For wild, sea-run brown trout (Salmo trutta) smolts, the physiological consequences of abrupt transfer to seawater and simultaneous challenge with copepodid larvae of the sea louse, Lepeophtheirus salmonis (Krøyer, 1837), were investigated in the laboratory. Analysis of osmoregulatory, metabolic, and stress markers allowed the derivation of a sublethal threshold burden of L. salmonis, above which the host suffers major physiological stress. Noticeable lice effects, consistent across all measured markers, were not observed until L. salmonis developed to the mobile preadult and adult stages. Preadult L. salmonis caused significant increases in plasma chloride, osmolality, glucose, lactate, and cortisol and a significant reduction in haematocrit. Piecewise linear statistical approaches allowed the determination of abrupt changes in these physiological markers, attributable to the intensity of L. salmonis infestation on individual fish, and identification of overall threshold lice burdens. Thirteen mobile lice·fish–1 (weight range 19–70 g) was a consistent breakpoint across several physiological measures. This information will provide a valuable, objectively derived tool to aid in the formulation of effective wild fisheries management policy concerning S. trutta conservation.
The efficiencies of fish passes specifically designed and constructed to facilitate the movement of a limited number of species and lifestages past structural barriers are likely to decline as site-specific conditions shift with a changing climate. There is a need to develop realistic fish passage criteria based on understanding swimming capability and behaviour of multiple species in relation to temperature and flow. The influence of temperature and discharge on behaviour and ability of groups of migrating adult river lamprey (Lampetra fluviatilis), a threatened species, to pass a small overshot or undershot weir at night was investigated using a large experimental flume. Lamprey approached the weirs less, and more often maintained station by using the oral disk to attach to structure, under high flows. Oral disk attachment was more commonly observed during tests employing the undershot weir. Upstream movement tended to be in close proximity to the channel walls and floor where, compared to the mid-channel, velocities were generally lower and velocity vectors more likely to be in a direction other than the bulk flow. Upstream movement was positively related to temperature, and was higher for the overshot weir. Weir passage rate was higher for undershot than overshot weirs, and negatively related to the maximum velocity at the weir. Passage rate was low when maximum velocities at the weir exceeded 1.5 m s À1 , although some fish passed at c.
m s
À1. Passage efficiency, the number of weir passes as a percentage of the number of approaches, was also higher for the undershot weir, but was not affected by discharge because lamprey approached less frequently at high flows. This study provides fish passage criteria under realistic conditions for an infrequently studied anguilliform species of conservation concern and provides a methodological perspective by which to improve fishway suitability for a wider range of species subject to changing climate.
The physiological consequences of premature migratory return to freshwater for wild sea-run brown trout (Salmo trutta) smolts infested with sea lice (Lepeophtheirus salmonis) were investigated in the laboratory. Osmoregulatory, metabolic, and stress markers were analysed in order to assess the potential consequences of transfer to freshwater, 19 days after the challenge with L. salmonis. Infestation intensity was significantly reduced following transfer to freshwater, and mortality rates were markedly higher in infested fish maintained in seawater vs. fish that were transferred to freshwater. Significant sea lice effects, consistent across a number of physiological markers, were apparent once L. salmonis developed to the mobile stages. Plasma chloride, lactate, and cortisol all were significantly higher than control values, and liver glycogen concentration was significantly reduced in infested fish in seawater. After return to freshwater, these physiological measures returned to control levels, but significant lice effects persisted for fish maintained in seawater. Premature return of infested sea-run brown trout to freshwater does, therefore, confer significant short-term physiological benefits across a range of osmoregulatory, metabolic, and stress markers.
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