Many coho salmon stocks (Oncorhynchus kisutch) have been in decline during the past three decades. Canada's most endangered salmon stock, the Thompson River coho salmon, is being studied extensively as managers attempt to reverse these population declines. Investigators are using acoustic telemetry to track the migratory behaviour and survival of the Thompson River (and other) coho salmon stocks. Coho salmon pre-smolts are relatively small compared with salmonid species that are typically studied using acoustic telemetry; therefore the identification of the appropriate sizes of fish and tags to use is critical. This study tested the effects of surgically implanting the three smallest sizes of acoustic tags currently available on the growth, survival, tag retention, swimming performance and physical condition of coho salmon pre-smolts for 300 days post-surgery. Maximum tag size to body size ratios ranged from 15-17% by fork length and 7-8% by mass for the three tag sizes (11 cm fork length for a 6×19 mm tag, 12.5 cm for a 7×19 mm tag, and 14 cm for a 9×21 mm tag). Based on our results, it is unlikely that coho salmon pre-smolts implanted with acoustic transmitters following these size guidelines would have poor survival in studies of freshwater migratory behaviour as a result of the surgery or the tag.

Eighty coho salmon Oncorhynchus kisutch smolts (40 wild and 40 hatchery-reared) were surgically implanted with acoustic transmitters and released into the Quinsam River over 2 days. Differences in physiology, travel time and migratory behaviour were examined between wild and hatchery-reared fish. In addition, tagged and control fish of both wild and hatcheryreared stock were raised for 3 months following surgery to compare survival and tag retention. Detection ranges of the acoustic receivers were tested in the river, estuary and ocean in a variety of flow conditions and tide levels. Receivers were placed in the river, estuary and up to 50 km north and south from the river mouth in the marine environment. Wild smolts were significantly smaller by mass, fork length and condition factor than hatcheryreared smolts and exhibited significantly higher levels of sodium, potassium and chloride in their blood plasma than hatchery-reared smolts. The gill Na þ K þ -ATPase activity was also significantly higher in the wild coho smolts at the time of release. Ninety-eight per cent of wild and 80% of hatchery-reared fish survived to the estuary, 8 km downstream of the release site. No difference was found in migration speed, timing or survival between smolts released during daylight and those released after dark. Wild smolts, however, spent less time in the river and estuary, and as a result entered the ocean earlier than hatchery-reared smolts. Average marine swimming speeds for wild smolts were double those of their hatchery-reared counterparts. While hatchery smolts dispersed in both a northward and southward direction upon entering the marine environment, the majority of wild smolts travelled north from the Campbell River estuary. The wild coho salmon smolts were more physiologically fit and ready to enter sea water than the hatchery-reared smolts, and as a result had higher early survival rates and swimming speeds.

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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