Stock discrimination of South African sardine (Sardinops sagax) using a digenean parasite biological tag

Spatial and temporal variation in infection of South African sardine, Sardinops sagax, by a metacercarial parasite was examined to test the hypothesis that the South African sardine population comprises multiple stocks. Samples of adult S. sagax were collected monthly from four commercial fishery landing harbours off the west and south coast and three indices of infection (infection prevalence, mean infection intensity and mean parasite abundance) were determined for each sample. Generalized linear models (GLMs) were used to assess the relative importance of putative stock (sardine caught off the west coast were assumed to be from the putative “western” stock whereas those caught off the south coast were assumed to be from the putative “southern” stock), season, year, fish size and all possible two-way interactions, where a binomial distribution was assumed for prevalence and a negative binomial distribution assumed for infection intensity and parasite abundance. The most parsimonious model for each index was selected based on Akaike's Information Criterion, and significant GLMs were derived for all three indices. Putative stock was the most significant contributor to explaining deviance seen in infection intensity and parasite abundance, and was the second most important contributor to explaining deviance seen in infection prevalence, with fish from the putative western stock having significantly (p<0.001) higher parasite loads compared to those from the putative southern stock. Temporal variability was also observed, with season and year also contributing significantly to all three models. Despite this temporal variability, the marked spatial difference in the distribution of infection of S. sagax by metacercariae supports the hypothesis of western and southern stocks of sardine off the coast of South Africa. The presence of infected large sardine off the south coast indicates that these putative stocks are not discrete and that some degree of mixing occurs between them, at least from the western to the southern area.