Mortality of Sardine and Horse Mackerel Eggs by Parasitism, Impact on the Survival of Early Life Stages

Sardine (Sardina pilchardus) and horse mackerel (Trachurus trachurus) are two key species in marine food webs and have a high commercial value in the Iberian Peninsula. The populations of these species are known to have exhibited marked fluctuations in abundance over the years, however the causes for such variability are still not totally understood. One of the identified sources of mortality in the early life phase, is the parasitism by Ichthyodinium chabelardi, an endoparasite of pelagic fish eggs, which leads to the host death. A total of 4000 ichthyoplankton samples, and over 30,000 eggs, from 10 surveys conducted by IPMA, in the Iberian Atlantic area, were analyzed to assess the percentage of eggs infected by the parasite (prevalence). The results showed interannual variability in the infection prevalence, with values between 0.80% and 4.32% for sardine eggs and between 0.82% and 10.97% for horse mackerel eggs. The number of samples with infected eggs varied between 2.53% and 15.32% in the case of sardine eggs and between 2.00% and 12.59% for horse mackerel eggs. GLM analyses highlighted location variables, as the main responsible for the spatial distribution patterns observed. Comparisons of infection prevalence and total egg mortality values obtained by the daily egg production method estimations clarify the relevance of I. chabelardi infection for sardine egg mortality.