Resources are heterogeneously distributed in nature, and many animals may breed in some areas but move to other areas to forage.
In some cases these animal movements may be obligate migrations, such as for Atlantic salmon Salmo salar L. In other cases the migration may be facultative, as is the anadromous behaviour of brown trout Salmo trutta L. Brown trout must reproduce in freshwater, and may fulfil all parts of its life cycle in freshwater.
However, where brown trout has access to the sea, it may smoltify at a size of 12 to 25 cm and make marine foraging migrations during late spring and summer to enhance growth and reproductive potential (Elliott 1994). This phenotype is referred to as sea trout.
The survival and growth at sea are key parameters in understanding population dynamics in anadromous salmonids (Elliott 1994, Aas et al. 2011). Although the sea migration may be rewarding, it also involvesincreased predation risk and infection risk by parasites and pathogens.
Changes in infection pressure may be brought about by human activities, such as the rapid growth of the salmon farming industry in recent decades (Krkošek et al. 2011, Serra-Llinares et al. 2014).
Mapping the individual be haviour of brown trout at sea is therefore essential in describing host−parasite interactions and behavioural responses to changes in infection pressure, and may provide links between activities in the aquaculture industry and the population dynamics of wild sea trout.
However, there is currently little detailed information on the behaviour of sea trout in the marine phase. The salmon louse Lepeophtheirus salmonis Krøyer is a marine ectoparasite of salmonids. It belongs to the subclass Copepoda and has 8 developmental stages: 2 naupliar stages which disperse by drift, an infective free-swimming copepodite stage, 2 attached chalimus stages, 2 preadult stages, and the mature and reproductive stage (Heuch et al. 2000, Boxaspen 2006, Hamre et al. 2013).
The louse is motile on the host in the preadult and mature stages. The infective stages feed on the skin, subcutaneous tissue, mucus and plasma of their hosts. Salmon lice occur naturally in cold temperate waters in the north ern hemisphere (Boxaspen 2006), but due to the growing salmon fish farming industry, lice densities in coastal waters have increased dramatically (Bjørn et al. 2001, Finstad & Bjørn 2011, Serra-Llinares et al. 2014).
This in turn has increased infection pressure on sea trout, and may have contributed to the recent decline in sea trout populations along the Norwegian coast (Anonymous 2009, Finstad et al. 2011).
Salmon lice infestation causes osmoregulatory stress to the host, resulting in changed levels of haematological parameters, reduced appetite, growth and food conversion efficiency (Boxaspen 2006, Costello 2006). This may affect their host’s survival directly as a consequence of lost physiological functionality, or indirectly through added effects of secondary viral and bacterial pathogens (Bjørn et al. 2001, Fast et al. 2006) and/or altered host behaviour (Krkošek et al. 2011). As indicated by experimental work, farm experience and surveys of patterns across populations, more than 5 to 10 lice per fish (> 0.1 lice g−1) can or will become pathogenic (Costello 2006, Wagner et al. 2008).
It is likely that salmonids through natural selection have developed behavioural adaptations in order to avoid or reduce lice infestation (Gjerde et al. 2011). The facultative anadromous migration of sea trout implies that the length of the marine phase may beinfluenced by the rewards and risks associated with this life history choice. Salmon lice survival decreases with decreasing salinity (Connors et al. 2008), and salmon lice may actively avoid waters with salinity < 20 ppt (Heuch 1995).
Hence, sea trout have the potential to reduce or rid themselves of infestation by seeking low-salinity waters, such as estuarine surface waters or river water. The term ‘premature return’ has been coined for a sea trout returning to freshwater at an earlier time than expected if it was not infected (Birkeland 1996, Wells et al. 2007). A premature return is costly for the sea trout as it reduces growth and reproductive potential (Birkeland 1996, Wells et al. 2007, Fjørtoft et al. 2014).
Reduced growth furthermore increases predation risk, as it extends the time a fish is able to size-dependent predation (Werner & Gilliam 1984). Reduced growth has been shown to be associvulner ated with reduced marine survival in sea trout (Jonsson & Jonsson 2009). Although several works have focused on sea trout premature returns to freshwater as a response to salmon lice infection, little has been done to investigate the behavioural responses while at sea. Given the importance of sea trout behaviour in mitigating the effects of salmon lice, further research on migration patterns at sea is required.
In the present study, we examined the behaviour of sea trout with varying degrees of salmon lice infestation. The movements of sea trout examined for infestation level and tagged with acoustic transmitters were monitored in a receiver array covering both marine and freshwater habitats. Half of the expermental group were treated with a pharmaceutical prophylaxis designed to reduce salmon lice infestation.
The infestation development in prophylaxistreated and untreated fish was quantified in a separate net cage experiment. It was hypothesized that (1) high salmon lice infestation would influence the movement pattern of the fish in terms of distance to river outlet and swimming depth, (2) untreated fish would use freshwater and/or low-saline habitats more than treated fish and (3) untreated fish would have a higher mortality rate than treated fish. To inform the reader on the salmon lice infection pressure in the study area, we also report infection control measures obtained from the Norwegian National Salmon Lice Monitoring Program.
The infestation pressure was expected to be high during the study, as a consequence of high salmon biomass in the closest fish farms resulting from a new rollover regime among aquaculture production zones in the study system (Serra-Llinares et al. 2014).
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