Thermo- and photoregulatory swimming behaviour of caged Atlantic salmon: Implications for photoperiod management and fish welfare

High host densities dilute sea lice Lepeophtheirus salmonis loads on individual Atlantic salmon, but do not reduce lice infection success

“…Salmon swimming behaviour and their vertical position in the water column is strongly influenced by light levels (Tang et al 2011), and can be manipulated by using artificial light (Juell et al 2003, Oppedal et al 2007, Frenzl et al 2014. Sea lice copepodids are phototactic and gather near surface waters during the day, while also actively avoiding low salinities (Heuch et al 1995, Costello 2006).…”

Atlantic salmon Salmo salar instantaneously follow vertical light movements in sea cages

“…Frenzl et al 2014), or environmental conditions. Temperature is known to override light attraction in Atlantic sal mon (Oppedal et al 2007), but it may not have been the driver for the behaviour of the surface-positioned subgroup in this study as warmer depths optimising salmon thermoregulation (Johansson et al 2006) were in the lower cage sections. It is possible that the surface fish were infected by salmon lice (0.85 lice fish −1 detected before the start of our study) and selected shallow lowsalinity areas to self-treat against these freshwatersusceptible parasites (Gjelland et al 2014).…”

“…Artificial light effectively modifies salmon swimming depth in sea cages at night (Juell & Fosseidengen 2004, Oppedal et al 2007). Sudden vertical light adjustments reposition salmon (Juell & Fossei-ABSTRACT: Atlantic salmon in marine farming environments are exposed to potentially detrimental conditions through space and time.…”

“…For instance, light attractiveness fades in temperature-stratified environments when optimal temperatures for salmon thermoregulation (~17°C; Johansson et al 2006) occur outside illuminated depths (Oppedal et al 2007). Attraction also diminishes if lights are of lower intensity (Stien et al 2014).…”

Sea caged Atlantic salmon display size-dependent swimming depth

“…This in order to secure proper management and fish welfare related to stocking density and environmental conditions (Johansson et al 2006(Johansson et al , 2009Oppedal et al 2007Oppedal et al , 2011aOppedal et al , 2011b, amount of food and pellet size, size sorting of fish, and to detect underlying causes of arrested growth. Also, a Corresponding author: ole.folkedal@imr.no this information is highly valuable for the farmer when preparing for slaughter and sale of the fish, when reporting to authorities and during control regimes by authorities.…”