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Ocean-going juvenile salmonids heavily infected with salmon louse, Lepeophtheirus salmonis, have been observed prematurely returning to freshwater. This change in salinity preference may be an attempt either to regain osmotic balance or to remove the lice. For either hypothesis to be true, freshwater habitats must provide infected fish with a higher net fitness than saltwater habitats. The objectives of this study were to use behavioural titration to quantify the energetic cost of different salinities to infected and uninfected pink salmon (Oncorhynchus gorbuscha) and to determine if infection alters salinity preference. Results demonstrate that infection changes the salinity preference of fish from saltwater to freshwater. The cost paid by these freshwater-preferring infected fish foraging in saltwater increased with lice density during trials conducted between 13-33 days after infection. Other infection-induced behavioural changes include a 14-fold increase in the jumping frequency of infected versus control fish and a decrease in foraging between 13 and 33 days after infection.Résumé : On a observé que certains jeunes salmonidés fortement infestés de poux du saumon, Lepeophtheirus salmonis, en route pour l'océan retournent prématurément en eau douce. Ce changement de préférence de salinité peut être une tentative pour retrouver un équilibre osmotique ou pour se débarrasser des poux. Pour que l'une ou l'autre de ces hypothèses se vérifie, il faut que l'habitat d'eau douce procure aux poissons infectés une fitness nette plus grande que les habitats d'eau salée. Les objectifs de notre étude sont d'utiliser le dosage comportemental pour mesurer le coût énergétique des diverses salinités chez des saumons roses (Oncorhynchus gorbuscha) infectés et sains et pour déterminer si l'infection modifie la préférence de salinité. Nos résultats démontrent que l'infection change la préférence de salinité des poissons, de l'eau salée vers l'eau douce. Lors d'essais menés entre 13 et 33 jours après l'infection, le coût payé par ces poissons qui préfèrent l'eau douce, mais qui se nourrissent en eau salée, augmente en fonction de la densité des poux. Parmi les autres changements comportementaux induits par l'infection, on note une augmentation de l'ordre de 14 fois de la fréquence des sauts chez les poissons infectés par comparaison aux poissons témoins et une diminution de la recherche de nourriture entre les jours 13 et 33 après l'infection.[Traduit par la Rédaction] Webster et al. 680

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Summary1. The influence of salinity, temperature and physiological development on habitat choice by juvenile salmon is poorly understood. We measured differences in the net energetic cost of habitats that differed in salinity or temperature using behavioural titration of juvenile salmon and correlated these costs with body size and osmoregulatory enzyme activity to quantify how costs change with physiological development. 2. Juvenile salmon showed a strong preference for saline water (27‰ or 15‰ vs 0‰) and for cold water (9 °C vs 14 °C). It was estimated to be 0·15 W and 0·11 W more costly for salmon to forage in fresh water than in 15‰ and 27‰ salt water, respectively, and 0·09 W more costly to forage in warm than in cold water. 3. We expected fish to prefer 15‰ salt water to fresh water regardless of enzyme activity because they are iso-osmotic with this salt concentration. In contrast, preference for higher salt concentrations should increase with enzyme activity. Consistent with our expectations, enzyme activity was not correlated with preference for 15‰ salt water, but was positively correlated with preference for 27‰ salt water. 4. The cost of changing salinity vs changing temperature were very similar, indicating that knowledge of both of these costs are necessary to understand juvenile salmon habitat choice.

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