Infection threshold to estimate Lepeophtheirus salmonis-associated mortality among juvenile pink salmon.

A threshold of lethal infection was estimated from previous controlled laboratory exposures to be 7.5 Lepeophtheirus salmonis g(-1) for pink salmon Oncorhynchus gorbuscha averaging < 0.7 g. This threshold was used to assess the risk of mortality caused by L. salmonis among pink salmon of the same size class in the Broughton Archipelago, Canada from 2005 to 2008. Virtually all (> or = 98.9%) pink salmon collected in late March belonged to this size class, and this proportion declined to < or = 1% by early July. The proportion of these small pink salmon with infections equal to or exceeding the threshold declined from 4.5 in 2005 to 0% in 2008, coincident with an overall decline in parasite prevalence and intensity during this period. In 2005 and 2006, this proportion was greatest in March (7.8 and 1.1%, respectively) whereas in 2007, the proportion exceeding the threshold was greatest in May (2.9%). In 2008, no infections exceeded the threshold. Parasite development coincided with fish migration through the study area. The declining risk between 2005 and 2008 was possibly related to changes in ocean conditions such as temperature, to changing treatment practices for this parasite on salmon farms, or to changes in the abundance or distribution of non-farmed hosts. The concept of a threshold of L. salmonis infection density may be used to assist in the management and conservation of juvenile pink salmon in the Broughton Archipelago region.