No abstract

This study examined expression levels of two heat shock proteins, hsp72 and hsp78, in white muscle of steelhead (anadromous rainbow trout Oncorhynchus mykiss) parr collected between 25 July and 4 August 2000 at 11 sites in the Navarro River watershed, California. The goal was to determine whether site‐specific thermal conditions were causing cellular stress responses in resident fish. The results demonstrated a highly significant sigmoidal relationship of the inducible isoform hsp72 and a linear relationship of the constitutive isoform hsp78 with water temperatures. Laboratory experiments showed that hsp72 was induced in fish from coldwater as well as warmwater sites by exposure to water at 25°C. Stream temperatures above which significantly elevated hsp72 levels were detected in field‐collected fish were 18–19°C in terms of both short‐ and long‐term averages and 20–22.5°C in terms of daily maximum averages. The highest hsp72 levels were measured at warmwater sites with the largest diurnal temperature fluctuations (≥6.5°C). The two dominant factors influencing water temperature were air temperature and the degree of shade provided by riparian vegetation. Our data suggest that elevated hsp72 expression levels in steelhead parr from several tributaries are indicative of cellular stress caused by thermal conditions during summer months.