Effects of lumpfish size on foraging behaviour and co-existence with sea lice infected Atlantic salmon in sea cages

• We assessed size effects of foraging of lumpfish and co-existence with Atlantic salmon.
• Lumpfish from the smallest size class had a higher consumption of naturally occurring food items, including sea lice.
• Increased growth in salmon co-habiting the two smallest lumpfish size groups was observed.
• Small lumpfish (initial size approx. 20 g) may represent a viable option for long term delousing of Atlantic salmon.

To assess possible size effects of foraging of lumpfish and co-existence with Atlantic salmon with particular interest to the sea lice grazing efficiency, eight sea cages (5 × 5 × 5 m) were stocked with 150 Atlantic salmon with a mean (± SD) weight of 538 ± 14 g. Six of the cages were stocked with 15 lumpfish each (10% density), with two cages for each of three different size groups of lumpfish. Three duplicate groups of lumpfish with an initial mean (± SD) weight of 22.6 ± 0.7 g (small), 77.4 ± 3.6 g (medium) and 113.5 ± 2.1 g (large) were used in the study. Two cages without lumpfish acted as controls. Sea lice infestation levels were recorded at two to four week intervals for 159 days. To determine the diet preferences of lumpfish in the cages gastric lavage was performed at the same time intervals. Behaviour and growth of the lumpfish was assessed throughout the study period and mean weight of the Atlantic salmon measured at the start and end of the study period. From day 35 and onwards growth was higher for the small lumpfish group compared to the two other lumpfish size classes. Lumpfish from the smallest size class had a higher consumption of naturally occurring food items, including sea lice, compared to the other two size classes. Growth stimulation in salmon co-habiting the two smallest lumpfish size groups was observed. Signs of sexual maturation were found in the medium (13%) and the large (20%) size groups. Based on present data small lumpfish (initial size approx. 20 g) have a higher overall preference for natural food items compared to larger conspecifics. Although the sea lice infestation rate was low in the study (< 0.5 lice salmon− 1) final lice burden was 40% lower in salmon groups stocked with small lumpfish compared to the control group without lumpfish.Statement of relevanceThe data presented here are highly relevant for Aquaculture as the effective use of lumpfish for biological delousing of salmon is very important for commercial aquaculture of Atlantic salmon.