The mechanism of energy allocation over reproduction and somatic growth in female North Sea plaice, Pleuronectes platessa L.

Abstract

The mechanism of surplus production allocation over reproduction and somatic growth was studied in female plaice using market sampling data, tagging data and a tank experiment. Somatic growth peaked between March and October and main gonad growth occurred between October and January. However, an estimated 50% of the gonad growth was subsidized from body reserves built up during the growing period. Therefore processes of somatic growth and the building up of energy reserves for gonad growth take place simultaneously. Market sampling data and a tank experiment showed that pre-spawning energy reserves were positively correlated with somatic growth but also indicated that a twofold increase in somatic growth only results in an increase in energy reserves of less than 10%. Analysis of the relation between back-calculated somatic growth and reproductive investment in individual females did not reveal a significant effect of somatic growth on size-specific fecundity, gonad weight and pre-spawning body weight. It is concluded that size-specific reproductive investment is not, or only slightly, enhanced by an increase in surplus production. In the tank experiment females, which did not grow by more than 20% of the initial body weight, did not develop their gonads, but this phenomenon of skipping a spawning season does not seem to occur in the North Sea population. The results of this study are summarized in a hypothetical model of the mechanism of surplus production allocation that is based on physiology and can be tested with tank experiments. The model postulates that surplus production is used to maintain body energy reserves at a threshold level that increases from a post-spawning low to a pre-spawning high. Surplus production exceeding the demands for maintaining body energy reserves at the threshold level is channeled into somatic growth. Implications of the mechanism for the changes in biological parameters in relation to reproductive strategies are discussed.