NOTE Arrizabalaga et al.: Uncertainity in stock assessments of Thunnus alalunga due to assumed stock structure 



145 



tions is not at lat. 5°N, but farther south, or that some 

 amount of migration exists between them. Because no 

 diagnostic loci were found by Arrizabalaga et al. (2004), 

 the estimated proportions from each stock in the Gulf 

 of Guinea sample could not be precisely determined and 

 the true nature of mixing between north and south At- 

 lantic albacore has yet to be fully determined. In spite 

 of this uncertainty, the present exercise has made it 

 possible to explore the response of biomass trends in 

 different plausible discrete-stock scenarios and stock- 

 mixing scenarios. 



Although it is not possible to determine where the 

 real line lies between populations, we can conclude from 

 our knowledge about the current low level of reported 

 equitorial catch and the size structure of this catch and 

 if we assume a limit of latitude 0° north or latitude 5°S 

 that our current perceptions of the stock structure of 

 this species are probably accurate. On the other hand, 

 several migration scenarios fitted the observed catch-at- 

 age and abundance indices better than the scenario of 

 no migration. In all these scenarios, SSB trends were 

 very similar and values did not differ significantly from 

 the ones in the base case; therefore it can be concluded 

 that, although some rate of migration between stocks 

 likely exists, the perception that we have about stock 

 status, assuming there is no migration, is quite realistic. 

 In other words, uncertainty in northern and southern 

 Atlantic albacore stock assessments associated with the 

 assumed stock structure does not seem to be important, 

 given current biological knowledge and fishery data. 



The highest variations in SSB were observed for 

 northern Atlantic albacore in scenarios with high mi- 

 gration rates from south to north, showing high levels 

 of SSB at the end of the study period. However, the ob- 

 served difference in SSB levels with respect to the base 

 case in the South Atlantic was not that pronounced, 

 showing that northern Atlantic albacore biomass is 

 more sensitive to biomass input from the south than 

 vice versa. This result occurs because the minimum 

 level of SSB in the north Atlantic in 1987 coincides in 

 time with the maximum SSB in the southern stock, 

 which is an order of magnitude higher. In this case, a 

 migration rate of 10% from south to north would imply 

 the input of approximately half the biomass present in 

 the north at that moment, leading to a more rapid re- 

 covery of historic levels than under the null migration 

 assumption. However, the existence of such important 

 migration rates from south to north seems unlikely 

 given the observed catch-at-age and abundance indices 

 for both stocks. 



The present analysis allows for the increase in con- 

 fidence levels regarding stock assessment results for 

 northern and southern Atlantic albacore obtained with- 

 in ICCAT, assuming that stocks are separated at lat. 

 5°N and that there is no migration between them. This 

 information is essential in order that the catch- and 

 effort-related management measures that are in force 

 for Atlantic albacore remain effective. Nevertheless, ad- 

 ditional hypotheses, such as migration between North 

 Atlantic and Mediterranean albacore, or between South 



Atlantic and Indian Ocean albacore, should be inves- 

 tigated further as future research findings are made 

 available. Moreover, it should be noted that migration 

 between stocks could vary among years, and a yearly 

 based assessment of genetic mixture, based on DNA 

 analysis, would be more useful for quantitative stock 

 assessments. 



Acknowledgments 



We are grateful to Clay Porch for his help in the use of 

 VPA-2box and statistical considerations regarding the 

 interpretation of the results. Mauricio Ortiz, Hilario 

 Murua. Dorleta Garcia, two anonymous referees, and 

 the scientific editor also made helpful suggestions. We 

 are also grateful to the Spanish "Comision de Ciencia y 

 Tecnologia" (CICYT) and the "Departamento de Agricul- 

 tura y Pesca" of the Basque Government for the funding 

 of the MAR98-0233 and RP2002023 research projects, 

 respectively. 



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