Spatial variation in demography of an estuarine teleost: implications for population and fishery assessments

Ca Gray

doi:10.3354/ab00623

Abstract

Length- and age-based information is used to assess the exploitation status of har- vested populations of fish worldwide. This study examined variability in length-at-age, length and age compositions and derived demographic parameters of Acanthopagrus australis (Sparidae) captured in beach seines in 3 estuaries across 5 yr. The method of ageing fish by counting opaque growth zones on sagittal otoliths was validated by field and aquaria studies. Length-at-age rela- tionships varied according to gender, age class and estuary. Females older than 3 yr had a signif- icantly greater mean length-at-age than males, and this was consistent across all estuaries. For both genders, mean length-at-age of fish older than 4 yr was greatest in the southernmost estuary, and least in the northernmost estuary, suggesting differential growth dynamics among estuaries. Observed longevity differed among estuaries, ranging from 14 to 22 yr. Variations in length com- position of retained catches were subtler than those for age composition. The average age of fish in catches varied among estuaries from 4.2�5.1 yr to 6.8�8.2 yr even though fish within 5 cm of the minimum legal length dominated catches in all estuaries. Estimates of total and fishing mortality varied 3-fold among estuaries, with fishing mortality exceeding natural mortality. Variability in demographic characteristics was generally greater among estuaries than among years within each estuary. These results show that the population demographics and ensuing assessments should not be extrapolated across estuaries or from one estuary to the entire A. australis population. Length is a poor predictor of age and future sampling of populations must be age-based and strat- ified to account for estuary-specific variation in demography. Failure to account for such varia - bility could confound assessments and management deliberations necessary for determining the most appropriate harvest and conservation strategies for such species.

Highlights

Length- and age-based information provide some of the most important data on life history characteristics used to assess the exploitation status of harvested populations of fish and to examine the appropriateness of different fishery and conservation management strategies (Hilborn & Walters 1992, Lai et al 1996, Quinn & Deriso 1999, Babcock et al 2013)
The mean Marginal increment (MI) value of wild-caught A. australis in CR was high in November and decreased to a low in February, after which it increased to attain a maximum in September and October (Fig. 1A)
For aquaria-held juvenile A. australis, the OTC stain in sectioned otoliths was visible under UV light for 145 individuals

Summary

Introduction

Length- and age-based information provide some of the most important data on life history characteristics used to assess the exploitation status of harvested populations of fish and to examine the appropriateness of different fishery and conservation management strategies (Hilborn & Walters 1992, Lai et al 1996, Quinn & Deriso 1999, Babcock et al 2013). Some tropical reef-associated fish display differing rates of growth, longevity and age characteristics across spatial scales ranging from among localized clusters of patch reefs within a reef system to among reef systems across broad geographic regions (Newman et al 1996, Meekan et al 2001, Williams et al 2003, 2007, Ruttenberg et al 2005). Some estuarine-associated teleosts display widely differing growth and demographic parameters among estuaries of differing geomorphic and environmental conditions (Sarre & Potter 2000, Bedee et al 2002, Walsh et al 2010). The extent of spatial variations in length-at-age and demography among populations can vary temporally, depending on rates of immigration and emigration (Sinclair et al 2002, Williams et al 2007)

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Conclusion