Date of Award
Program or Major
Doctor of Philosophy
The Atlantic bluefin tuna, Thunnus thynnus, is a highly migratory species capable of traversing great distances throughout the North Atlantic Ocean, but spawning is known to occur only in the Mediterranean Sea and the Gulf of Mexico/Straits of Florida. The regulatory body charged with managing Atlantic bluefin tuna, the International Commission for the Conservation of Atlantic Tunas (ICCAT), currently recognizes two spawning stocks, eastern and western, separated by a management line at 45° W. The eastern stock spawns from May through July in the Mediterranean Sea with an age at first maturity of 3--4 years. Due in part to a moratorium on fishing for bluefin tuna in the Gulf of Mexico, considerably less research has been conducted on the western spawning stock, and, subsequently, knowledge about basic biological characteristics of this stock is lacking. The age at maturity for western bluefin tuna has been reported as 5--16 years and is a topic of debate among fisheries managers. For stock assessment and management, ICCAT assumes an age at maturity for western bluefin tuna of 9 years. While only two spawning grounds are known, fish of reproductively mature size routinely do not return to either of these two locations during the presumed spawning season indicating additional spawning grounds may exist.
Atlantic bluefin tuna were sampled on and off the known spawning grounds, and maturity status was determined for male and female fish. All fish sampled on the spawning grounds (n=250) had mature gonads, and the spawning season in the north/central Gulf of Mexico was defined as April--June. Histological analyses showed a peak in oocyte maturation, and thus spawning activity, in May in the Gulf of Mexico sampling region.
Actively spawning fish from the Mediterranean Sea were compared with those from the Gulf of Mexico. Realized fecundity and spawning periodicity were found to be similar for both stocks, but the western spawning stock sampled in the north/central Gulf of Mexico spawns one month earlier than the eastern stock.
Fish sampled far from the known spawning grounds provided further information about the reproductive condition of western bluefin tuna. The youngest female and male to show signs of maturity had estimated ages of 6 years and 5 years, respectively. About one quarter of all females sampled contained vitellogenic or early atretic oocytes, and based on rates of atresia, it is unlikely these fish spawned in the Gulf of Mexico.
These results provide more extensive information about the reproductive biology of western Atlantic bluefin tuna and revise the age at maturity for the western spawning stock. This lower size and age at maturity, coupled with new results from endocrinology and electronic tagging data, suggests alternative spawning grounds exist, and more comprehensive spatial and temporal sampling is necessary to understand the reproductive biology of Atlantic bluefin tuna.
Knapp, Jessica M., "On the reproductive biology of the western Atlantic bluefin tuna, Thunnus thynnus" (2012). Doctoral Dissertations. 696.