Spatiotemporal variability in the reproductive biology of yellowfin tuna (Thunnus albacares) in the eastern Pacific Ocean

Abstract

The reproductive biology of yellowfin tuna (Thunnus albacares) was investigated through at-sea sampling of ovaries from 1728 females, across 12 length classes from 40 to 160 cm fork length, from four spatial strata (northern (N), central coastal (CC), central offshore (CO), and southern (S)) of the eastern Pacific Ocean, caught by purse-seine vessels from January 2009 to November 2016. Histological evaluations of the ovarian tissues provided the foundation for the estimates of length-specific reproductive characteristics. Spawning was widespread from about 25°N to 16°S between 78°W and 148°W, occurring throughout the year from 5°N to 15°N. North of 20°N, spawning took place during April–December, and south of 5°S, spawning took place during January–June. Spawning was observed for fish captured in dolphin, floating-object, and unassociated sets. Although spawning occurred at sea-surface temperatures from 25 °C to 32 °C, 73.0% of the spawning took place between 27.0 °C and 29.0 °C. Results of fitting a general additive model (GAM) to the maturity-at-length data indicated significant spatial variation in maturity, with estimated lengths at 50% mature of 90.6 cm in the S area, 83.2 cm in the N area, 78.6 cm in the CC area, 70.9 cm in the CO area, and 81.6 cm for the combined data. Results of fitting GAMs to maturity-at-length data sets from the current study and from a previous study executed during 1987–1989, indicated significant temporal variation for the CO, CC, and S areas, and the combined data sets. The estimated lengths at 50% mature for the combined data, for the same spatial strata, from the earlier samples and from those in the current study were 90.4 cm and 78.6 cm, respectively. The observed batch fecundities from 146 fish, based on counts of late-stage migratory-nucleus or hydrated stages of oocytes, ranged from about 54 thousand oocytes for an 87 cm female to about 7.2 million oocytes for a 153 cm female. The average size of the 146 females was 118 cm, and the average batch fecundity was about 2.3 million oocytes. The estimated mean relative fecundity for 136 females, which were weighed, was 60.95 oocytes per gram of body weight. The fraction of all mature females with postovulatory follicles was 0.56, equivalent to a mean spawning interval of 1.79 d, whereas for reproductively active females the fraction spawning per day was 0.81, equivalent to a mean spawning interval of 1.23 d.