The treatment of sea louse infestations using a chitinolytic enzyme composition, particularly in salmon or trout and in farmed fish aquaculture is disclosed. The enzymes hydrolyse or digest chitin and may be chitinases, chitobiases, chitin deacetylases, chitosanases and lytic polysaccharide monooxygenases, particularly one or more of ChiA, ChiB and ChiC. The sea lice may be the parasitic crustaceans Lepeophteirus salmonis (Lepeophteirus salmonis salmonis and Lepeophteirus salmonis oncorhynchi) or Caligus elongates. The treatment may be performed at a temperature of between 4°C and 20°C, a pH of between 7.0 and 9.0, the sea lice are is exposed to the composition for up to 24 hours for each dose. Preferably the fish are Salmo salar, Salmo trutta, Oncorhynchus clarkii, Oncorhynchus gorbuscha, Oncorhynchus keta, Oncorhynchus kisutch, Oncorhynchus masou, Oncorhynchus mykiss,Oncorhynchus nerka, Oncorhynchus tshawytscha or Salvelinus species.
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