Ectoparasitic sea lice (Lepeophtheirus salmonisandCaligus elongatus) infestations of wild, adult, one sea-winter Atlantic salmonSalmo salarreturning to Scotland

Caligid ectoparasitic copepods are major pathological pests on cultured Atlantic salmonSalmo salarL., and their population biology has been well studied in the farm environment. The ecology of caligid infestations of wild salmon is, by contrast, rather poorly understood. We monitored return migrant one sea-winter wild Atlantic salmon in Scotland annually for infestations of 2 caligids,Lepeophtheirus salmonis(Krøyer) (between 1998 and 2005) andCaligus elongatusNordmann (between 1999 and 2005). Prevalence ofL. salmoniswas 100% in all years, whereasC. elongatusprevalence ranged from 90 to 100%. Abundances fluctuated markedly between years andL. salmonismean abundance (min. to max.: 17.4 to 31.0) was significantly greater than forC. elongatus(min. to max.: 2.9 to 23.8) in all except one year. A positive association in abundance of the 2 species, for individual fish within any one year, indicates weak or absent competitive effects on abundances for individual hosts. Individual fish within any one year appeared similarly vulnerable to infestation by either species, although Taylor’s power regression showed clear differences in density-related patterns of overdispersion amongst hosts for the 2 species. Host condition factor (expressed either as Fulton’s index,K, or the relative mass index,W R) showed significant variation among years. Parasite species abundances were not, however, determined by host condition factor, and poor condition fish were no more likely to carry high infestations than were high condition fish.