RECENT OBSERVATIONS ON THE CHANGE OF REPRODUCTIVE TRAITS OF JAPANESE BUTTERFISH, PSENOPSIS ANOMALA, IN WATERS OFF NORTHEASTERN TAIWAN

摘要

The Japanese butterfish, Psenopsis anomala, is one of the most important fish species for trawl fisheries in Taiwan. This species has been intensively exploited in recent decades; however, how the species adapts to this impact by changing its life history traits is poorly known. In this study, we evaluated the change in the reproductive traits of the Japanese butterfish based on samples collected 25 years apart from the waters off northeastern Taiwan. There was a significant change in the mean length of the fish caught between 1984 and 2008. The spawning season was similar for both periods, but energy reserves in the muscle and liver both decreased in 2008, especially prior to the onset of spawning. The sizes at first sexual maturity for both sexes decreased slightly, whereas the total and relative fecundities for females increased substantially. The sex ratio was skewed toward females in the pooled annual samples, as well as in most months of 1984, but this pattern was not found in 2008. It appears that P. anomala may compensate for the loss of population biomass by producing numerous smaller eggs and may spawn more egg batches even under lower body mass conditions. These changes are a possible response to the impact of intensive fishing pressure, which has affected not only P anomala but also its prey in the ecosystem. The implications for the management of the demersal fishery resources of this region were further discussed in this study.