In the tidal freshwaters of Virginia, U.S.A., the blue catfish (Ictalurus furcatus), an introduced piscivore, derives a significant proportion of its nutrition from spawning anadromous fish (genus Alosa, including blueback herring (A. aestivalis), American shad (A. sapidissima), and alewife (A. pseudoharengus)). Because the Alosa are not continually available to I. furcatus, there is an isotopic turnover, defined as change in isotope composition due to growth and metabolic tissue replacement, in I. furcatus tissues associated with the diet switch from freshwater to anadromous fishes. However, isotopic turnover rates for ictalurid fish are unknown. This study determined the maximum isotopic turnover rate of channel catfish (Ictalurus punctatus) tissues and compared this maximum rate with that of I. furcatus captured in the field over the 3-month Alosa spawning run. Maximum turnover rates for 13 C were 0.014 and 0.017‰ per day in muscle and blood. For 34 S, rates were 0.017 and 0.020‰ per day in muscle and blood, respectively. Isotopic turnover of muscle carbon reflected growth rate, but sulfur did not match growth as well. Ictalurus furcatus captured in the field showed no enrichment during the Alosa spawning run owing to slow turnover and variable diet. In aquatic ecosystems that have migrating prey, exploitation by predators may be underestimated using isotopes because of slow tissue turnover.Résumé : Dans les eaux douces intertidales de la Virginie, aux États-Unis, Ictalurus furcatus, un poisson piscivore introduit, retire une proportion significative de son alimentation des poissons anadromes du genre Alosa, dont le l'Alose d'été (A. aestivalis), l'Alose savoureuse (A. sapidissima) et le Gaspareau (A. pseudoharengus), qui viennent y frayer. Parce que les Alosa ne sont pas toujours disponibles à I. furcatus, il se produit un virement isotopique dans les tissus de la barbue, c'est à dire un changement dans la composition isotopique reliée à la croissance et au renouvellement métabolique des tissus consécutif à un remplacement dans le régime des poissons d'eau douce par des poissons anadromes. Cependant, les taux de renouvellements isotopiques chez les poissons ictaluridés sont inconnus. Noua avons déterminé le taux maximal de renouvellement isotopique dans les tissus de la Barbue de rivière (Ictalurus punctatus) et l'avons comparé avec le taux maximum observé chez les I. furcatus capturés en nature durant les 3 mois de la montaison de reproduction des Alosa. Les taux maximaux de renouvellement du 13 C sont respectivement de 0,014 et de 0,017‰ par jour dans le muscle et dans le sang, alors que les taux maximaux de 34 S sont respectivement de 0,017 et de 0,020‰ par jour dans les mêmes tissus. Le taux de renouvellement isotopique du carbone musculaire reflète le taux de croissance, mais celui du soufre ne suit pas la croissance d'aussi près. Les I. furcatus capturés en nature ne font preuve d'aucun enrichissement durant la montaison des Alosa à cause du faible taux de renouvellement et du régime alimentaire variabl...

During recent decades, the Tuckahoe Creek (Virginia) watershed has been altered by human activities, including road and bridge construction, commercial and residential development, and riparian losses. We used historical (1958) and recent (1990) data to evaluate the hypothesis that long‐term, low‐intensity urbanization affects a warmwater stream fish assemblage in ways usually attributed to acute, high‐intensity perturbations. In 1990, fish species diversity of Tuckahoe Creek was significantly lower (P < 0.05) than it had been in 1958, and abundance was substantially lower for all species and trophic guilds. Four cyprinids, one ictalurid, and one percid present in 1958 were not collected in 1990. Jaccardˈs community similarity coefficient (species composition by site between 1958 and 1990) was negatively correlated with near‐stream development of the watershed at six study sites (r = –0.84; P < 0.05). Some characteristics of the feeding ecology of selected fish species, including consumption of terrestrial prey and diet overlap, exhibited slight differences between 1958 and 1990. Observed changes in the fish assemblage were consistent with our hypothesis. Our findings also suggest that the lack of invasions by exotic fishes may not always indicate undisturbed stream conditions.

In the tidal freshwaters of Virginia, U.S.A., the blue catfish (Ictalurus furcatus), an introduced piscivore, derives a significant proportion of its nutrition from spawning anadromous fish (genus Alosa, including blueback herring (A. aestivalis), American shad (A. sapidissima), and alewife (A. pseudoharengus)). Because the Alosa are not continually available to I. furcatus, there is an isotopic turnover, defined as change in isotope composition due to growth and metabolic tissue replacement, in I. furcatus tissues associated with the diet switch from freshwater to anadromous fishes. However, isotopic turnover rates for ictalurid fish are unknown. This study determined the maximum isotopic turnover rate of channel catfish (Ictalurus punctatus) tissues and compared this maximum rate with that of I. furcatus captured in the field over the 3-month Alosa spawning run. Maximum turnover rates for δ13C were 0.014 and 0.017‰ per day in muscle and blood. For δ34S, rates were 0.017 and 0.020‰ per day in muscle and blood, respectively. Isotopic turnover of muscle carbon reflected growth rate, but sulfur did not match growth as well. Ictalurus furcatus captured in the field showed no enrichment during the Alosa spawning run owing to slow turnover and variable diet. In aquatic ecosystems that have migrating prey, exploitation by predators may be underestimated using isotopes because of slow tissue turnover.

Recent investigations into the nutrient cycling of coastal ecosystems have suggested that migratory or anadromous fish could be important vectors of marine nutrients. Anadromous fish have assimilated marine nutrients that would contribute to the nutrient budgets of freshwater systems by excretion, gamete release, or the decay of post-reproductive carcasses. However, the extent to which freshwater predators utilize marine material is not well understood. In systems where anadromous fish temporarily constitute a major portion of the fish community, they may contribute substantially to the diet of piscivorous fish and other predators. Here we show the contribution of anadromous blueback herring, shad, and alewife (Alosa) to diets of large, non-indigenous piscivorous catfish (Ictalurus furcatus) using δS and δC. The spawning anadromous Alosa, captured in tidal freshwater, had enriched δS and δC values compared to resident, native freshwater species. As a result of consuming the anadromous Alosa, the I. furcatus isotope signature shifted towards the marine signal. The isotope analysis revealed that anadromous fish contribute substantially to the diet of most captured I. furcatus. The percentage of anadromous Alosa carbon and sulfur that was incorporated into I. furcatus (≥38 cm total length) ranged from 0 to 84% and 10 to 69%, and had means (±SD) of 42 (±24) and 43 (±16)%, respectively. Although the δN signal of marine-derived biomass is enriched by approximately 3‰ relative to terrestrial or freshwater biomass, it was not as useful as δS and δC for nutrient source owing to trophic fractionation. This study demonstrates that anadromous fish may be a significant source of nutrients to tidal freshwater apex predators.

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