Influence of Drake Passage oceanography on the parasitic infection of individual year-classes of southern blue whiting Micromesistius australis

Southern blue whiting Micromesistius australis occurs around the southern coasts of South America and has been known to undertake summer feeding migrations to the Antarctic Peninsula and South Orkney Islands. The largest stock spawns to the southwest of the Falkland Islands and has been the subject of a,major fishery on the Patagonian Shelf since 1978. Fish are infected with cysts of the Myxosporean parasite Kudoa alliaria which make the flesh commercially unattractive for fillets. A 10 yr study initiated in 1989 established that prevalence of the parasite is over 80% for fish older than 1 yr, and that average infection intensities are about 14 cysts per fish. We constructed a combined generalised linear/generalised additive model (GLM/GAM) of parasite abundance in M. australis. The combination of up to 30 ages and 8 yr of data spanning 10 yr allowed us to investigate rates of parasitism in 33 individual cohorts. Strong age and cohort effects in the model implied that the fish acquire parasites in their first 1 to 1 1/2 yr of life and that parasite abundance is set at these ages for the lifetime of the cohort. Several cohorts have statistically significantly lower parasite abundance than the majority, and these instances are roughly coincident with the 7 yr periodicity of sea-ice and sea surface temperature (SST) fluctuations in the Drake Passage. There are significant correlations between SST in the Drake Passage, the duration of sea-ice around Signy Island (South Orkney Islands) with a 1 yr time lag and parasite abundance in a cohort. We hypothesise that these correlations represent changes in the distribution and density of adult M, australis in warm and cold years, which thereby influences the density of the parasite field that juvenile fish encounter.