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Determination of the effects of increasing sea lice infestation on key osmoregulatory and stress parameters for sea trout smolts

The physiological effects of simultaneous abrupt seawater entry and sea lice infestation were investigated in wild sea trout smolts (Salmo trutta). In a series of laboratory experiments designed to mimic environmentally realistic conditions, we investigated the time course of physiological responses arising from varying intensities of infestation with the sea louse, Lepeophtheirus salmonis. Osmoregulatory, metabolic and stress physiology parameters were assessed alongside observations of general epithelial integrity in skin and gill tissue. Significant lice effects, consistent across all markers, were not apparent until L. salmonis had developed to the “mobile” preadult and adult stages. Mobile L. salmonis caused significant increases in plasma concentrations of chloride, glucose, lactate, cortisol and plasma osmolality and a significant reduction in haematocrit. In addition, deterioration of skin and gill epithelium and increased numbers of chloride cells, mucus cells and proliferating cells were evident. The novel application of piecewise linear models allowed, for the first time, the derivation of a threshold lice intensity, above which the host fish suffered sublethal physiological stress. This analysis identified a consistent breakpoint of 13 L. across several physiological markers. Identification of this threshold is an essential prerequisite in formulating effective wild fisheries management and stock conservation policy.

Reported by

University of St Andrews, Gatty Marine Laboratory
East Sands
KY16 8HL St Andrews, Fife
United Kingdom
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