Fish welfare in offshore salmon aquaculture

Abstract

To accommodate further growth in the Atlantic salmon aquaculture industry, new production sites may well be established at more exposed locations along the coast or even offshore. Here, fish will encounter strong currents and powerful waves, which are avoided at traditional sheltered locations. Exposed locations offer several advantages and necessitate new technological advancements. However, the most crucial question is whether Atlantic salmon are able to thrive in these more extreme environments. In this review, we describe how strong water currents affect the physiology, behaviour and ultimately the welfare of the fish. If ambient current speeds exceed swimming capacities, fish become fatigued and get stuck on the cage wall leading to unacceptable welfare. The swimming capacity will depend on both the magnitude and duration of the current speeds encountered. Moreover, several environmental and biological factors modulate swimming capabilities, where temperature, body size and health status are particularly important to consider. A series of empirical studies are subsequently used to formulate welfare guidelines with regard to biological limits in exposed aquaculture. In addition, owing to the growing popularity of cleaner fish in salmon aquaculture, we also evaluate their usefulness at exposed sites. Overall, Atlantic salmon is a powerful sustained swimmer, and based on available site surveys of ocean currents, we conclude that the prospects for responsible farming at exposed sites looks promising. However, cleaner fish species such as lumpfish and ballan wrasse are poor swimmers and are therefore not recommended for deployment at exposed sites.

Factors associated with baseline mortality in Norwegian Atlantic salmon farming

“…A limitation to our study was that we were not able to assess other environmental variables due to a lack of nation-wide data. As pointed out by others 7 , 72 , reduced oxygen levels, strong water currents, predation, and algal blooms can have consequences for fish welfare and mortality. Although the explored environmental factors cannot be controlled in salmon production sea cages, their association with fish health is useful to monitor mortality.…”

What is the limit of sustained swimming in Atlantic salmon post smolts?

“…Our assessment of sustained swimming limits of fish in fixed velocity tests for up to 72 h is substantially longer than any previous work on salmonids that we are aware of. The interest in swimming limits at longer timescales has emerged only recently with the trend towards exposed offshore salmon aquaculture (Bjelland et al 2015, Hvas et al 2021a. In these harsher farm environments, prolonged periods of inescapably fast water currents may occur and become a serious fish welfare concern (Remen et al 2016).…”

“…This process requires technological advancements and novel management strategies and, more importantly, thorough assessments of whether the fish can thrive in harsher environments. Since strong and persistent water currents are considered the greatest threat to fish welfare at exposed aquaculture sites, recent efforts have focussed on de-veloping guidelines for acceptable current conditions based on the behavioural and physiological impacts on Atlantic salmon (Hvas et al 2021a). In calm weather or low currents, farmed Atlantic sal mon swim in circles in the cages at speeds chosen by the fish themselves (Johansson et al 2007, Op pedal et al 2011.…”

Addressing the welfare needs of farmed lumpfish: Knowledge gaps, challenges and solutions

“…The species has a relatively low swimming ability 70,101 and has difficulty withstanding fast currents, [102][103][104] and it is, therefore, likely to struggle to both swim and cling to surfaces in salmon net-pens which are often situated in areas with strong currents. 35 Lumpfish prefer low-intensity blue lighting 27,74 and cling to dark structures, 104 but there is very little practical guidance that can be used to inform best practices.…”