Considerable gains in reliable delousing efficacy of farmed cleaner fish (ballan wrasse Labrus bergylta and lumpfish Cyclopterus lumpus) can be achieved by improving the behavioural quality of hatchery reared cleaner fish. Delousing behaviour is known to be highly variable between individual ballan wrasse and lumpfish within a population. Sea lice foraging is a difficult trait to phenotype, and here fish personality and coping style are used as a proxy for delousing ability that can be used to select good delousers for ongoing domestication programmes.
A tank bioassay was used to predict the behaviour components of ballan wrasse and lumpfish. Individual fish (n = 60 per species) were presented with consecutive stimuli (shelter/open water, novel object, mirror). Fish were video recorded and videos were analysed to predict levels of activity, anxiety, sociality, boldness and aggression for each fish. Then, each fish was moved to a tank containing 3D salmon models with 0, 1 or 6 model sea lice. Manual observations were recorded to test the interactions of cleaner fish with salmon models as a proxy for real salmon and lice. Results show that overall lumpfish are bolder than ballan wrasse as they are quicker to leave the shelter (P < 0.05), approach the novel object more often (P < 0.05) and engage with the salmon models more than the wrasse (P < 0.001). Both species prefer model salmon with more sea lice (P < 0.001).
In a similar experiment, individual ballan wrasse and lumpfish (n = 24 per species) were tested using the same behaviour bioassay as previously except videos were analysed using the R package “EBimage”. The fish were then individually cohabited in a tank with 10 Atlantic salmon (Salmo salar) infected with adult sea lice (Lepeophtheirus salmonis, mean n per fish = 10). Sea lice on the salmon were counted at 0h, 48h, 96h and 144h to calculate delousing rates per cleaner fish and allow delousing rates to be correlated with behaviour components. Delousing in both species was highly variable but only 27% of the wrasse and 17% of the lumpfish deloused more than 20% of lice after 144h. Wrasse delousing was positively correlated with approaches to the novel object and negatively correlated with mean velocity, whereas lumpfish delousing was not strongly correlated with behaviour.
Delousing ability between individual ballan wrasse and lumpfish is highly variable, and these results show that a limited number of fish within farmed populations are good delousers. Whether personality and delousing ability are heritable traits is yet to be determined, but simple behaviour tests could be used to predict delousing ability in cleaner fish for selecting potential broodstock for breeding future generations of farmed cleaner fish.