Elsevier

Aquaculture

Volume 502, 15 March 2019, Pages 332-337
Aquaculture

Welfare, behaviour and feasibility of farming Atlantic salmon in submerged cages with weekly surface access to refill their swim bladders

https://doi.org/10.1016/j.aquaculture.2018.12.065Get rights and content

Highlights

  • Salmon post-smolt in submerged sea-cages coped well with a regime of weekly surface access over 8 weeks.

  • Three hours of weekly surface access for salmon in submerged sea-cages was enough for swim bladder filling.

  • Repeated surface access of salmon in submerged sea-cages is a promising solution for solving key production problems.

Abstract

Farming Atlantic salmon (Salmo salar L) in submerged cages may be a way for the industry to avoid surface related problems such as salmon lice and make new farming sites available. However, salmon do not cope with long term submergence and regularly need to access the surface to refill their swim-bladders and maintain buoyancy. We tested how salmon coped with submerged cages that were lifted to the surface once each week to allow surface access. Three submerged cages were stationed with a roof depth at 10 m for a period of 8 weeks and measures of welfare and behaviour were compared against three standard cages. All cages held approximately 1700 newly sea-transferred smolts. Submerged fish swam 1.4× to 3.4× times faster, schooled tighter and swim bladder volumes declined gradually from the start to the end of each week. When cages were lifted and the surface became available, an immediate burst of jump and rolling behaviour showed that the salmon had been negatively buoyant. Nevertheless, no evidence of acute buoyancy problems occurred during submergence and similar growth rates and welfare scores were recorded. Our results indicate that farming salmon in submerged cages with weekly surface access is viable and could assist in solving key production problems.

Introduction

Farming of Atlantic salmon in coastal waters currently occurs almost exclusively in surface-based sea-cage systems, where conditions are suitable for culture for most of the time. However, unsuitable conditions at the surface periodically arise, due to a variety of biological factors that affect fish health and welfare or physical factors that affect farming structures (see Table 4 in Dempster et al., 2009). Submerged sea-cages may provide an alternate technology platform to address many of these problems. Holding fish deeper may allow a more consistent environment to be accessed without harmful biological agents, such as jellyfish or algae blooms, poor dissolved oxygen or temperature. Likewise, submerging sea-cages beneath storms can reduce the forces from large waves and strong surface-based wind driven currents on cages and mooring systems, and thus minimize the risk of escape events (Jensen et al., 2010) and the considerable risk storms create to the welfare of fish (Bjelland et al., 2016). Since the main structure is under water, another advantage of submerged sea-cages is that they minimize visual “pollution” of the seascape and may therefore create less conflict with coastal communities (Dempster and Sanchez-Jerez, 2008).

For Atlantic salmon, one of the most important production advantages submerged cages may offer is that they contain fish below the surface “belt” of the infective stage of salmon lice Lepeophtheirus salmonis. Salmon lice copepodites are positive phototactic and thereby attracted towards the surface, but they are yet repelled by brackish surface water as common for fjord farming sites (Heuch et al., 1995; Penston et al., 2008). Recent experimental data demonstrates that salmon held deep in sea-cages catch far fewer lice than salmon held in surface-based cages (Stien et al., 2016; Samsing et al., 2016; Oppedal et al., 2017). Salmon farmers using surface-based cages must treat salmon repeatedly with chemotherapeutants, thermal or mechanical removal methods during a production cycle to remain below allowable levels (Torrissen et al., 2013). The industry is therefore in urgent need of new production methods that limit infestation by sea lice.

Farming salmon in submerged sea-cages is, however, not straightforward. Salmon have a physostomous swim-bladder and must frequently go to the surface to gulp air to refill it and maintain their buoyancy (Dempster et al., 2009). Korsøen et al. (2009) found that keeping salmon in submerged sea-cages lead to empty swim-bladders within three weeks. Salmon with empty swim bladders exhibit sub-optimal behaviours, such as ‘tail-down, head-up’ swimming, which leads to poorer appetite (Fosseidengen et al., 1982; Dempster et al., 2008, Dempster et al., 2009; Korsøen et al., 2009, Korsøen et al., 2012a). Long-term submergence below 10 m reduced the growth rate of salmon markedly, and recorded markers of physical condition, including fin, skin and vertebrae, were compromised (Korsøen et al., 2009). Some studies have, however, indicated successful coping with both short-term repeated submergence (2, 3 and 4 days) and continuous submergence for 17 and 22 days (Dempster et al., 2008, Dempster et al., 2009). In these studies, salmon coped with submergence by swimming faster with no abnormal behaviour, or reduction in fin or skin condition. A possible farming protocol is, therefore, to have salmon in submerged cages for the most of time to avoid rough weather and minimize infection by sea lice, and lift the cage up to the surface at regular intervals to allow the salmon to refill their swim-bladder.

Here, we tested the coping ability of newly seawater transferred Atlantic salmon to repeated (8 times), short-term (7 days) submergence in winter (short days) to a relatively deep depth (10 m) and compared measures of welfare, behaviour and production performance with fish grown in standard surface-based cages.

Section snippets

Experimental set up

The study was performed from 18th of November 2014 to 14th of January 2015 at the Institute of Marine Research aquaculture field station at Solheim in Masfjorden, Western Norway (60°N). Six medium-scale (12 m × 12 m wide) commercial sea-cages were used; each stocked with ~1700 newly sea transferred (4 days) Atlantic salmon (Salmo salar L.). The three treatment cages were 24 m deep with a net roof at 10 m to keep the salmon submerged. The three control cages were 14 m deep to have the same

Environmental variables

The fjord environment was stratified, with a halo- and a thermo-cline generally present between 1 and 5 m (Fig. 1). Temperatures above the thermocline ranged from 2 to 9 °C (cold water), while the warmer water below stayed around 11 °C (Fig. 1A). The salinity was in periods as low as 0 ppt in the upper 1 m, but generally around 22 ppt (brackish water) above the halocline and 33 to 34 ppt below (Fig. 1B).

Growth and performance

At the end of the experiment, weight (control: 110 ± 1.6 g vs. submerged: 113 ± 1.3 g, p

Discussion

Key production parameters of importance to industry performance, such as mortality, SGR and physical condition of the salmon, were not compromised by repeated submergence of fish below 10 m depth over an 8-week period. These parameters integrate the commercial performance of the fish, and thus indicate that submergence with weekly surface access re-instatement is a viable option for the industry, at least for salmon of small size. The welfare index score was also consistently better in

Acknowledgment

The authors would like to thank Jan Olav Fosse, Kristian Dahle, Grethe Thorsheim, Linda Oldeide, Tone Vågseth and Jan Erik Fosseidengen for technical assistance, and one anonymous referee for comments that improved the manuscript.

The EU AQUAEXCEL (FP7/2007/2013, grant agreement No. 262336) and the Centre for Research Innovation in Aquaculture Technology (CREATE-841005) provided funding for the trials. The experiments were conducted in accordance with laws and regulations of the Norwegian

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