Assessment of a shell bank and associated substrates as nursery habitat of postsettlement red snapper

Abstract

Trawl surveys were conducted in 2000 and 2001 to examine patterns of distribution and abundance of postsettlement red snapper ( Lutjanus campechanus) on a shell bank, Freeport Rocks Bathymetric High (FRBH), in the northwestern Gulf of Mexico. In addition, otolith-based methods were used to determine age, hatch-date, growth and mortality of new recruits associated with FRBH. Date and region were significant factors affecting density of red snapper in 2000. Peak densities of red snapper were observed in July and August, and mean density among habitat types (shell bank, inshore mud, offshore mud) was similar (range: 50–52 ind hectare −1) in 2000. Alternatively, a habitat effect was detected during a limited survey conducted in 2001, with density higher on the shell bank than inshore or offshore mud habitat. Postsettlement red snapper were first detected at approximately 16 mm standard length, and individuals less than 20 mm were present in all habitats. Estimated ages of red snapper ranged from 26 to 121 d, with new settlers (≤20 mm) typically less than 28 d. Predicted hatch dates ranged from early April to mid August with a single peak occurring from late May to early June. Growth rate for the April–May cohort (0.817 mm d −1) was similar to the June–July cohort (0.830 mm d −1). Habitat-specific differences in growth were observed, and rates were highest for individuals from the inshore habitat (0.881 mm d −1). Mortality rates ( Z) during the early post-settlement period were approximated using catch curves, and early life mortality of red snapper was 12.1% d −1 ( Z=0.129). While the difference in mortality between cohorts was negligible, a habitat-specific difference in mortality was observed. Mortality rate of red snapper inhabiting the inshore mud habitat ( Z=0.045, 4.4% d −1) was lower than rates observed for individuals on the shell bank ( Z=0.120, 11.9% d −1) or offshore ( Z=0.099, 9.3% d −1) habitat. Individuals residing in the inshore habitat had significantly higher growth rates and significantly lower mortality rates, suggesting that recruitment potential was higher for these individuals.