Diel vertical distribution of early marine phase juvenile pink salmon (Oncorhynchus gorbuscha) and behaviour when exposed to salmon louse (Lepeophtheirus salmonis)

We observed diel vertical migration patterns in juvenile pink salmon (Oncorhynchus gorbuscha (Walbaum, 1792)) and tested the hypothesis that fish behaviour is altered by exposure to sea lice copepodids. Experiments involved replicated field deployments of a large (9 m) plankton column, which provided a vertical distribution enclosure under natural light and salinity conditions. Diel vertical distributions of juvenile pink salmon were observed during the first 3 weeks of seawater acclimation in both the presence and the absence of the ectoparasitic salmon louse (Lepeophtheirus salmonis (Kroyer, 1838)). Immediately upon entering seawater, juvenile pink salmon preferred the top 1 m of the water column, but they moved significantly deeper down the vertical water column as seawater acclimation time increased. A significant diel migration pattern was observed, which involved a preference for the surface at night-time, compared with daytime. When fish in the column were exposed to L. salmonis copepodids for 3 h, 43%-62% of fish became infected, fish expanded their vertical distribution range, and significant changes in vertical distribution patterns were observed. Nous avons observe les patrons de migration verticale journaliere de jeunes saumons roses (Oncorhynchus gorbuscha (Walbaum, 1792)) et verifie l'hypothese selon laquelle le comportement des poissons est modifie par l'exposition aux copepodites des poux de mer. Nos experiences comprenaient la mise en place en nature de deux grandes (9 m) colonnes a plancton qui fournissaient des enclos pour une etude reiteree de la repartition verticale dans des conditions naturelles de lumiere et de salinite. Nous avons observe la repartition verticale journaliere des jeunes saumons roses durant leurs trois premieres semaines d' acclimatation a l'eau de mer a la fois en presence et en l'absence de poux de mer parasites (Lepeophtheirus salmonis (Kroyer, 1838)). Immediatement apres leur entree en eau de mer, les jeunes saumons roses preferent le premier metre superieur de la colonne d'eau, mais ils se deplacent vers des profondeurs significativement plus grandes a mesure que s'accumule leur temps d'acclimatation. On observe un patron significatif de migration journaliere, avec une preference pour la surface durant la nuit, par comparaison avec le jour. Lorsque les poissons dans la colonne sont exposes a L. salmonis pendant 3 h, 43%-62% des poissons deviennent infectes, les poissons accroissent la portee de leur migration verticale et il se produit des changements significatifs dans les patrons de repartition verticale. [Traduit par la Redaction]
Introduction Commercial aquaculture, which has grown to the point of providing nearly half of the fish consumed by humans (FAO 2009), has not come without concerns. These concerns include impacts [...]