Field Estimation of a Lethal Oxythermal Niche Boundary for Adult Ciscoes in Minnesota Lakes

Ciscoes Coregonus artedi are coldwater stenotherms that are sensitive indicators of ecological stressors, such as eutrophication and climate warming, that reduce coldwater habitat. Temperature and oxygen profile data were collected during cisco mortality events at 17 lakes during an unusually warm summer in Minnesota in 2006. Combinations of temperature and oxygen from the profiles were mapped onto a two-dimensional niche space to directly quantify an oxythermal lethal niche boundary for ciscoes. Quantile regression was used to estimate the niche boundary that described lethal combinations of oxygen and temperature for cisco. The fitted lethal oxythermal niche boundary exhibited a curvilinear interaction between lethal temperatures and lethal oxygen concentrations. The lethal temperature under normoxia (about 8 mg/L) was estimated to be 24 degrees C. Lethal temperatures were progressively less at lower lethal oxygen concentrations (e.g., the lethal temperature was 23.0 degrees C at 5.0 mg/L, 22.0 degrees C at 3.0 mg/L, and 19.5 degrees C at 1.0 mg/L). The lethal oxythermal niche boundary provides a basis for quantifying coldwater cisco habitat under current and future climate warming and water quality conditions.