Accumulated snow and summer temperature -: critical factors for recruitment to high mountain populations of brown trout (Salmo trutta L.)

In lake Krokavatn, 1236 m a.s.l., only six strong year-classes of brown trout appeared during the period 1970-2003. Eleven year-classes were missing during the same period. The observed recruitment failure depended significantly on the accumulated snow depth in April, whereas the mean August temperature in the year of birth was significant for the appearance of strong year-classes. Size of the young-of-the-year (YOY) trout at onset of the winter seemed to be crucial for survival. Also little snow and low temperatures during the winter may have led to recruitment failure, as small nursery streams may freeze completely under such conditions, as happened during the winter of 1995/1996. Thus, the recruitment to brown trout populations in western high mountain areas of Norway seemed to be strongly affected by accumulated snow depth and summer temperatures. A climate change with more winter precipitation, as predicted for the present century, may therefore be detrimental to recruitment. However, warmer summers may increase recruitment to levels that lead to overpopulation, but also to establishment of brown trout populations at higher elevations than today.