Optimising handling in salmon aquaculture (2): The effect of mesh type on cryptic lesions and hygiene

Abstract

Atlantic salmon, Salmo salar, interact with humans during recreational and commercial activities, in both freshwater and marine environments. This involves routine hand netting, with a modest body of literature proving that handling techniques which reduce abrasion also minimises scale loss and likely improves fish welfare. In a recent study, the use of a rubber mesh, compared to a knotless equivalent, was shown to reduce scale loss in two size cohorts of the study species during routine movement between tanks. The current study aimed to investigate this further via a Fluroscein dye technique, now commonly used to visualise cryptic damage to the mucus membranes of fish (eyes and more recently, skin). Although the technique was shown to work using easily available and cost-effective consumables, negligible damage was observed in any of the individually handled salmon regardless of mesh type. Previous studies have shown that the extent of observable skin damage is influenced by species, behaviour and anaesthetic technique. Although encouraging for this particular stock, it is unknown if higher net capture densities (i.e. more than one fish per net) or stock undergoing smoltification would be so robust. Scale loss data collected simultaneously proved a link between fish size and the extent of scale loss. A positive correlation was of individual smolts netted with knotless mesh. This was not significant for rubber mesh which showed low scale loss regardless of fish size, suggesting a further benefit of using the latter mesh type and supporting the findings of previous studies. Finally, in vitro microbiological study exposed sterile mesh fragments to effluent tank water allowing absorbance and adherence of bacteria to the mesh matrix. Subsequent incubation in sterile saline then permitted estimation of bacterial transfer between the exposed mesh to a further medium. Knotless mesh transferred more Vibrio spp. than rubber mesh, and this was significantly greater for total heterotrophic bacteria. This is likely due to the greater surface area and absorbance of infected water found in the filaments of knotless mesh. Encouragingly, following exposure to a standard aquaculture disinfectant, both mesh types were found to be sterile. These findings further support the likelihood that rubber mesh reduces scale loss in Atlantic salmon, and also suggests reduced microbial transfer when rubber mesh is used for routine handling.