Temperature- and income resource availability-mediated variation in reproductive investment in a multiple-batch-spawning Japanese anchovy

Multiple batch spawning over a season should have an advantage for maternal fitness in unpredictable environments, but the manner in which females allocate resources to reproduction is not well understood. We explored the effects of temperature and food availability on reproductive traits in Japanese anchovy Engraulis japonicus using several physiological analyses. Under ad libitum food availability, temperature had a significant effect on batch intervals by changing the growth rate and size of oocytes during vitellogenesis. The latter is likely to result in variations in the size and total nutritional content of eggs. Relative batch fecundity, however, was not significantly influenced by temperature regimes. Reproductive effort per spawning was significantly higher at lower temperature, caused by the production of eggs with higher nutritional content, while total reproductive effort in a given period of time increased in water at a higher temperature due to shortening of batch intervals. Short-term food manipulation also had a significant effect on reproductive output. Insufficient income resources could lead to prolonged batch intervals, but the relative batch fecundity remained constant independent of the temperature regime. Our findings suggest that female Japanese anchovy may maintain reproductive effort per spawning relative to the prevailing temperature at the expense of more spawning events in a nutritionally harsh environment. Thus, although the total reproductive effort of well-fed specimens increased in water with increasing temperature due to shortened batch intervals, this variation may be caused by a compensatory response to the level of income resources available.