Action: Use natural control agents: cleaner wrasse
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- Two studies in Ireland found mixed effects of cleaner wrasse on sea lice numbers infesting salmon. One controlled study found corkwing and goldskinny cleaner wrasse were as effective at controlling lice infestation as chemical treatments. One replicated, controlled study found rockcook cleaner wrasse were ineffective at preventing lice outbreaks.
Commercial salmon farms are susceptible to outbreaks of sea lice. Traditionally, control of sea lice has involved pesticide use, either through oral administration or as a bath treatment. Cleaner wrasse feed on several species of sea lice. Therefore they have been investigated as an alternative to pesticides as a control agent.
Supporting evidence from individual studies
Between 1991 and 1992, a controlled study (Deady et al., 1995) at two salmon farms off the west Irish coast found the numbers of lice, Caligus elongates, infesting salmon, Salmo salar, were low in cages containing corkwing and goldsinny cleaner wrasse. Similar levels of lice were found in wrasse stocked cages and those chemically treated with a pesticide. Infestation levels were on average five lice per fish. Cleaner wrasse were as effective as chemical treatment at wrasse: salmon ratios as low as 1:250. The first farm used eleven ‘Polar Circle’ cages (12m depth, 20m diameter). One cage was stocked with 188 cleaner wrasse and 47,000 salmon smolts. All other cages were given chemical treatment for the duration of the study. At the second farm, two groups of eight square ‘Turmec’ cages (10m depth, 10m width) were stocked with 5,000- 8,000 salmon smolts each. Eight cages were stocked with 500-800 cleaner wrasse. The remaining cages were chemically treated for sea lice. On both farms, random samples of 15 salmon were removed from each cage every 1-2 weeks to monitor lice infestation levels.
In 1992, a replicated, controlled study in Ireland (Tully et al., 1996) found rockcook cleaner wrasse failed to consistently reduce numbers of lice, Caligus elongates, in sea-caged salmon, Salmo salar. When compared to control cages, the wrasse controlled infestation levels, preventing an outbreak on only one of the seven sampling periods. Each of five circular salmon cages (50-70m circumference, 10m depth) was stocked with between 13,000 and 24,000 salmon smolts. Wrasse were added to four cages at wrasse: salmon ratios between 1:37 and 1:49. Wrasse were not added to the control cage.
- Deady S, Varian S.J.A. & Fives J.M. (1995) The use of cleaner- fish to control sea lice on two Irish salmon (Salmo salar) farms with particular reference to wrasse behaviour in salmon cages. Aquaculture, 131, 73-90
- Tully O., Daly P., Lysaght S., Deady S. & Varian S.J.A. (1996) Use of cleaner- wrasse (Centrolabrus exoletus (L.)) and Ctenolabrus rupestris (L.)) to control infestations of Caligus elongatus Nordmann on farmed Atlantic salmon. Aquaculture, 142, 11-24