Spatiotemporal variability in the reproductive dynamics of Skipjack Tuna (Katsuwonus pelamis) in the eastern Pacific Ocean

The reproductive dynamics of skipjack tuna (Katsuwonus pelamis) were investigated by sampling 8136 fish caught by purse-seine vessels in the eastern Pacific Ocean from January 1996 to January 1998. The overall sex ratio deviated significantly from the expected 1:1 ratio, with a greater number of males. Histological evaluations of ovaries from 3732 females provided the foundation for the estimates of length-specific reproductive characteristics. Spawning was widespread from about 19 °N to 12 °S and from 79 °W to 136 °W, and continuous throughout the year between about 15 °N and 10 °S. Spawning was observed for fish captured in unassociated sets, floating-object sets, and dolphin sets. Although spawning occurred from 24 °C to 30 °C, 76.9% took place between 26 °C and 29 °C. Results of fitting a general additive model to the maturity-at-length data indicated significant spatial variation in maturity, with estimated lengths at 50% maturity of 56 cm north of 15 °N, 53 cm between 15 °N and 5 °S, and 47 cm south of 5 °S. The significant spatial variation in estimates of relative fecundity (oocytes per gram of body weight) is attributed to the differences between relative fecundities north of 15 °N and between 15 °N and 5 °S. The estimated average batch fecundity, based on counts of late-stage migratory-nucleus or hydrated stages of oocytes from 129 fish, was 296,475 oocytes, and the estimated mean relative fecundity was 54.9 oocytes per gram of body weight. The estimated fraction of mature females in the population spawning per day was 0.21, equivalent to a spawning frequency of 4.76 d, whereas for actively spawning females, the fraction spawning per day was 0.85, equivalent to a mean spawning interval of 1.18 d.