Fish Thermal Regulation
It is often assumed that all fish, as “coldblooded” vertebrates, are always at the same temperature as their environment. The large surface area of the gills, which exchanges gases so efficiently, also exchanges heat efficiently. As the blood circulates through the gills every 30 sec to 2 min, heat produced from body metabolism can be quickly carried away from the fish by the water ventilating the gills.
However, the British physician John Davy noticed in 1835 thatskipjack tuna caught for food were 10°C warmer than the water in whichthey were caught. Field studies also revealed that certain fishes “pre- ferred” waters of particular temperatures over other, equally accessibleareas. Thus, two possible methods of some internal temperature controlbecame apparent: behavioral and physiological thermoregulation.
Behavioral thermoregulation concerns the movements of fishesfrom one water mass or area to another characterized by a warmer orcooler temperature. As temperature affects rates of metabolism anddigestion so profoundly, some fishes may “select” aparticular temperature to conserve energy or to run their metabolicmachinery (i.e., enzymes) at its most efficient temperature. For example,Brett (1971) found that sockeye salmon (Oncorhynchus nerka) select awarm temperature (15°C) at a depth of approximately 11 m to digestthe food eaten at their dusk feeding during the short Canadian summernights. In contrast, the fish go deeper (37 m) to 5°C water between thedawn and dusk feeding periods. Thus, during the longer daytime period,the fish conserve energy by lowering their body maintenance energyrequirements in the colder water.
Human-induced thermal changes in aquatic environments havefurther stimulated investigations of fish thermoregulatory behavior. Forexample, Neill and Magnuson (1974) consistently captured bluegill(Lepomis machrochirus), largemouth bass (Micropterus salmoides),longnose gar (Lepisosteus osseus), small rockbass (Ambloplites rupestris),pumpkinseed (Lepomis gibbosus), large yellow bass (Morone mississip-piensis), and carp (Cyprinus caipio) in the heated outfall plume watersof an electrical power generating plant located on Lake Monona,Wisconsin. Evidence was collected that localized food sources (e.g.,zooplankton for the bluegill, small fish for the longnose gar) influencedthe preference of some of these species for the thermal plume area,which was 2°C to 4°C warmer (median temperatures) than adjacentareas during middle to late summer. However, concurrent laboratorystudies using a shuttlebox apparatus showed that the consistent plumeresidents tested had higher preferred temperatures than the one specieswhich consistently avoided the thermal plume, yellow perch (Percaflavescens).
Physiological thermoregulation in fishes, to a significant degree,is exhibited only by several continuous-swimming species. Each of the”warm-bodied” species leads a pelagic marine existence and has heat-exchanging retia mirabilia to conserve heat produced by the fish’smetabolism (Carey et al. 1971). These fish also have the major arteriesand veins for blood transport between the heart and gills and the heatexchanger located close to the skin.
This enables them to transport the cool (i.e., near water temperature) blood to and from the heat exchanger without absorbing much ofthe heat produced by the swimming muscles. . Essentially,by means of a honeycomb arrangement of afferent and efferent blood vessels, heat (instead of gas) is exchanged by convection across the wallsof these many vessels running parallel to each other. No gases are ex-changed in the heat-exchange retia because there is no localized acidifica-tion of the blood, such as by lactic acid in the swimbladder rete. Also,the larger vessel diameter (ten times as large: 0.1 mm rather than 0.01mm) and thicker vessel walls (compared with the swimbladder retecapillaries) further slow the diffusion of oxygen molecules, whichdiffuse at a rate ten times slower than heat. Because of the counter-current flow, the metabolic heat is efficiently conserved in the rete,which surrounds the red swimming muscles. The efficiencyof this heat exchanger is 95% as a thermal barrier between the gills andred muscle in skipjack tuna Although the number and position of heat-exchanging retia variesamong the various tunas and mackerels (Scombridae) and Iamnid sharks(e.g., shortfin mako [Isurus oxyrinchus]) that possess retia, all of themare capable of fast, continuous swimming. As warm muscles can con-tract faster than cool ones, presumably the heat exchanger allows thesepredatory fishes to exert more swimming thrust and thus outswim thesquid and smaller fishes that compose their diet. For example, thegrouper (Epinephelus), which does not have the special circulatoryadaptations for metabolic heat conservation, has an internal temperatureof 0.3°C above that of the water it lives in, whereas the swimmingmuscles of albacore tuna (Thunnus) show a 12°C elevation. “Warm-bodied” fishes, however, do not have constant bodytemperatures (such as in mammals or birds) but have temperatures thatfluctuate with that of the environment. The core temperature of thelargest of these “warm-bodied” species, the bluefin tuna, seems leastaffected by environmental temperature, apparently because of the greatthermal inertia that is inherent in large bodies. In some situations thismay work to their disadvantage by causing overheating of the musclemass when exercising. Thus larger tuna adopt a cyclical pattern of depthdistribution to “cool off” below the thermocline if surface waters aretoo warm for continuous occupancy.
The ability of “warm-bodied” fishes to sense changes in theirthermal environment may operate either by neural comparisons oftemperature of the water (external surface of the fish) with that deepinside the body or by the blood temperature gradient across the heatexchanger