The existence of differences in the isotopic composition of food webs supporting larval fishes during the pre- and post-settlement stages allows for the use of stable isotope ratios as tracers of recent settlement. A method based on measurements of Del13C and Del15N of larval tissues was developed to examine the settlement of red drum, Sciaenops ocellatus, following their migrationfrom coastal areas to estuarine nursery habitat. Larvae subjected to dietary shifts in the laboratory exhibited fast changes in isotopic composition as a result of rapid biomass gain. Based on published growth rates, it was estimated that the isotopic composition of a newly settled larva should differ 1-2 d after settlement, and that equilibration with estuarine food sources should be complete after 8-10 d.

An empirical model was constructed to estimate size at settlement (Lsett; mm standard length) and time since settlement (Tsett; d) for natural populations. Model results can be used to distinguish between new and recent settlers and to estimate individual settlement dates, thus greatly increasing the temporal resolution of settlement patterns. The model relies on characterization of the isotopic composition of pre- and post-settlement larvae, the contribution of growth and metabolic turnover to the rate of isotopic change, and growth rates. The model was validated by simulating a settlement event in which laboratory reared larvae were transferred to cages deployed in the nursery habitat. Accounting for growth and metabolic turnover led to accurate estimates of Lsett and Tsett. Sensitivity analysis indicated that model results are sensitive to metabolic turnover, the isotopic composition of pre-settlement red drum and growth rates, suggesting that these parameters must be thoroughly and accurately characterized.

The model was utilized to examine the size distribution of new settlers and to estimate the daily settlement during the fall of 1999. The peak settlement size of red drum was 6-8 mm SL, although they settled at a range of sizes (4-11 mm SL). Newly settled red drum were detected on a daily basis during the peak of the settlement season (October), suggesting that the supply of larvae to the estuary determines the abundance of newly settled red drum and may therefore have substantial implications for the recruitment of this species to estuarine areas.