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Abstract: Submersible sea-cages represent an alternative method to standard surface-based cages, and could help solve several production problems that exist in surface-based farming, such as heavy storms, algal and jellyfish blooms and attacks by predators. In this study, we investigated growth, haematological, biochemical and immunological parameters of European sea bass (Dicentrarchus labrax) farmed in surface and submersible cages, to compare the physiological status under different rearing conditions. In addition, we evaluated the use of erythrocyte HSP70 levels as a biomarker of fish welfare. The study was conducted in two submerged and two surface cages filled with D. labrax. No significant differences were observed in the growth rate and the relative condition factor between the two groups. Significantly higher levels of serum cortisol, haematocrit plasma glucose and total plasma proteins were measured in surface-based cages. Conversely, fish in submerged cages showed higher haemolytic activity and lysozyme levels in both mucus and kidney than surface reared sea bass. Results obtained in this study indicate that submergence could be a favorable condition for sea bass farming, suggesting mariculture in submerged net cages as a promising system that allows producers to minimize stress (included that from predation by fish-eating birds) and therefore favor animal welfare. [Copyright &y& Elsevier]

Abstract: Fish-farming structures are widespread in coastal waters and are highly attractive to wild fish. Several studies have estimated that tons to tens of tons of wild fish aggregate around fish farms. These estimates assumed that the majority of wild fish are concentrated immediately beneath farms, although this assumption has never been explicitly tested. We tested the hypothesis that abundances of wild fish would be greatest immediately beneath farms and progressively diminish with distance at 4 full-scale coastal salmon (Salmo salar) farms in Norway. At each farm, fish were counted with a video-camera system at 5 different distances from the cages (farm=0m, 25, 50, 100 and 200m) throughout the water column on three separate days. Combined across all locations and times, the total abundance of wild fish was 20 times greater at the farm than at the 200m sampling distance. Saithe (Pollachius virens) dominated assemblages at all 4 farms and were consistently significantly more abundant at the farm than at the 25–200m distances. This ‘tight aggregation’ around farms corresponds to the reliance of saithe on waste feed when they school near farms. In contrast, patterns of distribution of both cod (Gadus morhua) and poor cod (Trisopterus minutus) varied among farms, with either highest abundances at the farm or a more even distribution of abundance across all 5 distances sampled. No specific pattern of aggregation was evident for the bottom-dwelling haddock (Melanogrammus aeglefinus). Our results suggest that the present 100m no-fishing zone around salmon farms protects the greatest proportion of farm-aggregated saithe and cod from fishing during the daytime. However, whether this reduces their overall susceptibility to fishing requires further research regarding nighttime distribution and movements. [Copyright &y& Elsevier]

Abstract: In the search for alternative farming methods, we investigated whether large salmon submerged below 10m in winter conditions behaved normally and performed as well as control fish held in standard surface cages. On average, 2345 salmon of ~3.5kg were kept in each of six 2000m3 sea-cages for 6weeks; three of which were submerged to 10–24m depth and three acted as surface controls (0–14m). Behaviour during both day and night was studied with echo-sounders, and underwater video cameras fitted with infra-red lamps. A sub-sample of fish from each cage was weighed, measured and assessed for fin and snout condition prior to and after the experimental period. In addition, the vertebral column of 50 fish from the control and submerged treatments were dissected and X-rayed to assess vertebral deformities. The submerged salmon seemed unable to re-fill any gas into the swim bladder, as a linear decrease in echo reflection to <5% of pre-submergence levels after 22days of submergence indicated loss of almost all gas from the physostomous swim bladders and negatively buoyant fish. Around day 22, submerged salmon swam at night time with a distinct ‘tail-down, head-up’ tilt (26°) compared to the horizontal swimming position of control fish (−3°). Average swimming speed (body length per second) of submerged salmon were 1.3–1.4 times faster (day: 0.77±0.02; night: 0.46±0.02, (mean±SE)) than control fish (day: 0.54±0.01; night: 0.37±0.02) both during day and night. Almost no mortality was seen, and the submerged salmon maintained similar diurnal vertical migrations as the surface fish, indicating that deep submergence did not exhaust the fish. However, submerged fish fed less efficiently, resulting in lower growth and reduced feed utilization. Fins and snouts of the submerged fish had small, but significantly more erosion than the control fish. Vertebrae in the tail region were significantly compressed in the submerged fish compared to control fish. This could be an early symptom of development of vertebral deformities. The results suggest that continuous submergence below 10m for longer than 2weeks reduces the welfare and performance of Atlantic salmon. [Copyright &y& Elsevier]

The article examines one research which determines the extent to which wild fish from surrounding waters aggregate around salmon farms across the farming regions in Norway. Wild fishes collected at nine farms and nine control locations were counted on three separate days in June-August 2007. After counting, it was revealed that Gadoid fishes, specifically Pollachius virens, Gadus morhua and Melanogrammus aeglefinus, were dominating farm-associated assemblages. Factors such as a heightened potential for the transfer of pathogens from salmon farms to wildfish as well as the altered availability of wild fish to fisheries are cited as two main reasons behind substantial aggregations of wild fishes at farms.

The article presents a study that determines the farm abundance and spatio-temporal distribution of Pollachius virens in a fjord system with concentrated salmon cage aquaculture in Øksfjord, Northern Norway. It notes on the use of video counting to estimate the abundance of the saithe at various depths, of acoustic transmitters to study its movement in the farms, and of passive listening stations to monitor its spatio-temporal distribution. It reveals the abundance of the saithe in both farms during the release of tagged saithe and discloses its periodic movement from release farm to natural areas. It entails that understanding fish movements can minimise the outbreaks of disease in fish farms and in the populations of wild fish in adjacent areas.

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. [Copyright &y& Elsevier]