Projections of Fish Survival in US Streams after Global Warming

To project fish habitat changes of 57 fish species under potential global warming, their suitable thermal habitat at 764 stream gaging stations in the contiguous US was studied. Water temperature records were available at these gaging stations. Global warming was specified by air temperature increases projected by the Canadian Center of Climate Modelling general circulation model for a. doubling of atmospheric C02. The thermal regime at each gaging station' was related to air temperature using a nonlinear stream temperature/air temperature relationship.' Suitable fish thermal habitat was assumed to be constrained by both maximum temperature and minimum temperature tolerances. Streams with weekly water temperatures between the maximum and minimum temperature tolerances of a fish species were considered to provide suitable thermal habitat for that fish species: Because information on minimum temperature tolerance of many fish species could not be found, both a 0 °C and a 2 °C lower temperature constraint were' applied. There is a 38% difference in the number of stream gaging stations, with suitable thermal habitat for warmwater fishes, when 0 °C or 2° C is used as the lower temperature constraint. The total number of stations with suitable thermal habitat for warmwat~r fishes is projected not to decrease under 2XC02 climate conditions, regardless of whether a 0 °C or a 2 °C lower temperature constraint is applied but a· northward spread of warmwater fishes under 2XC02 climate conditions is projected. For cold-water and cool-water fishes, the number of stations with suitable thermal habitat under a 2xC02 climate scenario is projected to decrease by 36% and 15%, respectively, with 0 °C as the lower temperature constraint. With a 2 °C lower temperature constraint,. more suitable habitat for cool-water fishes and little change for cold-water fishes is projected. It is therefore concluded that the values of the lower temperature constraint, in addition to the upper temperature consp-aint, have a large influence on thermal habitat projections.