Oxygen gradients affect behaviour of caged Atlantic salmon Salmo salar

T Oldham,Jo Fosse,T Dempster,F Oppedal

doi:10.3354/aei00219

Abstract

Dissolved oxygen (DO) conditions in marine aquaculture cages are heterogeneous and fluctuate rapidly. Here, by temporarily wrapping a tarpaulin around the top 0 to 6 m of a marine cage (~2000 m3), we manipulated DO to evaluate the behavioural response of Atlantic salmon Salmo salar to hypoxia. Videos were recorded before, during and after DO manipulation at 3 m depth while vertical profiles of temperature, salinity, DO and fish density were continuously measured. The trial was repeated 4 times over a 2 wk period. Temperature and salinity profiles varied little across treatment periods; however, DO saturation was reduced at all depths in all replicate trials during the tarpaulin treatment compared to the periods before or after. In 3 out of 4 trials, swim speeds were 1.5 to 2.7 times slower during the tarpaulin treatment than the before or after periods. Significant changes in vertical distribution of fish density and DO were observed between treatment periods in all replicate trials; salmon swam either above or below the most hypoxic depth layer (59 to 62% DO saturation). In a regression tree analysis, the relative influence of DO in determining fish distribution was 17%, while temperature (39%) and salinity (44%) explained the majority of variation. Our results demonstrate that salmon are capable of modifying their distribution and possibly activity levels in response to intermediate DO levels, but that DO is not a primary driver of behaviour at the saturation levels examined in this study.

Highlights

The energy yield of anaerobic glycolysis is only 10% that of aerobic metabolism (Hochachka & Somero 2014), animals depend on a consistent supply of oxygen from their surroundings to achieve optimal performance
Poor dissolved oxygen (DO) conditions are exacerbated in aquaculture cages due to restricted water movement, nutrient loading and locally increased biomass (Johansson et al 2006, 2007, Oppedal et al 2011b, Burt et al 2012), and are becoming more common as global temperatures rise (Gruber 2011)
Temperature and salinity varied little between treatment periods, whereas DO saturation was reduced by 10% on average during the tarpaulin treatment compared to the before and after periods in all replicate trials (Table 1)

Summary

Introduction

The energy yield of anaerobic glycolysis is only 10% that of aerobic metabolism (Hochachka & Somero 2014), animals depend on a consistent supply of oxygen from their surroundings to achieve optimal performance. In the aquatic environment where diffusion happens slowly and photosynthesis can only partially meet the metabolic demands of organisms in the surface waters (Richards et al 2009), dissolved oxygen (DO) concentration varies both vertically and horizontally with changing light, temperature, currents (Johansson et al 2007), wind and rainfall (Diaz 2001). Even among fish that avoid reduced DO concentrations, the point at which behavioural responses are initiated varies greatly with species, lifestyle and habitat (Whitmore et al 1960, Richards et al 2009)

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Discussion

Conclusion