Evaluation of winter severity on growth of young-of-the-year Atlantic salmon

Overwinter growth of young-of-the-year (age-0) Atlantic salmon Salmo salar was studied during two winters in central New York streams. We hypothesized that longer and colder winters would negatively influence the growth of age-0 Atlantic salmon. Further, we predicted that Atlantic salmon growth and survival would be positively correlated with their size before the winter period. Four cages were placed in streams at three sites in the winters of 2000 and 200 1, with six individually marked Atlantic salmon per cage. One cage was retrieved monthly at each site and the weight, length, and energy density of each fish were measured to calculate growth rate and condition factor. All fish survived the winter experiment irrespective of starting size. Mean energy density of fish at the conclusion of each experiment was positively related to fish weight at the start. Winter conditions were different between years, stream temperatures being colder in 2000 than in 2001 (overall winter mean [SD], 0.71 degrees C [2.487 degrees C] versus 1.93 degrees C [2.87 degrees C]), accompanied by less variable temperatures, lower discharge, and more ice cover. Growth rates were negative during both winters, weight loss being significantly greater in the year with wanner, more variable temperatures. Growth rates were also significantly different between months, weight loss being greater in December and January and less (or replaced by positive growth) by the end of the winters (March). Mean energy density of salmon increased during the colder winter of 2000 but declined in 2001. Based on the observations in the current study, lower threshold temperatures for feeding activity may be even lower than previously reported, possibly near 0 degrees C. We discuss the energetic implications of variable winter conditions on age-0 Atlantic salmon.