Infection of sardine eggs by the protistan endoparasite Ichthyodinium chabelardi off Portugal

Abstract

In 1997 and 1999 around 30% of the stations off Portugal sampled for sardine eggs Sardina pilchardus included eggs infected with Ichthyodinium chabelardi. A randomization test on the mean nearest neighbour distances of parasitized stations did not reveal evidence of significant spatial clustering. The mean prevalence of infection was c. 0·05 for both years, but the probability of parasite detection increased considerably with egg age. Eggs in their first day of development (before the embryo is formed) were not parasitized; most parasitized eggs were in the cohorts close to hatching. Although the reasons for age-dependent detection are unknown, if all parasitized eggs of the cohort ready to hatch were to die, infection by Ichthyodinium chabelardi would lead to mortality rates similar to those reported for the average daily mortality of sardine eggs and early larvae.