124 



Fishery Bulletin 98(1) 



Alternative spawning grounds 



Our results might also be otabined if small migrant 

 tuna of eastern origin spawn elsewhere in the west 

 other than the Gulf of Mexico. A recently discovered 

 concentration of medium and large tuna off the 

 coast of North Carolina from January through April 

 caused speculation that perhaps this concentration 

 represents another spawning area. However, the 

 lack of gonad development in a sample of seventeen 

 fish (weighing between 65 and 183 kg) suggested 

 that these fish were unlikely to spawn in the year 

 they were captured and were probably immature 

 (Belled). 



Several other workers have searched for evidence 

 of bluefin tuna spawning in the west Atlantic. Mather 

 et al. (1995) reported finding ripening small fish 

 but no larvae. If the overlap hypothesis does pre- 

 vail, then these potentially mature but nonspawning 

 smaller fish may be eastern migrants that although 

 capable of spawning, will return to the Mediterra- 

 nean before actually doing so. McGowan and Rich- 

 ards (1989) reported on the sporadic presence of 

 larvae in the Gulf Stream as far north as North Caro- 

 olina but concluded that most larvae found in the 

 Gulf Stream were either advected out of the Gulf 

 or spawned by tuna exiting the Gulf Furthermore, 

 they stated that conditions are poor for larval devel- 

 opment in the Gulf Stream and that the occasional 

 occurrence of larvae there does not indicate an addi- 

 tional spawning ground. On the matter of alternative 

 spawning grounds, the National Research Council 

 concludes that "extensive searching has detected 

 only two spawning localities: the Gulf of Mexico and 

 the Mediterranean Sea" (NRC, 1994, p. 18). 



Underreporting or low catchability ' 



Two other possibilities for the lack of small bluefin 

 tuna in the Gulf of Mexico catch are that they are 

 present in the Gulf of Mexico and are either being 

 caught but not recorded, or are not being caught 

 owing to a lack of appropriate fishing effort. To test 

 for the first possibility, we acquired records of all 

 bluefin tuna recorded by longline observers in the 

 Gulf of Mexico during 1993-95. Of 31 bluefin tuna 

 recorded by observers for which actual or estimated 

 weights were recorded, all were greater than 135 kg. 

 We also reviewed ICCAT data for the Japanese long- 

 line fishery in the Gulf of Mexico for the period 1973 



to 1981 and found that only 58 records out of 14,530 

 (0.4%) were for fish under 180 cm ( 135 kg). These 

 data are particularly significant in light of the fact 

 that there were no regulations concerning the reten- 

 tion and sale of small bluefin tuna during this period 

 as there have been in recent years. Therefore, the 

 Japanese would have had no incentive to intention- 

 ally misidentify or underreport small bluefin tuna. 

 Mather etal. (1995), after reviewing longline catches 

 in the Gulf and Caribbean prior to 1973, found only 

 fish larger than 185 cm. They also reported very 

 young bluefin tuna (less than 2 kg) in the Gulf of 

 Mexico from July into November (Mather et al., 

 1995); fish presumably spawned a few months ear- 

 lier. Similarly, Hisada and Suzuki (1982) presented 

 length-frequency distributions of Japanese longline 

 catches from the Gulf of Mexico which appear to 

 show essentially no fish smaller than 200 cm. 



The possibility that small bluefin tuna are present 

 in the Gulf but are not being caught cannot be com- 

 pletely eliminated. However, there is a considerable 

 accumulation of evidence that suggests that this is 

 highly unlikely. For example, although there cur- 

 rently is no directed fishery for either small or large 

 bluefin tuna in the Gulf, there is a widespread, year- 

 round yellowfin tuna (Thunnus albacares) longline 

 fishery. This fishery targets yellowfin tuna of the same 

 size as the small bluefin tuna of east Atlantic origin 

 that we hypothesize would be present in the Gulf if 

 the diffusion model is correct. This fishery does have 

 a bycatch of bluefin tuna, none of which have ever 

 been recorded by observers as less than 175 cm.^ 



Furthermore, longline operations in the northwest 

 Atlantic do catch small bluefin tuna, indicating that 

 they are potentially vulnerable to this gear. Cramer 

 and Turner*^ reported length frequencies for observer 

 data from the U.S. longline fishery in the northwest 

 Atlantic from 1992 to 1995, showing that over 30% 

 of fish hooked were less than 150 cm straight fork 

 length (Fig. 4). Similarly, catch data from the Japa- 

 nese northwest Atlantic longline fishery in the 1970s 

 and 1980s show that the catch dominated by blue- 

 fin tuna between 100 and 150 cm in several years 

 (Fig. 8 in Hisada and Suzuki, 1982). Although fail- 

 ure to catch a given species or size class in an area 

 can never rule out its presence, given the extent and 

 diversity of fishing activity in the Gulf, it is unlikely 

 that any significant aggregation of small bluefin tuna 



^ Belle, S. 1996. Biological sampling of bluefin tuna off Cape 

 Hatteras, North Carolina. Final report to the New England 

 Aquarium Corporation (NOAA requisition no. 43AANF503279I, 

 Boston, MA. 12 p. 



' Lee, D. 1996. National Marine Fisheries Service, Southeast 

 Fisheries Science Center, 7.5 Virginia Beach Drive. Miami. FL 

 33149. Personal commun. 



" Cramer, J., and S. C. Turner 1996. Standardized catch rates 

 for bluefin tuna, Thunnus thynnus, from the U.S. pelagic long- 

 line fisherv in the northwest Atlantic. ICCAT working docu- 

 ment SCRS/96/69. 



