MEPS 226:143-156 (2002)  -  doi:10.3354/meps226143

Characterization of settlement patterns of red drum Sciaenops ocellatus larvae to estuarine nursery habitat: a stable isotope approach

Sharon Z. Herzka*,**, Scott A. Holt, G. Joan Holt

Marine Science Institute, University of Texas at Austin, 750 Channelview Drive, Port Aransas, Texas 78373, USA
*Present address: Departmento de Ecología, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Ensenada, Baja California, México. E-mail: **Mailing address: PO Box 434844, San Diego, California 92143-4844, USA

ABSTRACT: A novel approach was used to study the settlement of red drum Sciaenops ocellatus larvae from the coastal Gulf of Mexico to a seagrass meadow in the Aransas Estuary, Texas, during an entire recruitment season (September through November 1999). Differences in the δ13C and δ15N of the food webs supporting red drum while in the planktonic and estuarine habitats were characterized and used to identify post-settlement individuals in the process of completing the isotopic shift between the planktonic and estuarine signatures (transitional larvae). An empirical model was used to estimate the rate of change in the isotopic composition of transitional red drum to back-calculate size at settlement (Lsett; mm standard length: SL) and time since settlement (Tsett; d). During the period of study, there were 4 to 5‰ differences in the δ13C and δ15N of planktonic larvae and ŒlargeŒ settlers that had equilibrated to estuarine food sources. Red drum captured simultaneously exhibited variability in Lsett and Tsett values, indicating that they had settled at a range of sizes and over several days. Examination of length-frequency distributions of Lsett estimates indicated that the smallest settlers were about 4 mm SL, peak size at settlement was 6 to 8 mm SL, and the largest settlers were 10 to 11 mm SL. Settlement dates derived from Tsett indicated that a brief settlement pulse occurred during the latter half of September and that new settlers appeared on a daily basis throughout October and early November. The relatively continuous settlement pattern suggests a consistent supply of potential settlers to the estuary. The approach used in this study provides a fine- scale temporal resolution for the examination of settlement patterns in marine fishes that exhibit a distinct habitat transition and consequent dietary shift during early life.

KEY WORDS: Settlement · Fish larvae · Stable isotopes · Sciaenops ocellatus · Turnover

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