Semelparous or iteroparous: resource allocation tactics in the ayu, an osmeroid fish

Ayu Plecoglossus altivelis is an annual osmeroid fish with a single breeding season. Its reproductive mode tends to vary such that large females spawn once, while small females spawn twice with an interval of some 2 weeks. The hypothesis was examined that certain female reproductive modes are governed by adequate resource allocation according to maternal body size, to maximize lifetime reproductive success. A series of rearing experiments revealed that immediately before spawning, larger females had relatively higher diversion from the soma and higher investment into the gonad, in contrast to smaller females which maintained higher protein synthetic activity for further reproduction. It was concluded that the extant plasticity in the reproductive mode that occurred in P. altivelis was an outcome of different tactics on the semelparity‐iteroparity continuum rather than maladaptive deviation from the optimum.