Stable isotope evidence for income resource allocation to egg production in the Japanese anchovy Engraulis japonicus

For multiple-batch-spawning fish species with indeterminate fecundity, income breeding might be more rewarding than capital breeding, because it can fine-tune their reproductive output in terms of maternal condition and offspring survival rate. Although income breeding has been suggested for many small and short-lived pelagic fishes, direct and quantitative evidence for resource allocation to egg production is lacking. This study determined the resource allocation strategy in the Japanese anchovy Engraulis japonicus under ad libitum food availability by using carbon and nitrogen stable isotope ratios (SIRs) as quantitative indicators. Initially, feed with low SIRs was provided and then switched to feed with high SIRs for monitoring the changes in the SIR of eggs during the spawning season. The effect of the latter feed on eggs was evident with the increasing SIRs for both carbon and nitrogen after the diet was switched, suggesting that income resources were primarily used over a season. However, the estimated transfer rates of the resources from food intake to eggs produced varied along with the degree of prevailing temperature. Our findings also suggested that, although well-fed female Japanese anchovy are largely income breeders, the resource allocation strategy used for egg production might be more complicated than previously thought.