Fecundity regulation strategy of the yellowfin tuna (Thunnus albacares) in the Western Indian Ocean

The oocyte development process and fecundity regulation (i.e. whether yellowfin tuna shows determi- nate or indeterminate fecundity) were analyzed to investigate the reproductive strategy of yellowfin tuna, Thunnus albacares. A total of 819 yellowfin ovaries were sampled at sea and at the Seychelles cannery dur- ing 2009 and 2010 from purse-seiners operating in the Western Indian Ocean. Histological analysis and automated computer-controlled image analysis software were used to study four main criteria applied for fecundity style determination: (a) oocyte size–frequency distribution, (b) number of cortical alveoli and total vitellogenic oocytes in different ovary maturation phases, (c) differences in mean diameter of tertiary vitellogenic oocytes, and (d) seasonal development of atresia.
The results revealed an asynchronous oocyte development and a continuous oocyte size–frequency distribution throughout all ovarian developmental phases over the spawning season. The percentage and number of cortical alveoli and total vitellogenic oocytes remained constant through the spawning season. The mean diameter of tertiary vitellogenic oocytes decreased as spawning progressed. Also, the incidence of atresia was higher at the end of the spawning season as a consequence of over-recruitment of oocytes during this period. These findings revealed that yellowfin tuna exhibit indeterminate fecundity.