Sea caged Atlantic salmon display size-dependent swimming depth

Abstract

-The present study investigates vertical distribution of fish size in three 12 m × 12 m wide and 14 m deep sea cages stocked with Atlantic salmon of average weight from 3.5 to 3.7 kg, at commercial densities between 15.6 to 16.2 kg m −3 , in Norway. For each cage, individual fish weight were estimated by three 0.6 m × 0.6 m measuring frames as fish swam through. The frames were positioned at 3, 6 and 9 m depth. Recordings were carried out over five days per cage in succession, and during relatively stable environmental conditions in late autumn 2010. In all cages, measured fish were 15−25% smaller at 3 m compared to the average weights at 6 and 9 m depth. Largest average weight difference between depths within one cage was 0.995 kg. The average weight at 6 and 9 m was higher at night-time compared to daytime. Fish at the lower end of the weight spectrum were predominantly registered at 3 m, while fish at the higher end of the spectrum were mostly registered at the greater depths. Fish of average size were well represented at all three depths. The fact that smaller fish swam shallower may be ascribed to natural behavioural traits and introduce an important consideration in representative sampling within commercial sea cages. In biomass estimations and sea lice counts size-dependent vertical stratification needs to be accounted for.

Behaviour in the toolbox to outsmart parasites and improve fish welfare in aquaculture

“…Current methods of estimating louse loads on salmon in sea cages are unlikely to be representative of the group (Heuch et al 2011), as many factors influence the vertical distribution of fish (such as preference, infestation status and body size; Oppedal et al 2011a;Folkedal et al 2012;Bui et al 2016b) and sample fish are mostly captured from the surface. Infestation levels are intrinsically linked to the welfare status of salmon, and therefore, behaviour can be used as an indicator for health and welfare if there is comprehensive understanding of how and why fish behave, coupled with knowledge of the mechanisms of such behaviours (Martins et al 2012).…”

Effect of pancreas disease caused by SAV 2 on protein and fat digestion in Atlantic salmon

“…Sampling of different subpopulations may be due to diseased and lethargic fish being less able to escape the sweep net. Or that different weight classes choose different depths as indicated in previous studies (Folkedal et al, 2012;Nilsson, Folkedal, Fosseidengen, Stien, & Oppedal, 2013). An additional cause may be that diseased fish move to the surface due to a higher temperature, where they are more likely to be sampled.…”

Salmon welfare index model 2.0: an extended model for overall welfare assessment of caged Atlantic salmon, based on a review of selected welfare indicators and intended for fish health professionals

“…Farmed salmon are normally sampled from the surface using different types of nets. Some known factors are likely to influence the sample precision: sizedependent swimming depth in sea caged salmon (Folkedal et al 2012), highest sea lice infestation rate of sea caged salmon in the upper cage meters (Hevrøy et al 2003), and the fact that chronically ill and moribund salmon position themselves close to surface and net wall (Stephen & Ribble 1995). Moreover, time of day with regard to hunger level is likely to influence the sample as there is a positive correlation between hunger level and surface attraction (Juell et al 1994).…”