Reproductive biology of yellowfin tuna (Thunnus albacares) in the northcentral U.S. Gulf of Mexico

The reproductive biology of yellowfin tuna (Thunnus albacares) was investigated from fish collected throughout 2000–2017 in the northcentral U.S. Gulf of Mexico (GOM; n = 1135 females, 776 males), predominately from recreational anglers fishing off the Mississippi River (93 %). Histological evidence, along with mean gonadosomatic index (GSI) values, showed that peak spawning occurred from May to August, with the highest mean GSI value for both sexes observed in May (females, 1.52 %; males, 0.57 %). During the reproductive season, the amount of active spermatogenesis in spawning capable males varied monthly as observed by the progression of germinal epithelium (GE) subphases (i.e., early-GE, mid-GE, late-GE), with the mid-GE subphase observed most frequently throughout the peak spawning season. The upper and lower 95 % confidence intervals among length at 50 % maturity (L50) estimates had the largest degree of separation between the physiological (cortical alveoli, L50 =1002 ± 7.18 mm CFL) and functional maturity thresholds (primary or secondary vitellogenesis; L50 =1071 ± 4.89 mm CFL), indicating that the physiological maturity threshold should be used with caution as it may underestimate L50. Using the postovulatory follicle (POF) method, the minimum spawning interval was estimated during peak spawning months for all functionally mature females at 2.10 days, with a minimum of 17.30 % daily spawning. Batch fecundity estimates for 24 females was linearly related to size and ranged from 37,956–6.2 million eggs per batch. These data allow for U.S. GOM reproductive parameter estimates to be incorporated, for the first time, into future Atlantic yellowfin tuna stock assessments done by the International Commission for the Conservation of Atlantic Tunas.