Physiological responses of farmed Atlantic salmon and two cohabitant species of cleaner fish to progressive hypoxia

Malthe Hvas,Frode Oppedal

doi:10.1016/j.aquaculture.2019.734353

Malthe Hvas, Frode Oppedal

Open Access

https://doi.org/10.1016/j.aquaculture.2019.734353

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Abstract

To mitigate salmon lice infestations in Atlantic salmon (Salmo salar) sea cages, deployment of cleaner fish have become a widespread strategy. However, species of cleaner fish may experience poor welfare in the highly fluctuating farm environment owing to differences in physiological adaptations and niche requirements. In particular, occurrences of reduced oxygen levels are common in salmon cages. The purpose of this study was therefore to compare hypoxia responses of Atlantic salmon and two commonly used cleaner fish species, the lumpfish (Cyclopterus lumpus) and the ballan wrasse (Labrus bergylta). We used respirometry to measure metabolic rates (MO2) during progressive hypoxia down to 20% oxygen saturation. In addition, we also measured key haematological parameters before, during and after hypoxia exposure. While all fish survived exposure down to 20% oxygen saturation, distinct differences in metabolic and haematological responses were found, reflecting species specific adaptations and lifestyles. In Atlantic salmon, MO2 was independent of ambient oxygen levels until 27% saturation, after which it decreased linearly. In lumpfish, MO2 steadily decreased throughout the hypoxia trial. In ballan wrasse, MO2 was notably lower than in the other species and unaffected by the levels of hypoxia encountered. Hypoxia induced changes in plasma cortisol, plasma lactate and plasma osmolality were substantially greater in Atlantic salmon compared to both cleaner fish species. This suggests that similar magnitudes of hypoxia exposure were more stressful to Atlantic salmon. Hence, neither cleaner fish species should be in immediate danger as long as hypoxia levels that are known to be detrimental to Atlantic salmon are avoided. However, lumpfish had markedly reduced activity levels at the early onset of progressive hypoxia, and is therefore likely to require near normoxic conditions to efficiently function as cleaner fish.

Highlights

The global production of farmed Atlantic salmon (Salmo salar) has increased from 0.2 million tonnes in 1990 to above 2 million tonnes since 2012, and in terms of total economic value it is presently the largest commodity in finfish aquaculture (FAO, 2018, 2019)
The purpose of this study was to compare the physiological responses to environmental hypoxia between Atlantic salmon, lumpfish and ballan wrasse subjected to similar experimental conditions
Atlantic salmon and lumpfish were of comparable size classes with weights or 164 ± 6 g and 185 ± 5 g, respectively

Summary

Introduction

The global production of farmed Atlantic salmon (Salmo salar) has increased from 0.2 million tonnes in 1990 to above 2 million tonnes since 2012, and in terms of total economic value it is presently the largest commodity in finfish aquaculture (FAO, 2018, 2019). Substantial costs are associated with controlling salmon lice infestations in sea cages (Abolofia et al, 2017), and lice outbreaks may harm wild salmonids (Krkošek et al, 2011, 2013).Various control measures against salmon lice are currently in use and include thermal, mechanical and chemical treatments. These methods can be harmful to the fish and have been associated with increased mortalities (Overton et al, 2018a, 2018b). Most wrasse species deployed in sea cages are wild-caught and currently it is only the ballan wrasse (Labrus bergylta) that is cultured successfully for cleaner fish deployment (Skiftesvik et al, 2013; Powell et al, 2018)

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