REPRODUCTIVE BIOLOGY OF WESTERN ATLANTIC BLUEFIN TUNA 1 



Raymond E. Baglin, Jr. 2 



ABSTRACT 



Ovaries of bluefin tuna, Thunnus thynnus, were collected from the Gulf of Mexico, Florida Straits, 

 Middle Atlantic Bight of the western North Atlantic, and off the northeast coast of the United 

 States. There was relatively little development towards maturity in age 1 through age 7 fish from 

 the Middle Atlantic Bight as evidenced by low gonosomatic index values and histological 

 examination of ovaries. Well-developed ovaries were present in giant bluefin tuna from the Gulf of 

 Mexico and Florida Straits, with heaviest spawning occurring in May. For bluefin tuna measuring 

 205-269 cm fork length and 156-324 kg round weight, the average number of eggs measuring 0.33 

 mm in diameter and larger was estimated at 60.3 million, and the average number of eggs 

 measuring 0.47 mm in diameter and larger was estimated at 34.2 million. 



Atlantic bluefin tuna, Thunnus thynnus, are 

 seasonally distributed over most of the North 

 Atlantic. They are found from Newfoundland to 

 Brazil and from Norway to the Canary Islands 

 (Gibbs and Collette 1967). 



In the western Atlantic, a sport fishery for 

 bluefin tuna exists off the east coast of the United 

 States from Maine through North Carolina and 

 along the western Bahamas and the eastern coast 

 of Canada. Also, a substantial commercial 

 bluefin tuna fishery exists in the western 

 Atlantic. There is purse seining along the east 

 coast of the United States from Massachusetts to 

 North Carolina and a handline and harpoon 

 fishery off Massachusetts and Maine. A sub- 

 stantial Japanese longline fishery is present off 

 the east coast of the United States and in the Gulf 

 of Mexico. 



In the eastern Atlantic, a sport fishery for 

 bluefin tuna exists around the Canary Islands 

 and a substantial commercial fishery occurs off 

 Europe and North Africa. Purse seining is 

 conducted off the Atlantic coast of Norway and 

 Morocco, the Mediterranean coast of France, the 

 Adriatic coast of Italy and Yugoslavia, in the 

 Tyrrhenian Sea off Italy, and occasionally in the 

 North Sea off Denmark. An important hook-and- 

 line bait fishery occurs in the Bay of Biscay off 

 France and Spain, off Morocco, the Azores, the 



'Contribution Number 81-34 M, Southeast Fisheries Center 

 Miami Laboratory, National Marine Fisheries Service, 

 NOAA, Miami, FL 33149. 



2 National Marine Fisheries Service, NOAA, c/o Alaska 

 Department of Fish and Game, P.O. Box 686, Kodiak, AK 

 99615. 



Canary Islands, the Mediterranean coast of 

 Spain, and occasionally off Turkey. Trap 

 fisheries were present off southern Portugal, 

 southern Spain, and the Straits of Gibraltar, as 

 well as along the Mediterranean coast of 

 Morocco, Tunisia, and Sicily. There is a 

 significant Japanese longline bluefin tuna 

 fishery in the Mediterranean Sea, Bay of Biscay, 

 and off western Europe. 



There has been a substantial reduction in the 

 Atlantic-wide catch of bluefin tuna from 38,500 1 

 in 1964 to 12,500 t in 1973 with no large 

 reductions in effort (Miyake et al 3 .) A number of 

 studies have been made, and are continuing, to 

 understand the reason for this decline (Parks 

 1977; Shingu and Hisada 1980; Parrack 1980). 

 Of the various aspects of the dynamics of fish 

 populations, the measure of reproductive 

 potential is of primary importance since it is a 

 basic determinant of productivity. It is used to 

 separate subpopulations, to estimate mortality, 

 and, with ichthyoplankton data, to estimate 

 stock size. 



Two major bluefin tuna spawning areas are 

 located in the Atlantic approximately 4,000 mi 

 apart: In the Gulf of Mexico (Richards 1976; 

 Montolio and Juarez 1977; Rivas 1978) and the 

 Florida Straits (Rivas 1954; Baglin 1976) during 

 April, May, and June; and in the Mediterranean 

 Sea during May, June, and July (Frade and 

 Manacas 1933; Rodriguez-Roda 1964). Al- 



3 Miyake, M. P., A. De Boisset, and S. Manning (compilers). 

 1974. Int. Comm. Conserv. Atl. Tunas, Stat. Bull. 5. 

 Unnumbered pages. 



Manuscript accepted May 1981. 



FISHERY BULLETIN: VOL. 80, NO. 1, 1982. 



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