The strong effect of temperature on biochemical and physiological processes drives fishes to select environmental temperature sat which they can function efficiently (Coutant 1987). Because different physiological processes may have different optimal temperatures, the temperature selected by a fish often represents a compromise, or “integrated optimum”(Kelsch & Neill 1990). Fishes probably select temperatures that maximize the amount of energy available for activity, or metabolic scope (the difference between standard and maximum metabolic rates). Of course, habitat selection in the wild involves a compromise between temperature requirements and other important factors, such as dissolved oxygen levels, food availability, current velocity, substrate type, and avoidance of predators and competitors(see Coutant 1987). Temperature is, however, a very strong determinant of habitat choice by some fishes. Temperature sensitive radio transmitters surgically implanted in the body of trout revealed that when the water temperature of a New York stream exceeded 20°C, the fish selected cooler micro habitats within the river, such as tributary confluences and areas of groundwater discharge. The body temperature of Brook Trout was up to 4°C below river temperature, whereas Rainbow Trout had body temperatures up to 2.3°Cbelow river temperature (Baird & Krueger 2003).
Numerous laboratory investigations have shown most fishes select temperatures close to those to which they havebecome accustomed (see Kelsch & Neill 1990). There area few exceptions, however. Chum Salmon (Salmonidae)and Blue Tilapia (Cichlidae) show very narrow and constant temperature preferences regardless of acclimation temperature, and guppies (Poeciliidae) show a slight decline in preferred temperature with increased acclimation temperature(see Kelsch & Neill 1990). The physiological ability to adapt to different temperatures to the point of shiftingtemperature preference may reflect the climate in which a species evolved (Kelsch & Neill 1990). Species that evolved in areas with substantial seasonal changes in temperature, such as the Bluegill (Centrarchidae) of temperate North America, need the biochemical and physiological ability to shift temperature optima. More tropical species, such as guppies and tilapia, and Coldwater fishes, such as salmonids, probably have not had to respond to selective pressures that would favor individuals that can make these kinds of adjustments.
Temperature preferences can change as fishes grow, leading to different life stages of a given species utilizing different thermal niches. For example, juvenile Striped Bass (Moronidae) prefer temperatures around 25°C,whereas large adults will select cooler temperatures, around 20°C (Coutant 1985). This onto genetic shift in temperature preference has important implications for the success of efforts to introduce this highly prized sport fish into various reservoirs and estuaries. A body of water that is ideal for the success and growth of young fish may be thermally unsuitable for large adults, which may congregate in small areas of slightly cooler water (often 18–20°C)such as near underground spring inputs or in the hypo limnetic waters of stratified lakes and reservoirs (see Temperature, oxygen, and water flow). Extreme crowding can lead to increased susceptibility to disease and over fishing. It also can lead to locally depleted food supplies and subsequent poor growth and reduced fecundity. The thermal preference may be so strong that starving fish will not leave cooler deep waters to feed on abundant prey in warmer surface waters (Coutant 1985).
Strong thermal preferences probably are the result of natural selection resulting in fishes selecting habitats that offer them the best chances for growth and reproduction. This physiological constraint on habitat selection can become a liability, however, particularly in the face of human alterations of aquatic environments. In summer the deep, cooler hypolimnion of warm reservoirs can be attractive to large Striped Bass. As summer progresses, however, these deep waters can become low in oxygen, leading to fish mortality. Coutant (1985) discusses evidence for and implications of this temperature–oxygen habitat squeeze on Striped Bass populations in several diverse habitats, including freshwater and coastal systems..
Power plant cooling systems often discharge heated water into lakes and rivers, thereby altering their thermal structure. This can cause fishes to congregate in areas that may not be ultimately beneficial. For example, if the plant shuts down for a few days during the winter, fish that had become acclimated to the warmer water are suddenly left stranded in a cold environment and can die. Hydroelectric dams often release deeper, cooler water from an upstream reservoir. Fishes that congregate in these cooler hypo limnetic waters may be more susceptible, therefore, to being drawn through the turbines and injured or killed. There lease of cooler water through a hydroelectric dam also can attract downstream fishes to the tailrace water during the warm summer months. The concentration of fish can create an attractive sport fishery, but it also can lead tooverfishing and subsequent depletion of brood stock. in some “pump back” hydroelectric dams, large motors run turbines in reverse to push water back to the upstream side of the dam when power is not needed. When more electricity is needed, such as during periods of peak demand, this water is released again to generate electricity. The attraction of fishes to the foot of the dam during periods of power generation can set the stage for high fish mortality if those fishes are drawn through the turbines as water is pumped back to the upstream side of the dam(Helfman 2007).
The combination of cooler temperatures and high turbulence can cause water that is released from dams to become supersaturated with gases, especially nitrogen and oxygen. The blood of fishes living in these areas also can become supersaturated because of gas diffusion across the highly permeable gill membrane. When these fishes move to warmer, less turbulent areas, the gases come out of solution and form bubbles in the blood. This gas bubble disease(similar to “the bends” in humans) can cause blocked and ruptured blood vessels, resulting in disorientation and death.
Thermal preferences also may cause fishes to congregate in areas with high levels of toxic pollutants, as has been reported for Striped Bass in the San Francisco Bay-Delta area. Uptake and bioaccumulation of some of these contaminants has been correlated with poor growth, high parasite loads, and decreased reproductive potential(Coutant 1985).
The impact of temperature preferences on fish habitat selection is a good example of links among fish physiology,behavior, ecology, and conservation. The effect of temperature preferences on the success of introduced Striped Bass also demonstrates the importance of basic physiological and behavioral information, as well as a thorough understanding of the habitat, when considering ecosystem manipulation or species introductions.