Age, Growth, and Sex Ratio of American Eels in Brackish-Water Portions of a South Carolina River

Abstract

American eels Anguilla rostrata (yellow phase; N = 462) collected from brackish-water portions (range of salinity: 0-23ø/w) of the Cooper River, South Carolina, ranged in length from 26.0 to 68.7 cm and in weight from 32 to 767 g. The length-weight relationship is log•0W = -5.85 + 3.07 1og•0L; W = wet weight (g) and L = total length (mm). Age estimates (inland years) from 442 pairs of otoliths indicated a mean age of 4.3 years and range of 1-12 years. Growth of American eels from brackish-water areas of the Cooper River was faster than growth of the species from northeastern United States and Canada. American eels from this southern habitat appear to migrate back to sea after ages 6-7, sooner than eels from northern locations. Histological examination of gonads from 458 specimens revealed that females outnumbered males 23 to 1. These results were consistent with most previous investigations and suggest that females dominate American eel populations in brackish-water and freshwater habitats in North America. The American eel Anguilla rostrata is a catadromous species that is harvested in brackish and freshwater areas in South Carolina and other eastern-seaboard states (Lane 1978). Although age-and-growth information is available for American eels inhabiting fresh waters of South Carolina (Harrell and Loyacano 1982) and other locations farther north (Ogden 1970; Gray and Andrews 1971; Hurley 1972; Facey and LaBar 1981), similar data for brackish-water American eels are limited to Newfoundland

A synthesis of the ecological processes influencing variation in life history and movement patterns of American eel: towards a global assessment

“…1 for a summary of general patterns). Empirical data show that the proportion of males increases at lower latitudes (Helfman et al 1984), and a latitudinal cline in somatic growth rates (faster growth in southern latitudes) irrespective of sex (Hansen and Eversole 1984;Oliveira 1999), but also show that size at maturity increases with latitude in females but not in males (Helfman et al 1987;Oliveira 1999). These general patterns, also observed in European eel reproducing in the Sargasso Sea (Moriarty and Dekker 1997), support the hypothesis that silver eel metamorphosis and emigration are more dependent on sex-specific length than age (Helfman et al 1987;De Leo and Gatto 1996).…”

“…Exceptions to the above patterns are represented by higher proportions of males reported in northern subpopulations (e.g., Vladykov 1966; Krueger and Oliveira 1997;Jessop et al 2006) including freshwater systems (Facey and Helfman 1985;Krueger and Oliveira 1999), high variation in sex ratios over small geographic areas (e.g., Krueger and Oliveira 1999;Oliveira et al 2001), predominance of males in freshwater habitats (e.g., Winn et al 1975;Naismith and Knights 1990), and female predominance in brackish water (e.g., Hansen and Eversole 1984;Naismith and Knights 1990;see Fig. 3 for a summary of these relationships).…”

“…Arguably, existing evidence supports the hypothesis that eels evolved a migratory behaviour in response to the relative habitat productivity prevailing in their geographical range (Edeline 2007)-temperate eels living at higher latitudes with higher oceanic productivity exhibit a greater probability of remaining in lower reaches in brackish water than eels living at lower latitudes ) with higher freshwater productivity. But other factors must be involved since general patterns defining the variation in life history traits and movement patterns as functions of latitude (Helfman et al 1987;Tesch 2003) or ocean-freshwater productivity gradients (Jessop et al 2004) show some inconsistencies across the entire geographical range of the species (e.g., Hansen and Eversole 1984;Krueger and Oliveira 1999;Oliveira et al 2001). Since panmixis would preclude the segregation of genotypes, local differences in life history patterns would be most likely phenotypic in origin (Jessop et al 2004) and triggered by biotic and abiotic factors associated with local habitats.…”

Anguilles estuariennes et fluviales : apports de l'otolithométrie

“…Un modèle similaire de croissance en phases a été proposé pour l'anguille américaine (HANSEN et EVERSOLE, 1984 ;HELFMAN et al, 1984) et pourrait être liée aux modifications physiologiques résultant des processus de maturation (FERNANDEZ-DELGADO etal., 1989 ;VOLLESTAD, 1992). Ces hypothèses demandent à être testées sur un échantillon étendu, représentatif des structures de la population.…”

“…La variété des sites colonisés entraîne une forte variabilité des structures démographiques (APRAHAMIAN, 1988), du sexe-ratio et de la croissance (VOLLESTAD, 1989). En outre, l'anguille est un poisson migrateur amphihalin : sa capacité de déplacement au sein d'un bassin versant (pour le coloniser ou retourner en mer) est un paramètre déterminant de la durée du cycle biologique (HANSEN et EVERSOLE, 1984 ;HARO et KRUEGER, 1991). L'isolement, artificiel ou non, de fractions estuariennes et fluviales au sein d'une population d'anguilles est donc à même de conditionner l'efficacité de la colonisation des zones amont du bassin et d'engendrer des caractéristiques biologiques différentes.…”