NOTE Arrizabalaga et a\: Uncertainlty In stock assessments of Thunnus alalunga due to assumed stock structure 



143 



Stock assessment under the assumption 

 of migration between stocks 



Fits under several migration scenarios were more par- 

 simonious than under the assumption of no migra- 

 tion between stocks (Table 3). Scenarios 1 (p^g = 0.025, 

 Ps^=0.009), 2 (pi,,s = 0.05.psN=0l 3 (pvs = 0' Psiv=0-05), 

 4 (/)^,s = 0.05, p>,.^=0.018), 6 (p^s = 0.05, Ps^=0.05), and 7 

 (p,^g=0.1, pg^=0) showed lower AICc values than in the 

 base case (BO. 



Spawning stock biomass (SSB) values and trends 

 under scenarios 1, 2, 3, 4, 5, 6, and 7 were similar to 

 the ones observed in the base case, especially in the sec- 

 ond half of the study period (except for the SSB of the 

 southern stock over the last two years, which showed 

 more variability. Fig. 2). In contrast, scenarios 8 and 9 

 showed a very different pattern in the last half of the 

 series. In the north Atlantic, after the decline of the 

 SSB during the beginning of the 1980s, the recovery 

 was much more effective under these two scenarios, 

 reaching higher values at the end of the 1990s than in 

 the 1970s. Meanwhile, the SSB values for the southern 

 stock were only slightly lower than those under the 

 assumption of no migration, and under scenario 9 they 

 were higher than in the base case over the last two 

 years (Fig. 2). 



Discussion 



The method used to assess the uncertainty in the stock 

 assessments that is due to the assumed stock structure 



