The Winter Infection of Sea Lice on Salmon in Farms in a Coastal Inlet in British Columbia and Possible Causes

An increase in infections with two species of sea lice occurred on farmed salmon farthest up an inlet in the Broughton Island area of British Columbia during the winter of 2005/2006. The increase in the chalimus stage started at the end of November 2005 at a rate of 0.03 lice/day for Lepeophtheirus salmonis and 0.015 lice/day for Caligus clemensi. The increase in the infection started at a time of high salinity and low sea surface temperatures with very few gravid sea lice detected on the farmed fish. The mobile stages increased in early January about one month after the increase in the chalimus stage. Gravid lice increased in abundance about the time the farmed fish were treated with SLICE® in early February. This pattern of an increase in infection in the winter was similar in two nearby farms. In January of 2008 the three farms in the study area were either treated with SLICE® or the fish were harvested. Despite the reduced capacity of the salmon farms to produce sea lice, the juvenile stages remained abundant on sticklebacks ( Gasterosteus aculeatus ) around the farms in February and March of 2008. Sticklebacks were heavily infected with sea lice but were not a host of gravid lice. Trawl studies in the vicinity of the farms did not find an abundance of hosts except for sticklebacks. Some of the infection on the sticklebacks could originate from a low level of gravid lice on the farmed fish in the study area. However, we speculated that a major source of the winter infection on the sticklebacks could result from the transport of infectious stages in the deeper water that flows into the area as a consequence of the estuarine circulation. The source of these lice and the lice in the 2005/2006 study could be natural or from untreated fish farms farther down the inlet or both. Continued research is needed to understand the biology and population ecology of both species of sea lice in order to manage their production