Metazoan parasite communities in Alosa alosa (Linnaeus, 1758) and Alosa fallax (Lacépède, 1803) (Clupeidae) from North-East Atlantic coastal waters and connected rivers.

Metazoan parasites were studied in 96 Alosa alosa and 78 Alosa fallax from North-East Atlantic coastal waters and connected rivers (among them three sympatric sites) in order to increase knowledge on these anadromous endangered fish and measure the parasitic impact on host condition. All shads were infected by one to six metazoan parasite taxa among the 12 identified in the whole sampling, with a mean abundance of parasites higher for A. alosa (167 ± 10) than for A. fallax (112 ± 11). Helminths, mostly trophically transmitted, were the best represented (eight taxa, prevalence up to 99%) in contrast with crustaceans and Petromyzontidae that rarely occurred (four taxa, prevalence <6%). Despite some quantitative differences, metazoan parasite communities of A. alosa and A. fallax remained stable in composition whatever the host developmental stage, sex, sample site, and salinity. Among the nine parasite taxa harbored by each Alosa species, six were shared with some differences in distribution patterns including in sympatric conditions, suggesting increasing dissimilarities between A. alosa and A. fallax with the age. Information on feeding ecology provided by trophically transmitted helminths confirmed euryphagous opportunistic diet of immatures and adults of both shad species, and assessed feeding of adults during spawning migrations. Our study also revealed the significant negative impact of Hemiurus appendiculatus on A. alosa and Pronoprymna ventricosa on A. fallax. Because helminth parasites are omnipresent in the shads and decrease their fitness, parasitological data must be included in further investigations and management programs on A. alosa and A. fallax.