ENERGETIC CONSTRAINTS AND SIZE-BASED TACTICS - THE ADAPTIVE SIGNIFICANCE OF BREEDING-SCHEDULE VARIATION IN A MARINE FISH (EMBIOTOCIDAE, MICROMETRUS-MINIMUS)

Female breeding schedules in the viviparous dwarf surfperch (Micrometrus minimus) vary systematically with size: small females conceive and bear young later in the season than large females. We test two general hypotheses for this pattern: (1) energy reserves impose a proximate energetic constraint on the timing of reproduction, and thus small females breed late because they carry less stored energy: (2) small females postpone reproduction as an adaptive size-specific tactic to maximize reproductive success. The hypotheses are not mutually exclusive, and both appear to apply to the dwarf surfperch. Among yearling fish, those that conceive their broods early in the season do so at higher reserve levels than females that breed late in the season. Late breeding places less demand on stored energy, and postponement enables individuals with low initial reserves to breed without drawing energy stores down to lethal low levels. However, lipid reserves do not explain all of the size-based variation in breeding schedule. In fish of both age classes, small size causes a delay in breeding, independent of lipid reserves. This is consistent with our second hypothesis, that small females postpone as a tactic. A potential tactical payoff is offspring number because females grow throughout gestation, and fecundity increases with size, larger broods can be produced by delaying conception. We show that small females realize greater proportional increases in fecundity for equal delays in breeding.