of Mexico in September, and in Cuban 

 waters in September and October 

 (Potthoffand Richards 1970, Richards 

 1976). The major spawning of giant 

 bluefln in the Gulf of Mexico most 

 probably takes place in May and June, 

 with lesser occurrences in late April 

 and early July. 



The distribution of catches of lar- 

 val bluefin tuna along the transect of 

 the Florida Straits from Miami to 

 Bimini, with the largest catches at the 

 western and central stations, and none 

 in the station off Bimini, seems very 

 significant. It suggests that the majority 

 of the larvae have been transported by 

 the Florida current from areas in the 

 southwestern part of the Straits, and 

 possibly even in the southwestern Gulf 

 of Mexico. Conversely, it does not in- 

 dicate extensive spawning in the area 

 of the fishery for adult bluefin along 

 the northwestern edge of the Great 

 Bahama Bank. This in turn strongly 

 supports the belief that the majority of 

 the large adult bluefin which pass 

 Bimini in May and June have spawned 

 before arriving there, as did the studies 

 of their gonads (Rivas 1954, Baglin 

 1976). Rivas' (1954) alternative sug- 

 gestion that the northward migration 

 might include schools of spent fish, 

 which may take bait, and schools of 

 ripe fish, which do not, seems less prob- 

 able in view of this new information on 

 the distribution of larvae. It thus ap- 

 pears that this migratory passage off 

 the northwestern Bahamas is actually a 

 spent fish run, analogous to the "re- 

 turn" runs which occurred regularly 

 along the north coast of the Ibero-Mo- 

 roccan Bay and in some Mediterranean 

 localities. Rivas (1954), Potthoffand 

 Richards (1970), Richards (1976) and 

 Baglin (1976), on the basis of their 

 studies of gonads, larvae and juveniles 

 collected in or near the Straits of Florida 

 and the occurrences of spent adults 

 along its eastem edge, reached slightly 

 different conclusions about the prob- 

 able dates of spawning in that area. All 

 of their estimates, however, fell within 

 the months of April, May and June. 

 The total evidence suggests that some 

 spawning may have occurred in late 

 April, but that the bulk of it has taken 

 place in May and June. The termina- 

 tion of spawning may well have coin- 

 cided with the disappearance of the 



adult fish, which has usually occurred 

 during the last half of June. 



As noted above, the collections of 

 bluefin larvae in the central and west- 

 em parts of the Straits of Florida off 

 Miami suggest that spawning may have 

 occurred in the Straits south and west 

 of there, off the Florida Keys or possi- 

 bly in the southwestern Gulf of Mexico. 

 Bluefin tuna have very rarely been ob- 

 served off the Florida Keys (Section 

 IV), but, as Ju^ez ( 1 974b) pointed out, 

 the species tends to spawn in offshore 

 waters. Most of the very large sport 

 fishing effort exerted off the Florida 

 Keys has taken place on or close to the 

 continental shelf This might explain 

 the scarcity of observations of bluefin, 

 even if they had been spawning in the 

 deep waters off the Keys. 



E. RELATIONSHIPS BETWEEN 

 ENVIRONMENTAL FACTORS 

 AND SPAWNING BEHAVIOR 



1. Introduction 



Biologists agree that the spawning 

 behavior of bluefin is strongly affected 

 by environmental factors and that its 

 sensitivity to these factors is intensified 

 during the reproductive period. Opin- 

 ions as to the importance and the nature 



of the effects of various conditions, 

 however, are extremely diversified. 



Among the environmental factors 

 most frequently considered are the tem- 

 perature, salinity, density, transparency 

 and oxygen content of the water, winds, 

 tides, currents, atmospheric pressure, 

 rainfall, and abundance of plankton. 

 Most of this research has centered on 

 mature individuals. Less attention has 

 been paid to the effects of environmen- 

 tal conditions on the hatching of eggs 

 and the survival of the early stages. 

 With the present decrease in the spawn- 

 ing stocks, however, knowledge of these 

 factors may also be very important. 



2. Mediterranean Sea 



Since the ancient trap fisheries in 

 the Mediterranean Sea depended on 

 the spawning behavior of the bluefin 

 tuna, much attention has been paid there 

 to the effects of environmental factors 

 on this behavior. 



Roule(1914a, 1914b, 19I7),apio- 

 ncer in these studies, concluded that 

 the bluefin tuna was "stenothermic" 

 and "stenohaiine." These beliefs were 

 the basis of his "halo-thermic" theory. 

 Roule did not specify the limits, or 

 averages, of the temperatures and sa- 

 linities which were suitable for the blue- 

 fin. He did, however, note that the sen- 



60' 



BERMUDA 



t2:> 



Figure 70. Collection locations for the large, mature bluefin tuna collected during 

 exploratory fishing cruises of United States and Russian research vessels (see 

 Table 25). 



84 



