Documenting the settlement history of individual fish larvae using stable isotope ratios: model development and validation

The delta C-13 and/or delta N-15 of newly settled fish larvae should change from a planktonic signature to one reflecting estuarine food sources following settlement from pelagic to demersal nursery habitat. An empirical model based on measurements of delta C-13 and delta N-15 was used to estimate size at settlement (L-sett) and time since settlement (T-sett) for red drum (Sciaenops ocellatus; Sciaenidae), an estuarine-dependent species whose larvae migrate from coastal areas to shallow seagrass habitat in estuaries. The model relies on (1) the isotopic composition of the larvae prior to a dietary switch and following equilibration to a new food source, (2) growth rate measurements and (3) estimates of the contribution of metabolic turnover to isotopic change. To test the model, a settlement event was simulated by transferring settlement-size larvae (6 mm standard length; SL) reared in the laboratory to cages deployed in nursery habitat in the Aransas Estuary, TX, USA. Most of the changes in delta C-13 and delta N-13 (ca. 10 parts per thousand for both) occurring over the 15-day study period were attributed to growth, although metabolic turnover significantly accelerated the rate of isotopic change. When growth and metabolic turnover were incorporated into the model, accurate estimates of L-sett (within 1 mm SL) and T-sett (within 1-2 days) were generated. The isotopic composition of wild-caught pre- and post-settlement larvae collected during a 2-week period was also characterized. Variability in the delta C-13 and delta N-15 of potential settlers as a function of size and sampling location was small (< 1 parts per thousand), although there were temporal differences. For part of the study period, there was a distinct difference between the delta C-13 of newly settled larvae (-19.3 parts per thousand) and "large" individuals that had equilibrated to estuarine foods (-16.5 parts per thousand). Nitrogen could not be used as a tracer of settlement due to the absence of differences in delta N-15 of pre- and post-settlement larvae. The carbon data was used to estimate size and time since settlement. The most abundant L-sett for wild-caught fish was 5-6 mm SL. Using published growth rates for post-settlement red drum in the study area, it was estimated that settlement events occurred over several consecutive days. The estimates of size and time since settlement were sensitive to the effect of metabolic turnover, the isotopic composition of pre-settlement red drum and growth rate. Utilization of stable isotope ratios allows for the finer temporal resolution of the settlement dynamics of marine fish species. (C) 2001 Elsevier Science B.V. All rights reserved.