Critical timing for reproductive allocation in a capital breeder: evidence from sandeels

Life-history theory predicts a tradeoff in resource allocation between early maturation and growth in species which exhibit indeterminate growth. However, since life-history models assume that the fitness payoff of each developmental strategy at completion is known, they are unable to encapsulate the mechanics by which maturation is controlled under natural conditions. To determine the proximate conditions which influence variation in reproductive investment, we recorded somatic growth, sexual maturation and fecundity in individually marked first-time maturing lesser sandeels Ammodytes marinus. Cytological investigation revealed that the transition in gonad development from primary to secondary stages occurred between June and August in both sexes. Using logistical regression, variation in maturation could be attributed to both initial weight and observed differences in resource accrual during this period, suggesting that if growth prior to this period is inadequate in smaller fish then maturation remains inhibited. For those females that did mature, potential fecundity positively correlated with final body size. Our findings indicate that small changes in the timing of the availability of their zooplankton prey could explain the large regional variability in maturity at age exhibited by sandeels in the field.