Gill worm Ancyrocephalus percae (Ergens 1966) outbreak negatively impacts the Eurasian perch Perca fluviatilis L. stock of Lake Constance, Germany.

The monogenean gill parasites of the genus Ancyrocephalus (Creplin 1839; Ancyrocephalidae, Dactylogyridae) are monoxenous parasites of freshwater fish (Ergens 1966; Simkov a et al. 2003; Simkov a, Matejusov a & Cunningham 2006). Adult trematodes locate mainly on the gills of the host, where they produce eggs that are released into the aquatic environment. The eggs hatch into free-living, ciliated oncomiracidia, which seek a host and migrate as post-oncomiracidia to the gills, where they grow to adulthood (Chubb 1977; Cone & Burt 1981). Two species have been found infecting Eurasian perch, Perca fluviatilis L.: A. paradoxus and A. percae. Whereas A. paradoxus is found mainly on pikeperch Sander lucioperca L. and S. volgensis (Gmelin 1789) and only occasionally on Eurasian perch, A. percae, seems to be host specific to perch (Ergens 1966; Chubb 1977; Kritscher 1988; Moln ar & Sz ekely 1995; Moravec 2001; Simkov a et al. 2003, 2006). Other members of the order Dactylogyridae are well-known causes of fish epizootics, especially under stress conditions such as in crowded aquaculture facilities (Thoney & Hargis 1991). However, to the authors’ knowledge, there have been no reports so far concerning epizootic outbreaks of Ancyrocephalus sp. targeting Eurasian perch in wild or cultured fish populations. In Lake Constance, perch are the second most important species next only to whitefish in commercial significance. Routine surveys on perch cohort strength, growth and health have been conducted regularly over several decades on behalf of the lake fishery commission (Internationale Bevollm€achtigen Konferenz f€ ur Bodenseefischerei; www.ibkf.org), and these include assessments of parasite infection status. Three previous studies have investigated the parasite communities of various fish species in Lake Constance, including pikeperch and perch (Zandt 1924; € Ozcelik & Deufel 1989; Balling 1992). The authors conducted a parasite survey of perch in 2008, in which 255 adult perch from 10 sampling locations covering the whole lake area were investigated between March and December (data unpublished). No cases of Ancyrocephalus sp. infection were reported in any of these previous studies, strongly suggesting the parasites were absent from Lake Constance until recently. In the 2 years before this study, fishermen began to report abnormal lesions on perch caught in the lake. On closer inspection, in some fish, the isthmus was found to have been torn away from the lower jaw. During a routine parasite survey in spring 2012, the authors were able to confirm these observations and identified infection with a member of the genus Ancyrocephalus as the cause. Recognizing this as a new parasite in the locality, the current study set out to identify the species and assess the Correspondence J Behrmann-Godel, Limnological Institute, University of Konstanz, Mainaustrasse 252. 78457 Konstanz, Germany (e-mail: [email protected])