Parasite fauna of Baikal oilfishes (Comephorus spp.) (Cottidae, Scorpaeniformes) in Lake Baikal and its changes in the age series of the hosts

The big and little Baikal oilfish are endemic to the Baikal Lake, being the most numerous fish species there. Only these two species make up the group of pelagic fish of Lake Baikal. The Baikal oilfish are characterized by a dispersed vertical distribution, from the surface to a depth of 1000 m or deeper. This study provides a description of the current situation regarding parasite infestation in the big and little Baikal oilfish and, for the first time, provides an analysis of changes in the fauna and numbers of parasites in the age series of these endemics of Lake Baikal. There are 14 and 15 species in the parasite fauna of the big and little Baikal oilfish, respectively. In total, the parasite fauna of both host species is represented by 17 species, of which 12 species are common to both fish species. There is a high degree of similarity in the species composition of parasites in the Baikal oilfish, however, trematodes were found in the little Baikal oilfish only, which indicates differences in ecology between the two host species. A discrepancy was revealed in changes in the species richness of parasites in the age series of the little and big Baikal oilfish, namely: it increases in the first host, while in the second host, the species richness of parasites first increases, then decreases in older age groups. This discrepancy between the Baikal oilfish is associated with differences in the composition of the food bolus in the middle and older age groups of these fish.