Atlantic salmon (Salmo salar) in winter: "the season of parr discontent"?

Winter is a dynamic period. Effects of the winter regime on northern stream s and rivers is extremely variable and characterized by dramatic alteration s in physical habitat to which Atlantic salmon (Salmo salar) must acclimate and adapt to survive. In this paper, we synthesize recent advances in the biological and hydrologic/geomorphic disciplines, with specific reference t o Atlantic salmon overwintering in the freshwater portions of those running waters subject to freezing water temperatures. The specific requirements a nd adaptations for surviving winter at the three distinct life-stages in fr eshwater (egg, parr, kelt) are identified in relation to the characteristic s of three biophysical phases: early winter (temperature decline and freeze -up), midwinter lice growth and habitat reduction), and the break-up/warmin g phase. In a case study of Catamaran Brook (New Brunswick), a hydro-ecolog ical analysis was used to explain interannual variability in juvenile abund ance, especially for young-of-the-year salmon. A strong relation was found between winter discharge and interstage survival (egg to 0+, 0+ to 1+, 1+ t o 2+) in 5 of 6 years. That is, juvenile salmon abundance in summer was hig hest following winters with high streamflow, presumably a function of habit at availability, especially beneath ice cover. However, the lowest measured egg-0+ survival (9.2%) was related to an atypical midwinter, dynamic ice b reak-up triggered by a rain-on-snow event that resulted in severe scouring of the stream-bed and redds. Thus, interannual variability in Atlantic salm on parr abundance from 1990 to 1996 was largely explained by density-indepe ndent (environmental) constraints to winter survival. The complexity of str eam processes during winter underscores the need for interdisciplinary rese arch to quantify biological change.