Effects of salmon lice infection on the behaviour of sea trout in the marine phase

Abstract

Salmon lice Lepeophtheirus salmonis Krøyer may affect survival and growth of anadromous salmonids through physiological stress and/or behavioural changes. Using acoustic telemetry tracking, we investigated the behaviour of 30 infected sea trout Salmo trutta throughout the summer in a fjord with very high salmon lice infection pressure. Most of the tracked sea trout adopted a movement pattern expected to suppress salmon lice infestation, as they showed a strong preference for fresh or brackish water, spending most of the time close to a river outlet or even migrating into the river. Highly infested sea trout preferred shallower depths, associated with lower salinity. The fish lost to predation stayed further away from the river outlet than nonpredated fish, and were likely subjected to a stronger infection pressure. Half of the tracked group were treated with a salmon lice prophylaxis, emamectin benzoate. The effect of treatment on infestation was monitored in a separate group held in a sea cage and found to be moderate; the mortality in this group was associated with infestation by motile lice stages. In contrast, treatment was not found to have an effect on tracked fish behaviour. It is likely that some physiological and behavioural responses to high salmon lice infection pressure may be present even after a prophylaxis treatment, in particular when the treatment is given after exposure to salmon lice infection. We conclude that increased salmon lice infection pressure associated with altered salmon farming practice may have the potential to influence the marine behaviour and growth of sea trout.

Statistical and ecological challenges of monitoring parasitic salmon lice infestations in wild salmonid fish stocks

“…Another reason why salmon lice infestations, like many parasite infestations, are notoriously difficult to study is that the likelihood of a fish to be collected in the monitoring programme is dependent on its infestation level, since salmon lice influence the survival and behaviour of the fish (Finstad & Bjørn 2011). However, studies that include acoustic telemetry may shed light on fish movements in the fjords as well as behavioural differences between individuals with and without lice, as recently illustrated by Gjelland et al (2014).…”

Trophic niche similarity among sea trout Salmo trutta in central Norway investigated using different time-integrated trophic tracers

“…High parasite burdens have also been associated with pale gills, decreased body condition and host mortality in farmed S. trutta (Rubio-Godoy & Tinsley 2008), with high loads of intestinal helminths in particular seen as the probable trigger for long-term malnutrition leading to increased S. trutta emaciation and mortality (Mladineo et al 2009). High parasite loads may also affect fish behaviour, altering habitat use and foraging opportunity as infected individuals adopt movement patterns expected to suppress infestation rates (Gjelland et al 2014). Lowered levels of parasites observed at sites sampled here suggest reduced parasite-induced energetic and fitness costs for S. trutta in central Norway.…”

“…For rapidly growing salmonid fishes, the isotopic value of muscle will equilibrate to diet within the order of a few months (Perga & Gerdeaux 2005, Trueman et al 2005, Phillips & Eldridge 2006. As a consequence, muscle tissue is considered a useful surrogate for the study of temporally integrated feeding in anadromous salmonid fishes (Doucett et al 1999b, Etheridge et al 2008, van der Velden et al 2012, with δ 13 C and δ 15 N stable isotope ratios of a consumer reflecting the isotopic signatures of the prey consumed during the time period that the tissue was synthesised (Fry 2006). The δ 13 C signature provides an indication of the origin of the carbon source since littoral, pelagic and profundal carbon sources in freshwater have different δ 13 C signatures, de creasing along littoral− profundal gradients (Van der Zanden & Rasmussen 1999).…”

Sea trout adapt their migratory behaviour in response to high salmon lice concentrations

“…Because salmon lice do not tolerate low salinities for extended periods (Heuch, 1995;Wright, Oppedal, & Dempster, 2016), coastal migrating salmonids like sea trout (Salmo trutta L.) can actively reduce or get rid of a salmon lice infestation by seeking FW (Birkeland & Jakobsen, 1997;Gjelland et al, 2014;Wells et al, 2007). Nevertheless, spending time in the estuary or returning to a river has a cost of reduced foraging opportunity and efficiency (Birkeland, 1996).…”