Assessing Nursery Quality for Southern Flounder, Paralichthys lethostigma, through Fish Energy Content and Habitat Abiotic Conditions

Abstract

Estuarine nursery habitats are essential to the sustainability of many of the world's fisheries. Anthropogenic perturbations and watershed activities threaten the quality of these vital ecosystems, and the significance of nursery habitat loss has become so distressing that most fishery management strategies now place high priority on the protection and restoration of "Essential Fish Habitat" (EFH, Magnuson-Stevens Act, NOAA 1996). The inability to accurately evaluate habitat quality and fully understand the factors limiting habitat-specific productivity preclude the success of any management efforts to improve fishery recruitment through the protection and restoration of estuarine nursery habitats. The Albemarle-Pamlico Estuarine System in North Carolina (NC) provides important nursery habitat for many ecologically and economically important species, including southern flounder, Paralichthys lethostigma. Abiotic factors, in particular temperature, dissolved oxygen (DO), and salinity, almost certainly affect growth and survival at the levels experienced in NC nurseries, but the relative importance of these factors in determining nursery habitat productivity for southern flounder has yet to be determined. Two separate indicators of nursery habitat quality were compared: fish energy content and habitat abiotic conditions. The hypothesis that fish energy content represents the cumulative effect of the abiotic environment on the fish was tested to see if these independent but potentially related indicators could be used in assessments of nursery habitat quality. When measured on appropriate temporal and spatial scales, total fish energy content provided a measurable response of young-of-the-year (YOY) southern flounder to short-term environmental variability, and the results of this study suggest a strong potential for the utility of fish energy content in evaluations of nursery habitat quality for the species. Over a continuous 15 week sampling period from May to August 2006, YOY southern flounder energy content varied weekly in a single nursery habitat by a maximum of approximately 8%, and 51% of this weekly variability in energy content was explained by the weekly variability in temperature and DO measured in this habitat. Across multiple nursery habitats sampled biweekly from May to August 2005, the variability in YOY southern flounder energy content ranged from approximately 12 to 37%. The spatial variability in temperature, DO, and salinity measured across these different nursery habitats was often significant but minimal and did not unequivocally explain the measured variability in fish energy content. The results of this study suggest that fish energy content can provide an unbiased measure of the fish's perception of habitat quality and can be an alternative to other, more labor intensive and potentially biased studies that estimate juvenile growth through field enclosures or micro-otolith analyses. The conclusions of this study and the utility of fish energy content as an indicator of nursery habitat quality are likely applicable to other fish species whose juveniles utilize similar estuarine nursery habitats.