Microhabitat use by juvenile Atlantic salmon (Salmo salar ) sheltering during the day in summer

Abstract

Daytime snorkeling surveys were conducted in the Wardsboro Branch, a tributary of the West River, Vermont, U.S.A., in July and August 1996. We documented microhabitat use by 245 juvenile Atlantic salmon (Salmo salar) sheltering (i.e., concealed beneath the stream substrate) at water temperatures ranging from 17 to 23°C, well above 10°C, the maximum temperature at which young salmon are thought to shelter during the day. The majority (92%) of sheltering salmon were young-of-the-year salmon (YOY). Of the YOY observed, 45% were sheltering, while 55% were in the water column. In comparison, only 10% of post-young-of-the-year salmon (PYOY; age 1 or older) observed were sheltering, while 90% were in the water column. Sheltering PYOY occupied greater water depths and were found under larger substrate stones than were YOY. Sheltering salmon (YOY and PYOY) were not distributed in proportion to the available microhabitat. Salmon only sheltered beneath unembedded cobble or boulder substrate, and sheltering salmon were found in pool habitats 43% of the time. Daytime sheltering suggests that the current interpretation of juvenile salmon habitat use and behavior during summer is incomplete. The availability of suitable sheltering habitats may be a factor affecting juvenile salmon production.