight (Table 6). In a general sense, this implies that a temperature reductin f abut 0% is sufficient t accunt fr a reductin in mean weightat-age values as large as thse bserved frm 979 t 98. Fr the prevailing temperature regime, the change in size-at-age culd result frm a C decline in temperature frm abut t C. There is, hwever, n a priri reasn t expect a simple prprtinality, and it has been suggested (Ricker, 979) that the degree-day mdel shuld be mdified t Time x (Temperature - T) = Cnstant where T is a psitive temperature belw which n grwth takes place. If, in the case f cd, T is assumed t be C, a 0,0 C reductin in temperature frm. t.0 C wuld be sufficient t accunt fr the bserved reductin in size-at-age. TABLE 6. Calculated time (years) fr cd t grw t varius weight levels in 979 and 98, based n the weight-at-age data in Table. Weight Time() Time() (kg) 979 98 Rati.0.90.7 0.8...8 0.8.0.9 6.0 0.8.. 6.7 0.79.0.6 7. 0.76..99 8.0 0.7.0 6.0 8. 0.7 If these very rugh cnsideratins reflect situatins clse t reality, i.e. that temperature reductins as small as 0. t.0 C may cause reductins in size-at-age as large as thse which have been bserved fr cd, it will be quite difficult in practice t cnstruct a realistic grwth mdel due t the magnitude f the spatial and tempral temperature variatins in nature. Changes in size due t migratin Recent analyses f data relevant t determining the state f the cd stcks ff West Greenland fr management purpses have been carried ut under the assumptin that large numbers f cd migrate frm West Greenland t East Greenland and pssibly Iceland (Hrsted et al., MS 98; Crnus et al., MS 98; ICES, MS 98). These mass mvements have been interpreted as spawning migratins. Recent maturity data fr cd ff West Greenland indicate that the mean size-at-age f mature fish is nly slightly larger than that fr immature fish. Thus, the idea cannt be excluded that the reductin in mean size was caused, at least in part, by increased migratin f large mature cd away frm the West Greenland area. Nevertheless, the reductin in size-at-age seemed t affect all agegrups unifrmly, irrespective f the prprtin f mature fish in each age-grup. Cncurrently with the large migratins in recent years, there has been a gradual suthern displacement f the cd fishery ff West Greenland. This trend is clearly reflected in the catches f Greenland trawlers during 979-8 (Table 7). The suthern mvement f the stck cmplex may have affected the mean size-atage f cd in the regin under investigatin (Div. CDE). Length frequencies frm recent autumn bttm-trawl surveys by research vessels f the Federal Republic f Germany indicated that the smallest cd were fund in the nrthern areas (Div. BC) and that there was a gradual increase in size frm Div. C t F, with the largest fish ff East Greenland (Messtrff and Crnus, MS 98; Crnusetal., MS 98). Thus, a general suthern displacement f the stck cmplex wuld result in the mvement f relatively small cd t
Sci. Cuncil Studies, N., 987 TABLE7. Ttal catch f state-wned Greenland trawlers and percentage distributin by area ff West Greenland, 979-8. Catch Catch distributin by area (%) Year (tns) B C 0 E F 979 0,70 9.8. 6.6 0. 980 6,6 6.9.8 7.0.0 0. 98,6.9..9 98 0,98 0.6 9..0. 0. 98 6,.8.. 69.7 0.7 98,9..6. Div. CDE frm the nrth and the suthward migratin f relatively larger fish ut f the area, References BUCH, E. MS 986. A review f the hydrgraphic cnditins ff West Greenland in 980-8. NAFO SCR Dc., N. 8, Serial N. N6, p. CORNUS,H. P.,J. MESSTORFF,A.SCHUMACHER,H.H. HANSEN, Sv. Aa. HORSTED, J. M. JENSEN, and K. M. LEHMANN. MS 98. Status f the West Greenland cd stck and management cnsideratins. NAFO SCR Dc., N. 6, Serial N. N0, 6 p. HORSTED, Sv. Aa., J. M. JENSEN, J. MESSTORFF, and A. SCHUMACHER. MS 98. Stck size f cd at West Greenland by the beginning f 98 and prjectins f yield and stck size, 98-87. NAFO SeR Dc., N. 9, Serial N. N88, 0 p. ICES. MS 98. Reprt f the wrking grup n cd stcks ff East Greenland. ICES C.M. Dc., N. Assess: 6, 6 p. ICNAF. 976. Tabular summaries f fish and sea! catches, 960-7. ICNAF Stat. Bull., : -0. MESSTORFF, J., and H. P. CORN US. MS 98. Subarea cd: results f research vessel surveys cnducted ff West Greenland in 98 and 98. NAFO SCR Dc., N. 9, Serial N. N886, p. NAFO. 98. Tabular summaries f nminal catches, 969-8. NAFO Stat. Bull., : 7-. MS 986. Prvisinal nminal catches in the Nrthwest Atlantic. 98. NAFO SCS Dc., N., Serial N. N0, p. RICKER, W. E. 979. Grwth rates and mdels. In Fish physilgy (vl. 8, p. 677-7), W. S. Har, D. J. Randall and J. R. Brett (ed.), Academic Press, New Yrk, N.Y., 786 p. ROSENORN, S., J. FABRICIUS, E BUCH, and Sv. Aa. HORS TED. MS 98. Recrd-hard winters at West Greenland. NAFO SCR Dc., N. 6, Serial N. N0, 7 p.