Powell A (2021) Rubber net mesh reduces scale loss in farmed Atlantic Salmon, Salmo salar. Recent advances in animal welfare science VIII: Virtual UFAW Animal Welfare Conference, Online, 29.06.2021-30.06.2021.
Abstract Atlantic salmon, Salmo salar, are an economically and ecologically important fish species that interact with humans during farming, fishing, environmental monitoring and research operations. Routine handling in nets is an almost unavoidable consequence of these activities, exposing fish integument to net mesh. This can cause scale loss and subsequently exposes fish to pathogenic and osmoregulatory stressors.
To promote welfare and experimental refinement, a simple method was established to allow scale recovery, analysis and comparison, during netting and routine stock management. A study was performed in a controlled environment to investigate the effect of net mesh type (rubber coated or conventional knotless equivalent) and the number of fish per net (capture density) on scale loss for two size grades of salmon.
Up to 3 large adult salmon (mean 900 g) or up to 15 small smolts (mean 145 g) were briefly captured in hand-nets (both bag volumes ca. 7 L, mesh size 6 mm) during routine immersed stock movement between tanks, using a variety of circular transportation containers to allow at least 10 L volume per fish. After fish recovered and were removed, scales were collected and counted from containers, confirming that for both size grades scale loss was generally proportional to capture density.
For large adult salmon, scale loss significantly increased with capture density when knotless mesh was used, however the relationship was weaker and not statistically significant for adults handled in rubber mesh.
Small smolts also demonstrated significantly reduced scale loss when handled with rubber mesh compared to knotless mesh. Small smolts handled in knotless mesh also showed a more marked increase in scale loss, as capture density increased.
The scale recovery method suggests a low-tech and rapid approach to safeguard fish welfare, quantitatively compare net types and optimise husbandry techniques. This proof of principle study also suggests fundamental but simple improvements to fish handling and net design.