Fallowing of marine Atlantic salmon, Salmo salar L., farms as a method for the control of sea lice, Lepeophtheirus salmonis (Kroyer, 1837)

Abstract

Abstract. Infections of Lepeophtheirus salmonis (Krøyer, 1837) and Caligus elongates Nordmann, 1832 were monitored over a 20‐month period on four marine Atlantic salmon, Salmo salar L., farms in Scotland. Three farms were fallowed between harvesting and restocking, and thus, contained only one year class of fish at a time. The fourth site was not fallowed and contained two year classes of fish. Results show that fallowing led to low numbers of L. salmonis on newly introduced fish for several months after stocking, with much less need to use chemotherapy. New fish in the non‐fallowed site became very rapidly infected with L. salmonis and required chemotherapy within 2 months of stocking. Fallowing did not appear to be effective in restricting the abundance of C. elongatus. Longer periods of fallowing were more effective in controlling lice numbers.