Elsevier

Aquaculture

Volume 288, Issues 3–4, 20 March 2009, Pages 254-263
Aquaculture

Submergence of Atlantic salmon (Salmo salar L.) in commercial scale sea-cages: A potential short-term solution to poor surface conditions

https://doi.org/10.1016/j.aquaculture.2008.12.003Get rights and content

Abstract

Submergence of Atlantic salmon (Salmo salar L.) in commercial scale sea-cages (1600–2000 m3) affected their behaviour, but did not alter growth rates, food conversion ratios, appetite, condition factor or fin condition in comparison with control cages held under similar environmental conditions. Four sea-cages each held 3300–4200 Atlantic salmon of 0.45 kg; two cages acted as controls, while two were submerged for 22 days with the roof held at 3 m depth. Salmon in the control cages mainly swam at similar depths to those maintained by submerged fish, resulting in both treatments experiencing similar temperature, light, salinity and dissolved oxygen levels. Submerged fish swam an average of 1.6 times faster (0.88 BL s 1) and in more structured schools than control fish (0.55 BL s 1). Specific growth rates (1.1–1.3% day 1), appetite (0.8–1.2% body weight day 1) and condition factor (1.10–1.15) were high, while food conversion ratios (0.6–1.0) and the incidence of fin damage were low in all cages, with no significant differences between the control and submerged treatments. Our results highlight the potential for submergence of salmon in sea-cages for short periods to avoid negative surface events.

Introduction

Submerged or semi-submerged sea-cage culture has been successfully demonstrated for several species, including Pacific threadfin Polydactylus sexfilis (Ryan, 2004), cobia Rachycentron canadum (Rapp et al., 2007), Atlantic cod Gadus morhua and haddock Melanogrammus aeglefinus (Chambers and Howell, 2006). In contrast, salmonids, which dominate the global production of fish in sea-cages, are cultured solely in cages open to the surface. Examples when submergence may enable a better production environment to be accessed during poor surface conditions are numerous (Dempster et al., 2008). Moreover, submergence may provide opportunities to reduce specific environmental impacts related to salmonid culture in sea-cages, such as escapes during storms (Naylor et al., 2005) and sea-lice loads in coastal waters (Hevrøy et al., 2003). Despite these advantages, submerged systems for salmon have not been adopted by the industry because knowledge of how and when to submerge fish is lacking.

The uncertainty surrounding submergence is related to the assumption that salmon become negatively buoyant when submerged beneath a cage roof as they cannot access the surface to gulp air and fill their physostomous swim bladders (Dempster et al., 2008). Numerous previous attempts have been made to determine the effects of submergence on salmonids in tanks, small-scale cages and commercial scale sea-cages (summarised in Table 1). Trials in tanks and small-scale cages have caused the rapid onset of tilted swimming, loss of buoyancy control, and in some cases exhaustion and mortality (Fosseidengen et al., 1982, Ablett et al., 1989). The restricted space within such small test enclosures may have inhibited fish from compensating for negative buoyancy through swimming at sufficient speed to generate hydrodynamic lift through planing (Sfakiotakis et al., 1999). In contrast, trials in large sea-cages (Osland et al., 2001, Dempster et al., 2008) have demonstrated that salmon cope with submergence, as they feed actively and grow, although at a slower rate than surface control cages. Dempster et al. (2008) demonstrated that 1.7 kg salmon increased their swimming speeds by 1.5 times when submerged compared to fish in control cages, suggesting an energetic mechanism lies behind the lower growth rates observed. However, differences in growth could not be interpreted as the direct physiological effect of faster swimming alone as other confounding factors may have played a role. Temperature and light differences between fish held in surface control cages and the submerged cages deeper in the water column may have contributed to the growth differences observed. Further, in a trial into the effects of submergence conducted by Osland et al. (2001), in addition to differing environmental conditions between the submerged and control cages, feed intake was not measured, creating the possibility that fish in submerged and control cages may not have had similar access to feed.

No previous experiment has investigated the effects on growth, condition and behaviour of salmon in submerged cages when control cages experienced similar or poorer environmental conditions. In this experiment, we compared the growth, behaviour and fin conditions of Atlantic salmon submerged in commercial scale sea-cages with control cages during a period of the year when salmon in the control cages naturally chose to swim at similar depths as the submerged fish for the majority of the time. To induce swimming at similar depths at night, we used continuous artificial lighting, which is a standard management tool in commercial production of both spring-transferred (Endal et al., 2000) and autumn-sea-transferred (Oppedal et al., 2006) Atlantic salmon to reduce the sexual maturation of fish. Specifically, we tested the hypotheses that the behaviour, growth, food conversion ratio and incidence of fin damage of salmon in submerged cages differed from salmon held in cages where they could access the surface, under similar environmental conditions and with similar access to food.

Section snippets

Location and experimental design

The experiment was conducted at the Cage Environment Laboratory at the Institute of Marine Research field station, at Solheim, in Masfjorden, western Norway (60°N). Four commercial scale cages (12 m × 12 m × 14 m depth; approx. 2000 m3) were used. The two control cages were of a standard type used for commercial salmon production. The two submerged cages were modified standard cages in that a roof of black netting, which consisted of the same mesh as the cage, was sewn into the net-cage 3 m below

Environmental conditions during the trial

Temperature, salinity, dissolved oxygen saturation and light intensity varied with depth at the Solheim site. A distinct thermocline existed throughout the experimental period from days 1 to 10 and from days 19 to 28 (Fig. 1). The thermocline was evident at approximately 3 m depth, distinguishing colder water < 6 °C in the upper 3 m from warmer water of 8–9 °C below 3 m. While the thermocline did not break down, a body of warm water (9 to 11 °C) appeared in the top 1–1.5 m between days 11 and

Experimental conditions

Under the environmental conditions present during the trial, submerged and control salmon experienced broadly similar light and temperature regimes, as control fish preferred to swim at depths in the cage which were similar to those at which submerged fish swam. Little difference existed in the cumulative degree days experienced by fish in the submerged and control cages. Light levels experienced by submerged and control fish during the night were comparable and during the day, control and

Acknowledgements

Ole Fredrik Skulstad, Tone Vågseth and Jan Erik Fosseidengen are thanked for their technical assistance throughout the experiment. Funding was provided by SINTEF Fisheries and Aquaculture through the Intelligent Structures in Fisheries and Aquaculture Project and the Institute of Marine Research through the EU FastFish Project. The work was conducted in accordance with the laws and regulations controlling experiments and procedures on live animals in Norway following the Norwegian Regulation on

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