Fisheries Organization

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1 . NOT TO BE CITED WITHOT PRIOR REFERENCE TO THE ATHOR(S) Northwest Atlntic Fisheries Orgniztion Seril No. N351 NAFO SCR Doc. 81/VI/67 SCIENTIFIC CONCIL MEETING - JNE 1981 On Cod Stocks in NAFO Wters by A. Vzquez InstitutO de Investigciones Pesquers Bouzs, Vigo, Spin Introduction The three min cod stocks in the NAFO Are - 2J+3KL, 3N0 nd 3M - hve usully been studied independently. This pper is n overview of these three stocks tht, s strting point, present one similr feture - the low ctches fter Methods All of the informtion on ICNAF Sttisticl Bulletins for the period ws processed. The method described by Vzquez nd Lrrfiet (1980) ws used for clculting ctch-per-uniteffort (CPE) vlues for ech yer nd stock. Two different nlyses were crried out. In the first cse, ech stocks ws studied independently. Fishing power fctors for ech vessel ctegory nd for ech stock were clculted from the ctches in ech stock, nd CPE fctors for ech yer, nd other fctors, were clculted from the fishing power fctors. In the second cse,-the fishing power fctors were clculted from ll ctches in the three stocks, by ssuming tht fishing power is independent of the stock (i.e. we clculte stock-independent fishing power), but gin, for ech stock, CPE fctors nd other fctors were clculted, s in the first cse, but using these new fishing power fctors. The results were quite similr in both cses, nd so we use the results of the second one. For estimting equilibrium points of the reltionship between CPE nd effort, nnul efforts for ech stock were weighted fter the Fox (1975) method, using s the mximum fctor two different vlus: 8.5 nd 5.0. In this pper, if no reference is mde, fctor 5.0 is the one used. There ws no substntil difference between the results with the two fctors, for the purpose of this pper. Results CPE vlues were properly correlted with prticulr stock estimtions of other uthors s follows: Stock r2 Reference 2J+3KL N M* 0.78 Gvris (1980) Bishop nd Gvris (1981) Gvris (1981) * When using dt for 3M only, r 2 = 0.89

2 CPE vlues for 1979 nd 1980 in Div. 2J71-3k1 nd 3N0 'were deduced from the dt of these uthors, but we were frid, to do so for Div 3M. 'Effort vlues were poorly correlted with F vlues from VPA nlysis (r2' less thn 0.20). Fig. 1 shows the reltionship between,cpe nd effort nd equilibrium yield nd effort for the cod stocks in Div. '2J+3KL, 3N0 nd 3M. Numericl dt re given in Tble 1. Discussion From Fig. 1, it is difficult t o see liper reltionship between CPE nd effort for Div. 2J+3KL nd 3N0, nd the clcultion of such regressions mkes little biologicl sense. It must be remembered tht these points re eguilibrium,estimtions nd not nnul points. With nnul efforts, circuling distribution of.cpe -effort points would be expected when effort increses nd decreses consecutively, but there is no popultion dynmics theory, to explin this distribution of equilibrium pproximtions. It seems tht the three sets of d t show coincident development. The CPE ws high up to 1968, declined during , nd ws mintined t lower level fter If this prllelism exists nd if it is not cceptble tht in Div. 2J+3KL nd 3N0 the sme regression line cross the Points snd the points simult4ppusly, this will not be cceptble for Div. 3M lso, lthough for this stock the pqints seem distributed round the sme line. In other words, there re lso two different levels in Div. 3M but the points re grouped. According to this Pttern scheme, some observtions cn be mde. In the Div. 2J+3KL nd 3N0 stocks the high fishing effort in the lte 1960 s could be understood s the cuse for the decline from high level of CPE to lower one, but in the Div. 3M stock the decline occurred t low level of effort. The s me fct ws interpreted in Div. 3N0 in the Yers ( Vzquez nd Lrrrlet, 1980). After the decline from high level to low level of CPE, the e ffort decresed in Div. 2J+3KL nd 3N0 cod stocks but it incresed in the Div. 3M stock, nd the CPE vlues were mintined t bout the sme low level in the three stocks. All o f these fetures seem t o indicte n independence between the chnge in the different levels of CPE nd the effort. It seems cler tht the equilibrium sitution for these stocks re fr from the equilibrium stges ssumed by the generlized 'product ion Models, s tht of Schefer, but we re not devoted to think tht s uch sitution is non-equilibrte d or errtic condition becuse nture is lwys in equilibrium. Lrrnet ( 19 1) gives n PPlntion for the two levels of CPE in Div. 314(),nd 3M. This would result froln chnge in the stock-recr uitment relt ion -ship. Referring to the Edv. 2J+3KL cod stock (Fig. 1) it is not difficult to drw stright line with negtive regression coefficient, s the Schefer model ssumes, for the upper points. A negtive regression coefficient is not evident for line fitted to the lower points. Th e tendency of 1979 nd 19:80 points, if correct, would indicte recovery of the fishery to the upper level of CPE. Conclusions For the cod stocks of Div. 2J+3KL, 3N0 nd 3M, the reltionship between CPE nd effort does not correspond to liner AePendnce, As the Schefer model ssumes. In fct, two different equilib rium situtions cn be discerned. Historiclly, the decline froin hi gh to lower level of CPE does not correspond with high fishing effort nd vice vers.

3 For the Div. 3N0 nd 2J+3KL stocks, effort is ctully t low level, s compred with the lst 25 yers, nd there is no evidence tht, by mintining this effort level, the fishery would be improved (i.e. increse to higher CPE level). In Div. 2J+3KL, this increse could hve hppened if the provisionl dt for 1979 nd 1980 re correct. Acknowledgement I m indebted to Dr M. G. Lrrnet for his dvice in prepring this pper nd to Mr V. M. Hodder for his vluble help. References Bishop, C.A. nd S. Gvris Stock ssessment of cod in Division 3N0. NAFO SCR Doc. 81/11/11. Fox, W. W Fitting the generlized stock production model by lest-squres nd equilibrium pproximtion. Fish. Bull. 73(1): Gvris, S Assessment of the cod stock in NAFO Division 2J+3KL. NAFO SCR Doc. 80/VI/81. Gvris, S Assessment of the cod stock in Division 3M. NAFO SCR Doc. 81/11/12. Lrrnet, M. G Ecology nd fishing of cod stocks in Divisions 3M nd 3N0. NAFO SCR Doc. 81/11/6. Vzquez, A., nd M. G. Lrrnet Assessment of cod stock in Divisions 3N0. NAFO SCR Doc. 80/11/10.

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