Nemerson et a\. Spawning site fidelity In Thunnus thynnus 



125 



30 n 



25 



20- 



3 15 



10 



5- 



D 



n 



there would have been entirely 

 missed. 



Finally, we tested the sensitivity 

 of our results to a range of selec- 

 tivities of longline gear set in the 

 Gulf of Mexico. With selectivities 

 on fish smaller than 135 kg rang- 

 ing from to 1 (where the selec- 

 tivity on fish greater than 135 kg 

 is 1), the sensitivity analysis indi- 

 cated that at selectivities greater 

 than about 0.13, small fish were 

 significantly less abundant (chi- 

 square test, P<0.05 ) in the catch in 

 the Gulf than would be expected 

 given our assumptions and a 2% 

 east-to-west transfer rate. 



Future research on this topic 

 must seek to address both the 

 annual rate of trans-Atlantic 

 movement as well as the degree 

 of philopatry exhibited by mi- 

 grants to achieve a full under- 

 standing of the population dynam- 

 ics of east and west Atlantic bluefin 

 tuna. Currently, there are studies underway to iden- 

 tify nuclear and mitochondrial DNA markers that 

 may have variations specific to east and west popu- 

 lations (Graves et al., 1995). Examinations of otolith 

 chemistry (microconstituent analysis) may provide 

 information on stock differentiation and mixing 

 rates, and researchers are currently deploying archi- 

 val tags on bluefin tuna caught in the west Atlan- 

 tic, primarily off Cape Hatteras, North Carolina. ^'i" 

 These tags will record geolocation data and, if recov- 

 ered, should yield a complete record of each fish's 

 movement since its release. Finally, additional stud- 

 ies on the maturation schedules of fish in the east 

 and west are still needed. 



Clearly, it will take years before these studies 

 yield sufficient information on transfer rates and 

 philopatry to provide robust management advice. 

 The depleted state of bluefin tuna populations world- 

 wide, and in the west Atlantic particularly (Safina, 

 1993), make these issues of considerable practical 

 importance. Until such time as these questions are 

 answered definitively we believe that spawning site 

 fidelity should be assumed and the stocks managed 

 accordingly. 



Landed 

 Catch 



Observed 

 Catch 





■n 



70 90 110 130 150 



170 190 210 230 250 

 Fork Length (cm) 



270 290 310 330 350 



Figure 4 



Length-frequency distributions of bluefin tuna caught on longlines in the north- 

 west Atlantic (landings. n=403) and measured by observers on longline vessels 

 (observer. n = \\2) between 1992 and 1995 (see Footnote 7 in the main text). 



^ Prince, E. 1996. National Marine Fisheries Service, South- 

 east Fisheries Science Center, 75 Virginia Beach Drive, Miami. 

 FL 33149. Personal commun. 



10 Block, B. 1997. Hopkins Marine Station, Stanford Univer- 

 sity, Oceanview Boulevard, Pacific Grove, CA93950-3094. Per- 

 sonal commun. 



Acknowledgments 



We would like to thank Clay Porch and Dennis Lee 

 of the Southeast Fisheries Science Center, National 

 Marine Fisheries Service, for kindly providing assis- 

 tance in acquiring the data sets that made this paper 

 possible. 



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