Size limits for Ballan wrasse use in salmon farm cages

Abstract

Ballan wrasse (Labrus bergylta) is an important cleaner fish species used by the salmon farming industry for sea louse (Lepeophtheirus salmonis) control and demand for it is high. However, supply of wild-caught fish is limited and most ballan wrasse used by the industry are farmed individuals. Ballan wrasse can escape from aquaculture cages if netting mesh size is too large for retention. Further, mesh shape and state can also affect the size of ballan wrasse that can escape through the cage netting. Escaping fish may result in genetic contamination of local fish populations through hybridization and spread of diseases. Based on morphology measurements combined with fall through tests, the escape risk of ballan wrasse was predicted using computer simulation. The simulations considered fish morphology and tissue compressibility, and square meshes between 10 and 100 mm in stiff, semi-slack and slack states. The predictions showed that the escape risk increases with mesh size and mesh openness, peaking at an openness of ca. 90 %, and was highest for slack meshes. Size selectivity results from crowding experiments in the laboratory complied well with the simulation results and demonstrated the importance of considering mesh states other than stiff meshes when evaluating the potential escape of ballan wrasse through meshes in aquaculture cages. Finally, the morphology of wild and farmed ballan wrasse was compared to conclude that size selectivity predictions for farmed ballan wrasse cannot be used to predict the escape risk for wild fish because the latter exhibits a smaller cross section.