Introduction

Temperature is the most pervasive environmental factor influencing aquatic organisms. Nearly all fish species are, for practical purposes, thermal conformers. In other words, fish of most species are not able to maintain body temperature by physiological means, and their body temperatures fluctuate in close accord with the temperature of the surrounding water. Intimate contact between body fluids and water at the gills, and the high specific heat of water assure this near-identity of internal and external temperatures (Brill, Dewar & Graham, 1994). Consequently, fish species are largely dependent on behavioural control of their body temperatures (Beitinger & Fitzpatrick, 1979; Magnuson, Crowder & Medvick, 1979; Coutant, 1987). Behavioural thermoregulation is widespread among fish species, and in laboratory studies it has often been found that there is good agreement between preferred temperatures and temperatures at which the fish can grow well and perform efficiently (Brett, 1971, 1979, 1995; Beitinger & Fitzpatrick, 1979; Magnuson et al., 1979; Jobling, 1981b; Kellogg & Gift, 1983).

Natural water bodies will seldom, if ever, provide the fish with conditions under which maximum rates of growth can be achieved. Such water bodies will, however, present varied thermal environments offering a wide range of feeding opportunities. It is within such habitats that the ability to thermoregulate by behavioural means may be particularly beneficial to a fish, enabling it to reduce the potentially harmful effects of unfavourable thermal regimes on physiological performance.