Year-class strength, physical fitness and recruitment cycles in vendace (Coregonus albula)

Abstract

Vendace, an obligate zoo-planktivore through all life stages, is a key-stone species in many large lakes in northern Eurasia. Vendace often produces strong year-classes in cycles of various length. To better understand the factors that determine the emergence of strong year-classes we analyzed long time-series hydroacoustic data on abundance of young-of-the-year and older vendace, and representative samples of vendace from trawl catches measured for length, weight and age. Data were collected in the three largest lakes in Sweden – Lakes Vänern, Vättern and Mälaren – for 1995–2012, 1992–2012 and 2008–2012, respectively. The lakes range from ultra-oligotrophic to mesotrophic. In L. Vänern there is an extensive commercial fishery on vendace, whereas the fishery, at present, is small in L. Mälaren and negligible in L. Vättern. Size of mature vendace, expressed as 90% of Lmax, differed between the lakes and was positively correlated with lake nutrient levels. Strong year-classes did not occur in synchrony between lakes, not even between the two main basins within oligotrophic L. Vänern, and were less frequent in the ultra-oligotrophic and commercially unfished L. Vättern. Strong year-classes negatively affected physical fitness (Fulton's condition factor) of older fish. During years of low physical fitness in sexually mature fish no strong year-classes would appear. Recovery of physical fitness might take several years and depended on food availability, expressed as nutrient levels or population density. Not until fish had regained individual physical fitness and could allocate energy to gonads, other factors – as physical environmental conditions – might become important for the emergence of a strong year-class.