Pre-deployment acclimatisation of farmed ballan wrasse (Labrus bergylta) to sea-cage conditions promotes behaviour analogous to wild conspecifics when used as cleaner fish in Atlantic salmon (Salmo salar) farms

Abstract

Ballan wrasse (Labrus bergylta) are used as cleaner fish in commercial Atlantic salmon farming to remove ectoparasitic sea lice. While the delousing performance of wild wrasse is usually good, that of farmed wrasse is variable, possibly because of different conditions in hatcheries and sea cages. In this study, three passive-acoustic telemetry (PAT) trials were conducted at a salmon farm to compare the behaviour of wild and farmed wrasse and test the effect of acclimatising farmed wrasse to sea-cage conditions before deployment. Up to 40 fish were monitored simultaneously for 60–124 days by triangulating tag positions within a hydrophone array every 6–10 s. This data was used to assess fish depth and preferred cage locations and calculate activity, orientation and home ranges. Wild wrasse occupied shallower depths (13.3 ± 2.4 m) than farmed wrasse, which remained near the bottom of the cages (18.2 ± 1.7 m). Swimming activity was higher in wild wrasse with significant diurnal variations due to nocturnal quiescence on 68% of observed days. Wild wrasse rapidly developed large home ranges (616.8 ± 110.1 m2), preferring cage corners. Hatchery-and-sea-cage acclimatisation improved the behaviour of farmed wrasse; they rapidly moved up the water column (9.08 ± 2.05 m after 1 week) and established home ranges (514.3 ± 146.6 m2), and they developed diurnal activity patterns, which may indicate a positive response to acclimatisation. Acclimatising farmed ballan wrasse to sea-cage conditions positively improved and encouraged behaviours similar to those seen in wild wrasse, including diurnal rhythms and the establishment of home ranges, and is recommended for all farmed wrasse prior to deployment to improve delousing performance.