Atlantic salmon cope in submerged cages when given access to an air dome that enables fish to maintain neutral buoyancy
Introduction
Salmon lice (Lepeophtheirus salmonis) are a major debilitating issue for salmon farming in Norway (Costello, 2006) with treatment costs for the parasite now estimated to range between 230 and 590 $US/ton of product produced (Liu and Bjelland, 2014; Brooker et al., 2018). However, the concept that prevention may be better than treatment is gaining traction with the industry. Recent research has highlighted the benefits of barrier technologies to minimise encounter rates between host and parasite, which can result in a reduced need to treat fish for lice (Stien et al., 2016, Stien et al., 2018; Oppedal et al., 2017; Wright et al., 2017, Wright et al., 2018; Nilsen et al., 2017; Grøntvedt et al., 2018; Geitung et al., 2019). Barriers can be either spatial, physical, or a combination of both, depending on the scale required to lower host-parasite encounter rates. For example, Samsing et al. (2019) modelled the spatial distribution of lice abundance in salmon farming areas across Norway and identified potential “spatial firebreaks” to limit lice dispersal between farming regions. At a finer scale, i.e. individual farms, establishing physical barriers, via installing a tarpaulin skirt (Stien et al., 2018; Grøntvedt et al., 2018) or snorkel (Stien et al., 2016; Oppedal et al., 2017; Wright et al., 2017, Wright et al., 2018; Geitung et al., 2019) or using an enclosed cage (Nilsen et al., 2017) can reduce salmon-lice encounter rates. Furthermore, submerged cages, which keep salmon below the surface waters where infective stage lice are most prevalent (Johannessen, 1978; Costelloe et al., 1995; McKibben and Hay, 2004) have been tested with clear benefits and limitations of this technology identified (Dempster et al., 2008, Dempster et al., 2009; Korsøen et al., 2009; Sievers et al., 2018; Glaropoulos et al., 2019).
Submerged cage farming creates a spatial host-parasite barrier without the issues of reduced water flow and low oxygen levels that are often associated with tarpaulin skirts (Stien et al., 2012, Stien et al., 2016) or snorkel barrier technology (Oppedal et al., 2017; Wright et al., 2017). However, submersed salmon still require air to refill their physostomous swim bladders to maintain buoyancy and normal swim behaviour (Dempster et al., 2009; Korsøen et al., 2009; Korsøen et al., 2012a). The swim bladders of farmed Atlantic salmon deflate within 3 to 4 weeks without surface access (Dempster et al., 2009; Korsøen et al., 2009, Korsøen et al., 2012a). Adding an underwater air dome to a submerged cage enables salmon to refill their swim bladders and maintain normal swimming behaviour for weeks to months (Korsøen et al., 2012b; Nilsson et al., Unpublished results). However, there remain outstanding questions regarding the optimal air dome size for commercial sized submerged sea-cages, and whether fish welfare can be maintained for a full production cycle, as maintaining a continuous supply of air to a dome under water is not a simple engineering problem (Korsøen et al., 2012b).
Few studies have investigated air domes in submerged cages (Table 1). Ablett et al. (1989) and Korsøen et al. (2012b) tested 1 × 1 m air domes submersed 4 and 10 m below the surface, respectively, with low stocking densities (<2 kg m−3) and found salmon used the air dome to refill their swim bladders and exhibited normal swimming behaviour. Nilsson et al. (Unpublished results) tested a range of dome sizes (1 to 4 m diameter) held <0.5 m below the surface, with low (smolts stocked at ~0.3 kg m−3) and high stocking densities (large fish stocked at 15.5–24 kg m−3) which fall within recommended stocking densities for commercial sea-cage farming (Stien et al., 2013). They concluded that a 3 m diameter air dome was sufficiently large for medium-scale commercial cages (12 m × 12 m, 10,000 fish or 20 m × 20 m, 30,000 fish). To date this has only been tested at a shallow roof depth < 0.5 m from the surface with a maximum submergence period of 18 days, with no measurements of animal based fish welfare variables made.
Here, we used three cohorts of Atlantic salmon smolts naive to submerged culture conditions and held them for 5 to 7 wk. in 12 m × 12 m submerged sea-cages with a 10 m roof depth and access to a 3 m diameter octagon air dome. We measured salmon swimming behaviour and SWIM welfare variables (condition factor K, skin condition, and fin condition) to determine whether submergence with access to an air dome altered their behaviour, condition or welfare over time.
Section snippets
Submerged cage and air dome set-up
Submerged caged rearing of Atlantic salmon was conducted from April to August 2017 at the Marine Research Institute's sea facility at Solheim, Masfjorden, Norway (60°N). Masfjorden is a typical fjord salmon farming site with a strong pycnocline (Fig. 1). The sea cage used was approximately 12 m × 12 m × 30 m (volume ~4320 m3) fitted with a 3 m diameter octagonal air dome (surface area ~7.1 m2) and net ceiling set at 10 m below the surface. Using this cage set up, three consecutive trials
Results
The environment followed a normal seasonal pattern for the site with brackish water (11 to 28 ppt) in the top 2 to 4 m, coldest temperatures in the surface layer in spring switching to warmest surface layer in late spring and summer. Average temperatures in the warmest part of the submerged cage depth were 7, 9 and 13 °C in trials 1, 2 and 3, respectively.
Growth, condition and SWIM
Keeping salmon submerged below 10 m deep with access to an underwater 3 m diameter air dome did not alter specific growth rate or condition factor. Observed specific growth rates were comparable to growth rates expected under standard cage conditions for this fish size, time of year and water temperature during each trial and are within the normal range of SGR's for salmon grown in 12 m × 12 m sea cages in Masfjorden (e.g. Oppedal et al., 2006) and predicted growth rates for commercial salmon
Conclusion
Post smolt salmon up to 1.5 kg refilled their swim bladders via an underwater 3 m diameter octagonal air dome during submerged cage farming. This enabled them to maintain physical condition and normal swimming behaviour over a 5 to 7 wk. period. The results indicate that submerged cage farming where the cage roof is at a 10 m depth could be a viable option to create a spatial host-parasite mis-match between farmed salmon and salmon lice. Further work should focus on determining whether
Declaration of Competing Interest
None.
Acknowledgements
Funding was provided by the Norwegian Research Council project no Fôrdom 256326, Future Welfare 267800, Dypdom 296157 and internal Institute of Marine Research project 14597-04/ -14. We thank the crew at the Solheim sea facility, Kristian Dahle, Marita Laupsa, and Jan-Harald Nordahl, the Matre Aquaculture Research Station technical staff, and the supplier of the air-domes, Plastinvent (https://www.plastinvent.no/). This work was conducted in accordance with the laws and regulations controlling
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