Reproductive biology of female Pacific bluefin tuna Thunnus orientalis from south-western North Pacific Ocean

Pacific bluefin is a highly valuable pelagic species that inhabits a broad range in the North Pacific Ocean. The reproductive biology, especially for the spawning aggregation in the south‐western North Pacific Ocean, is not well understood. Thus, a total of 119 paired ovary specimens were collected from the Taiwanese longline fleet during the 1999 fishing season (late April through June) to gain a better understanding of important reproductivity‐related stock parameters associated with this species. The following conclusions were made: (i) condition factor decreased from late May to early June; (ii) the sex ratio might be 1:1 for spawners; (iii) the gonadosomatic index stayed at a relatively high level and markedly increased from late May to early June; (iv) histological examination of oocytes indicated that all specimens were sexually mature; (v) spawning activity appeared to start in May and peak in late May to early June; (vi) batch fecundity increased with fork length; and (vii) preliminary estimates of spawning frequency between batches ranged 2–4.5 days based on analysis of postovulatory follicles.