Net-biting and escape behaviour in farmed Atlantic cod Gadus morhua: effects of feed stimulants and net traits

Abstract

Preventing farmed fish from escaping sea cages is a major challenge for developing sustainable aquaculture practices. In this study, the individual behaviour of Atlantic cod Gadus morhua to inspect and bite the net was investigated in 4 replicate multi-patch experiments, using underwater video techniques and behavioural event recording. The experimental system included a tank enclosure where cod were presented with 6 separate net panels: duplicate pairs of plain control nets and 2 different treatment panels. Four holes were made in the treatment panels, 3 of which were mended with different repair techniques including short and loose thread ends and contrasting colour repair threads. Behavioural effects of feed stimulants were studied before, during and after a feed source was placed outside the net. The cod inspected and bit the net systematically, significantly focusing on loose threads or minor damage on the net, and the fish were capable of causing severe net damage within a few days. The observed behaviours were related to hunger level and feed stimulation, and increased significantly during times when a feed source was present outside the net wall. Few bites were observed, and almost no measureable damage was recorded to plain control panels and the area around unrepaired holes during the treatment. Repairs made with white and black threads, and loose, long ends were significantly more damaged than repairs made with short thread ends and threads of similar colour as the net. The experiments demonstrated how a behavioural approach might be used in interpretation of a practical problem in aquaculture, and the results will be useful in the development of technology and escape mitigation strategies.

Rapid morphological divergence of cultured cod of the northwest Atlantic from their source population

“…When cultured cod escape from net-pens, they interact with wild cod, and are subjected to the selective pressures of the natural environment (Moe et al 2007, Damsgård et al 2012, Zimmermann et al 2012. It is likely that the morphology developed by the cod in culture will be to some degree maladaptive in the wild, and thus any escapees will experience lower fitness than their wild counterparts, as has been seen in other species (Fleming et al 2000, McGinnity et al 2003, Meager et al 2010, Skaala et al 2012).…”

Wild Atlantic cod sperm motility is negatively affected by ovarian fluid of farmed females

“…In the relatively well-studied Atlantic salmon Salmo salar, most escape events are related to structural failures of equipment, either due to human error or harsh weather conditions (Jensen et al 2010). In Atlantic cod Gadus morhua, the same problems exist, but exploratory behaviour and net biting are important additional drivers of escape events (Moe et al 2007, Damsgård et al 2012, Zimmermann et al 2012), contributing to 10-to 20-fold greater rates of escape than observed in salmonids (Moe et al 2007). …”

Post-escape dispersal of juvenile Atlantic cod Gadus morhua from Norwegian fish farms and their potential for recapture

“…Cod exhibit exploratory behaviors that increase the chance of fish detecting a hole and escaping (Hansen et al 2008). In addition, cod can bite the cage netting materials (Moe et al 2007, Damsgård et al 2012, and this biting behavior can weaken the net ). At present, direct evidence of the negative ecological impacts of escaped cod is sparse.…”