Breeding critically endangered European weatherfish in captivity in Flanders (Belgium): a holistic approach also conserves threatened parasitic flatworms

Abstract

Despite its large Eurasian range and global Red List status of Least Concern, in Flanders (Belgium), European weatherfish Misgurnus fossilis (Teleostei, Cobitidae) is a critically endangered freshwater fish. Conservation actions for this species in Flanders include ex-situ captive breeding. The parasite fauna of the European weatherfish in Belgium was never studied. Conservation science often overlooks parasites or sees them as threats to the species targeted by conservation programs. Conversely, not only are parasites conservation-relevant in their own respect, a holistic approach to captive breeding recognises positive contributions of parasites to resilience of their farmed hosts. With this in mind, we investigated the parasite fauna of European weatherfish in the Flemish captive breeding program, focusing on monopisthocotylan flatworms. These helminths are the dominant ectoparasites of bony fishes, and mainly infest gills and external surfaces. Several species naturally parasitizing on European weatherfish were proposed to be threatened by extinction in Czech Republic and Slovakia following IUCN criteria in 1997. We hypothesized that captive breeding of European weatherfish also allows maintenance of native and potentially threatened species of these flatworms, and that certain artificial conditions may be associated with higher abundances of these parasites and possible fish mortality. Our parasitological survey, including morphological and genetic characterisation, found three monopisthocotylan species on captive weatherfish: Actinocleidus cruciatus (Dactylogyridae) and Gyrodactylus misgurni (Gyrodactylidae) on gills, Gyrodactylus fossilis on skin. All three are new records for Belgium, and were suggested to be conservation-sensitive in Czech Republic and Slovakia. Scrutinising conspecific hosts from 1932-1973 in the vertebrate collection of the Royal Belgian Institute of Natural Sciences supported the status of the former two species as native to Belgium. We further provide new molecular resources that can be used for detecting these policy-relevant parasites in captivity and in nature. In the hatchery, juvenile weatherfish indeed suffered from massive infection by one of these parasites, G. fossilis. As adult weatherfish maintained at lower densities still host this parasite without visible harm, this breeding program can be used as a case study to conserve several host-parasite combinations without jeopardising the fish. In view of the presumed rarity of these parasite species, and of the health benefits that helminths can even have for their hosts (e.g. in development of immunity), we propose conservation efforts to also consider monopisthocotylan flatworms when reintroducing weatherfish in nature, obviously in discussion with stakeholders.