Recovery of Endangered Species Act—listed salmonids in the Columbia River basin has relied upon the efficacy of the U.S. Army Corps of Engineer's juvenile salmon transportation program to move fish past Snake and Columbia River hydropower dams. The effectiveness of this program has been assessed by the indirect method of comparing smolt‐to‐adult returns. We present some of the first data and mortality estimates of barged and run‐of‐river (ROR) radio‐tagged juvenile spring–summer Chinook salmon Oncorhynchus tshawytscha after release in the lower Columbia River, representing years of study. Our data suggest that smolt mortality (1) is very low for ROR and barged fish between Bonneville Dam and the estuary proper, a migratory distance of 180 river kilometers (rkm); (2) occurs in the lower estuary (rkm 0–46); (3) varies more across dates within a year than between years or between passage types (barged or ROR); (4) increases with time within a season and increasing numbers of avian predators, including Caspian terns Sterna caspia and double‐crested cormorants Phalacrocorax auritus; and (5) is estimated to be 11–17% of all smolts annually. Preliminary evidence suggests that at least some smolt mortality is influenced by differential predation by avian predators on Chinook salmon infected with Renibacterium salmoninarum and possessing low smoltification levels (relatively low gill Na+,K+‐ATPase activity). Fish type (barged or ROR) did not appear to influence mortality because of avian predation. This project was also the first to identify avian predators as a major source of mortality for out‐migrant Columbia River basin salmonids.
. 1994. Influence of larval gizzard shad (Borsscama cepedianum) density on piscivsry and growth of young-of-year saugeye (Stizostedion vitreum x S. caraadense). Can. 1.Fish. Aquat. Sci. 51 : 1993Sci. 51 : -2082 Growth and survival of young-of-year saugeye (Stizostedion vitreum 9 x S. canadense 6) (stocked into Ohio reservoirs to create sport fisheries) are probably influenced by prey availability, variations in which may account for historically documented variability in stocking success. Because saugeye switch from a diet of zooplankton to fish once stocked, we sought to determine experimentally if saugeye size and available ichthyoplankton, i.e., larval gizzard shad (Borosoma cepediaraum), affected this switch and whether piscivory improved saugeye growth. In an enclosure experiment, saugeye (33.9 mm TL) immediately switched to piscivory when exposed to ichthyoplankton densities of 20 and l O~-m -~, growing faster when more gizzard shad were available. In another enclosure experiment, saugeye 30-49 mm TL consumed 14-rnm gizzard shad. In ponds (N = 4 ponds-treatment-') containing zooplankton and chironornids, we compared saugeye growth with and without larval gizzard shad and found, as in the first enclosure experiment, that piscivory improved saugeye growth. Neither saugeye size nor ichthyoplankton density influenced how quickly saugeye switched to piscivory. We conclude that managers should stock saugeye 230 mm 1-2 wk before peak ichthyoplankton densities to improve saugeye growth and survival by enhancing opportunities for exploitation of young-of-yeas gizzard shad.La croissance et la susvie de jeunes de l'annee issus du croisement entre le dor6 jaune et le dore noir (Stizostedisn vitreum B x S. canadense 8 ) (ensernences dans des reservoirs en Ohio pour la peche sportive) sont probabfement influencees par I'abondance des proies, dsnt les variations peuvent expliquer la variabilite historique du succ&s de I'ensemencement. Etant donne que ce dore hybride passe d'un regime form6 de zooplancton A un regime piscivore d&s I'ensemencernent, nous avons tent6 de determiner experimentalernent si la taifle du poisson et la presence d'ichtyoplancton, c.-a-d. la Barve de I'alose a gesier (Dorosoma cepedianum), influen~aient cette modification du regime et si le regime piscivose accelerait la croissance du dore. Dans une experience en enclos, les dor6s (LT = 33,9 mm) snt adopte immediatement un regime piscivore Borsqu'ils ont ete exposes h des densites d'ichtyoplancton de 28 et de 100-m-" et leur croissance s'est revklee plus rapide en presence d'une plus grande quantit6 d'aloses h gesier.Dans une autre experience en enclos, des dores de 30 A 49 mrn de LT ont consomm6 des alsses A gesier de 14 mm. Dans des etangs (N = 4 &tangs-traiternent-') renfermant du zooplancton et des chironomides, nous avons compare la croissance du dor6 avec et sans la presence d'aloses 2 gesier. lVous avons observe, comme dans la premi&re experience en enclos, que le regime piscivore activait %a croissance des dorks. N i la taille des dores, ni...
Abstract. -To explain variable stocking success of juvenile saugeye Stizostedion vitreum females x S. canadense males, we quantified survival of saugeyes in response to transport conditions, predation mortality, and prey resources at stocking in four Ohio reservoirs (162-1,145 ha) in 1991. May through October survival was 0.8-1 1.5%. We assessed stresses associated with transportation and handling by holding fish in small enclosures 5-10 d poststocking. Enclosure mortality varied greatly (18.9-63.5%). and because enclosures inadequately mimicked the reservoir environment, transportation and handling mortality were likely overestimated. Mortality resulting from resident piscivores was calculated via population estimates and poststocking sampling. Predation on age-0 saugeyes was minimal during poststocking sampling, ranging from 0 to 28.3% of the stocked population. Saugeye survival was statistically unrelated to zooplankton or ichthyoplankton (i.e., larval gizzard shad Dorosoma cepedianum) densities, although both ichthyoplankton density at stocking and saugeye growth showed positive trends with saugeye survival. Ichthyoplankton availability likely influences growth and survival of saugeyes; therefore, saugeye slocking should coincide with peak ichthyoplankton densities.
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