Long-term decrease of the vendace population in Lake Pluszne (Poland)—result of global warming, eutrophication or both?

Numbers of vendace (Coregonus albula L.) in Lake Pluszne were monitored twice a year from 1999 until 2013 by hydroacoustic methods and pelagic trawls. Since 2002, the numerical density of vendace has decreased continuously. Young-of-year fish were caught in large numbers only in 2001, when a strong year class was recruited, and very few in subsequent years, indicating that natural reproduction after 2001 was hampered. Observed changes in the vendace population were related to temperature and oxygen conditions and we suggest that mainly worsening hydrological conditions, associated with increased eutrophication, have been responsible for scarcity of coregonids.