Abstract
Sampling protocols and water quality sensors have been developed to assess fish health and welfare in recirculating aquaculture systems (RASs). Still, the use of fish-based non-invasive welfare indicators, reflecting the physiological state of the fish, is limited in this type of system. Cortisol, the major stress-coping hormone in fish, diffuses through the gills. Consequently, waterborne cortisol is a potential fish-based non-invasive welfare indicator in RAS. However, its use in commercial rearing systems is sparse. In this study, we evaluated water cortisol levels and feeding behavior as welfare indicators of newly inserted smolt in a commercial RAS for harvest size Atlantic salmon. The RAS consisted of two parallel fish rearing raceways, raceways 1 and 2, sharing the same water treatment with common outlets and inlets. The smolts were inserted in raceway 1 while salmon that have been in the system for 6 months or more were kept in raceway 2. The smolt insertion period was 3 days. Samples for water cortisol levels were withdrawn the day before, 1 and 3 days after the smolt insertion period. Smolt insertion resulted in elevated water cortisol concentrations in the entire system, with the highest values in raceway 1, one day after smolt insertion. Estimated cortisol production in newly inserted smolt decreased over time, was similar to what has been reported in salmon adapting to experimental tanks. Feeding behavior indicated that the appetite was not fully resumed in the newly inserted smolts, while the appetite of fish in raceway 2 was unaffected by smolt insertion. These results, obtained in a highly intensive commercial RAS, suggest that waterborne cortisol together with feeding behavior can be used as indicators for adaptive processes associated with stress resilience in farmed fish. Thus, they are promising non-invasive indicators for assessing the impact of potential stressors on fish welfare in this type of rearing system.
Highlights
Recirculating aquaculture systems (RASs) minimize the environmental impact and provide a venue to balance aquaculture growth and environmental/ethical considerations (Martins et al, 2010)
Since steroids are quite stable in water and play a central role in stress coping and health of fish, many of the studies on water-borne hormones focus on these types of hormones or their metabolites in the rearing water
The cortisol concentrations were changed significantly over time in raceway 1 [ANOVA: F(2,6) = 165, p < 0.001] and increased significantly compared to values before smolt insertion (p < 0.001). This increase was highest the day after smolt insertion, showing significantly higher values compared to water sampled 3 d after smolt insertion (p < 0.05), Table 2
Summary
Recirculating aquaculture systems (RASs) minimize the environmental impact and provide a venue to balance aquaculture growth and environmental/ethical considerations (Martins et al, 2010). Fish-based non-invasive welfare indicators are often based on behavioral and respiratory changes, and the use of other measures is limited (Baretto et al, 2022). Since steroids are quite stable in water and play a central role in stress coping and health of fish, many of the studies on water-borne hormones focus on these types of hormones or their metabolites in the rearing water (reviewed by Scott and Ellis, 2007). Several studies show a strong relationship between blood plasma levels of cortisol, the main stress-coping hormone in fish, and the release of free cortisol to the rearing water [for references see review by Scott and Ellis (2007)]. Its use as a non-invasive fish-based welfare indicator in commercial RAS is limited
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