poor evidence for a single Atlantic 

 stock. 



On the other hand, giant bluefln 

 marked off the Bahamas continue to be 

 recaptured occasionally off Norway. 

 Since 1969, however, more of these 

 fish have been recaptured in the west- 

 em Atlantic than in its eastern waters. 

 UnforUuiately, no estimates of the re- 

 cruitment of western Atlantic tuna to 

 the northern European fisheries more 

 recent than those of Tiews ( 1 964) for 

 the 1954-1962 season have come to 

 our attention. These estimates were 

 based on the body condition of the fish 

 landed during the late part of the sea- 

 son. 



It is very puzzling that 1 1 of the 1 5 

 returns from tfie 1,100 releases off the 

 Bahamas have been from outside the 

 western North Atlantic, whereas all of 

 the 50 returns from 1,662 releases of 

 large bluefin in New England and Ca- 

 nadian waters have been from within 

 the western North Atlantic. Further- 

 more, not one of more than 450 me- 

 dium bluefin tagged in offshore west- 

 em Atlantic waters has been recaptured 

 elsewhere. This might be explained on 

 the basis that fish from the eastern At- 

 lantic spawned in the Gulf of Mexico. 

 The same arguments against the Baha- 

 mas-Norway migration being a part of 

 the regular migratory pattern of the 

 westem Atlantic fish, however, could 

 be used against its being part of the 

 regular migratory pattern of the eastern 

 Atlantic fish. Also, not one of the large 

 and medium bluefin tagged off Nor- 

 way and Spain has been captured west 

 of longitude 10°W. The most probable 

 explanation of this phenomenon is that 

 the strong flow of the Gulf Stream sys- 

 tem, especially when the relatively slow 

 North Atlantic Current is fortified by 

 exceptionally strong westerly winds, 

 and the tendency of the tuna to swim in 

 favoring currents overrides all other 

 stimuli. Consequently, the fish do not 

 leave the current when they normally 

 would to reach their American feeding 

 grounds, and continue across the ocean. 



Other recent indications of the 

 separation of stocks include the dis- 

 covery of the major spawning grounds 

 of giant bluefin in the Gulf of Mexico. 

 The failure to find early stages of blue- 

 fin in the I bero- Moroccan Bay and ad- 

 jacent Atlantic and Mediterranean wa- 

 ters reinforces the view that the central 



Mediterranean is the prime reproduc- 

 tive area for eastem Atlantic Mediter- 

 ranean bluefin. Thus the major spawn- 

 ing grounds of the two proposed stocks 

 are almost as far apart as they could be, 

 in an east-west direction, in the Atlan- 

 tic-Mediterranean system. As noted pre- 

 viously, specific size composition data 

 on oceanic longline catches are mea- 

 ger. Shingu et al. ( 1 ')74), however, pre- 

 sented some very useful data on the 

 sizes and abundance of bluefin usually 

 encountered in large areas of the Atlan- 

 tic and Mediterranean. The area of mini- 

 mum catches in the Atlantic north of 

 latitude 15°N is between longitudes 

 20°W and 40°W. The catches there 

 consist mainly of large fish. Tliis might 

 be the area where the proposed westem 

 and eastem Atlantic stocks meet. The 

 catches in the equatorial region between 

 latitudes 1 5°N and 1 0°S consist mainly 

 of large fish, with a few mediums. South 

 of latitude I0°S, catches of Atlantic 

 bluefin are rare, and consist mainly of 

 large fish. These data suggest that the 

 South Atlantic is a marginal area for 

 bluefin from the North Atlantic, rather 

 than the habitat of a separate stock. Tlie 

 groups of bluefin which remain in equa- 

 torial waters during the northcm warm 

 season have probably found good feed- 

 ing areas in the frontal areas of the 

 currents, and remained in them instead 

 of migrating northward to coastal feed- 

 ing areas. Concentrations of feeding 

 tunas of other species in frontal areas 

 of the Pacific equatorial currents have 

 been reported (Blackburn 1965, Laurs 

 and Lynn 1977, Murphy and Shomura 

 1972). 



Sorting out the populations of 

 larger tuna in mid-ocean by conven- 

 tional tagging seems an almost hope- 

 less task. Possibly some of the more 

 advanced biochemical methods will be 

 more successful. 



We conclude from the niaterial 

 considered that the following stocks 

 constitute the most probable arrange- 

 ment: 



a) A westem Atlantic spawning 

 stock which is generally self sufficient 

 and relatively uncomplicated. The chief 

 problems remaining are to define its 

 wintering and spawning areas com- 

 pletely. Contingents of large fish from 

 this stock emigrate to the eastem At- 

 lantic fairly frequently. Contingents of 

 small fish do so only rarely. 



b) A more complex stock which 

 might be subdivided into eastem At- 

 lantic and Mediterranean sub-stocks 

 until the fish are about 9 years old. The 

 eastem Atlantic substock probably re- 

 cmits much of its strength from the 

 Mediterranean substock. The fish over 

 9 years old constitute an eastem Atlan- 

 tic Mediterranean spawning stock. 



c) Possibly one or more localized 

 Mediterranean stocks. The most prob- 

 able separate stock would be the one 

 which feeds and winters in the Sea of 

 Marmara and spawns in the Black Sea. 



E. RECOMMENDATIONS 



It is clear that there are many un- 

 answered questions relating to the stock 

 or stocks of bluefin tuna in the Atlan- 

 tic-Mediterranean system. As more data 

 become available, many of these ques- 

 tions are beginning to be answered or 

 at least indications of one choice are 

 becoming clearer. Tliere are some as- 

 pects of the bluefin's life history which 

 we feel call for additional studies to 

 more readily solve some of the gaps in 

 the data. Perhaps one of the most im- 

 portant in terms of stock stmcture is 

 one we have mentioned, to detenmine 

 what percentage of the "arrival" fish 

 which enter the Ibero-Moroccan Bay 

 spawn there and what percentage 

 spawns in the Mediterranean. Some 

 type of a sonar scanning program might 

 be established in the Strait of Gibralter 

 along with a tagging program (sonic if 

 possible) of some of the fish taken in 

 the "arrival" mn in the Bay. Environ- 

 mental conditions must be examined to 

 determine what effect tliey have. 



Knowledge of the distributive pat- 

 tems of the bluefin could be greatly 

 increased by more comprehensive sam- 

 pling of the sizes of the catches, par- 

 ticularly in the oceanic longline fisher- 

 ies. More attention should also be given 

 to recording occurrences of age blue- 

 fin. In the westem Atlantic, at least, 

 many fishermen apparently do not dis- 

 tinguish very small Thynnus thynnus 

 (hynnus from other small tuna-like 

 fishes. Simple and clear identification 

 keys should be distributed to individu- 

 als who handle large numbers of fish, 

 including dockmasters, fish handlers 

 and taxidermists, as well as fishermen. 



Tagging of age bluefin in key 

 areas, such as the western Mediterra- 



143 



