Is Growth a Reliable Indicator of Essential Fish Habitat
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Date
2005-12-08
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Abstract
Estuarine habitats serve as important nurseries for many species of juvenile fishes and invertebrates. Due to concerns about declining fishery stocks and habitat degradation, it is critical that the value of different habitats be evaluated, so that areas that are most important can be prioritized for habitat conservation and restoration. This dissertation used Atlantic croaker (Micropogonias undulatus) as a model species to quantitatively examine patterns of early juvenile nursery habitat use, factors influencing growth and mortality, and to test the widely held assumption that 'growth is a valid indicator of habitat quality'. A unique aspect of this study was consideration of how growth and survival of juvenile Atlantic croaker were influenced by the joint effects of individual (larval and juvenile growth) and cohort (density-dependent) characteristics, as well as biotic and abiotic characteristics of the nursery. Late-larval and early juvenile Atlantic croaker were collected weekly, using a 1-m beam trawl in river, upper-estuary, and creek habitats of two estuaries (White Oak River estuary and Newport River estuary, North Carolina, USA), sampled during two seasons (fall and spring) of two recruitment years (2001-2002 and 2002-2003). Subsequent analysis of otolith microstructure was used to provide estimates of growth rates, population age structure, and mortality. Juvenile Atlantic croaker were most abundant in oligohaline river nursery areas (primary nursery area), with the upper-estuary serving as an important 'refuge' habitat following periods of high freshwater outflow that forced fish down-estuary. In addition to management implications (i.e., time and area closures of shrimp trawling), such flood events have important implications for growth-based assessment of habitat quality: A positive covariance between larval and juvenile growth was found only after periods in which croaker were displaced from river nursery areas, when food was apparently in short supply. Atlantic croaker cohorts from both estuaries also exhibited evidence for selective mortality favoring survival of individuals that were faster growing as larvae and early juveniles, as well as evidence for density-dependent growth (i.e., slower average cohort growth rates at higher conspecific density). The results of this study indicate that growth is not a robust indicator of habitat quality. Unless factors such as previous environmental history, density-dependence, and selective mortality are accounted for, growth-based assessment of habitat quality may reach incorrect conclusions regarding the value of different habitats.
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Keywords
otolith, Atlantic croaker, selective mortality, habitat quality, density dependence
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Degree
PhD
Discipline
Marine, Earth and Atmospheric Sciences
