Introduction

In 1979, we proposed the idea of a thermal niche for fishes as analogous to niches for other resources such as food type and size (Magnuson, Crowder & Medvick, 1979). In our view, competition occurs for space with the appropriate thermal properties in a manner similar to competition for places suitable for refuge or spawning. We concluded that ‘fish do compete for thermal resources and that considerations of temperature as a resource are generally consistent with the characteristics of food as a resource.’

We defined the thermal niche of fishes as the preferred temperature ± 2 °C or ± 5 °C. This definition was derived from laboratory gradient studies which showed that fish spend two-thirds of their time within 2 °C and all of their time within 5 °C of their preferred temperature (Magnuson et al., 1979). A large number of, but not all, performance optima for individual species occur within these ranges (Magnuson et al., 1979; Jobling, 1981). In North America, freshwater fish have been grouped into three thermal guilds – coldwater, coolwater and warmwater – based on their temperature preference (Hokanson, 1977; Magnuson et al., 1979). Other criteria for the thermal niche, based on temperature acting as a controlling or a lethal factor, enrich the concept (see Fry paradigm below).

Global warming would be expected to alter the temperatures of lakes and streams and thus favour some species over others in relation to present conditions.