THE FINAL THERMAL PREFERENDUM OF FISHES: SHUTTLING BEHAVIOR AND ACCLIMATIONOVERSHOOT

William W. REYNOLDS

Biology Department, Pennsylvania State University, Wilkes-Bane PA 187o8 U .S.A .

Received June 10, 1977

Keywords: Final preferendum, temperature preference, fish, behavioral thermoregulation, preferred temperature, thermalpreferendum, acclimation

Abstract

The concept of final preferendum is reexamined in light of dataconcerning acclimation in a cyclic thermal regime . Because of`shuttling behavior' to temperatures above and below the meanpreferendum, and because of faster acclimation `upward' than`downward', which result in acclimation to a temperature ex-ceeding the mean of the cycle, a fish may finally gravitate to apreferendum which does not equal acclimation temperature . Itis suggested that a distinction be made between the 'crossover-point' preferendum, where preference and acclimation are equal,and the 'ultimate' preferendum to which a fish will ultimatelygravitate.

Fry (1947, P. 24) defined the final thermal preferendumof a fish species as I) that 'temperature around which allindividuals will ultimately congregate, regardless of theirthermal experience before being placed in the gradient,'and 2) 'that temperature at which the preferred tempera-ture is equal to the acclimation temperature .' One prob-lem with this convenient concept is that a fish does notsimply move to the species-specific final preferendumand then remain there, as in a taxis, but rather continuesto shuttle back and forth within a range of temperaturesabove and below the preferendum (characterized bysome statistical measure of central tendency-cf. Rey-nolds & Casterlin, 1976 ; Reynolds, 1977), sometimesmaking brief excursions into temperatures even exceed-ing the ultimate upper incipient lethal temperature (Rey-nolds & Thomson, 1974) . Because fishes acclimate muchmore rapidly to increasing than to decreasing tempera-

Dr. W. Junk b .v . Publishers - The Hague, The Netherlands

Hydrobiologia vol . 57, 2, pag. 123-124, 1978

tures (Brett, 1956), they become acclimated nearer to themaximum than to the mean of a cycled thermal regime(Heath, 1963; Lowe & Heath, 1969; Feldmeth et al., 1974 ;Otto, 1974). Bacon et al. (1967) noted that the thermalacclimation state of mosquitofish (Gambusia affinis)captured at 26.2'C in a thermal gradient exceeded that offish held at a constant 28'C, probably due to behaviouralcycling of acclimation temperatures, with excursions intohigher temperatures . Thus thermoregulating fish canbecome acclimated to a temperature exceeding the finalpreferendum, even when given a free choice .

The relationship of preferred temperature to acclima-tion temperature above the crossover point varies amongspecies, and may decrease, remain constant, or increasewith increasing acclimation temperature (Zahn, 1962 ;Fry, 1964). In some species, a decline in mean preferredtemperature occurs in fish left in a gradient more than 4days (Bacon et al., 1967; DeVlaming, 1971 ; Reynolds &Thomson, 1974) . This could result from acclimationtemperature exceeding preferred temperature due tobehavioural cycling, if these species (Gambusia affinis,Gillichthys mirabilis, and Leuresthes sardina, respective-ly) show a decrease in preferred temperature at acclima-tion temperatures above the crossover-point where thetwo are equal . This would lead to an initial overshoot ingravitation to the 'ultimate' preferendum from accli-mation temperatures below the crossover-point . Most(short-term, less than 4 days) laboratory experimentsmeasure crossover-point preferenda, while longer-termlaboratory experiments and field data probably reflectultimate 'gravitational' preferenda unequal to acclima-tion temperature (cf. Richards et al., 1977 for a discussionof methodologies and applications and a data tabula-

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tion) . It has further been noted (DeVlaming, 1971 ; Rey-nolds & Thomson, 1974) that the precision of thermo-regulation tends to decline over time in extended labora-tory experiments, similar to the lack of precision oftenseen in field thermal distributions . This could be attrib-uted to individual differences in behavioural cycling (andtherefore acclimation state) due to non-thermal factors .This could help explain why field thermal distribu-tions are often lower and less precise than laboratorypreferenda as commonly measured .

The above considerations imply that the ultimate tem-perature to which a fish will finally gravitate in a gradientmay not equal the crossover point at which preferenceand acclimation are equal . In future work it may benecessary to clearly differentiate between 'crossover-point' and `ultimate' preferenda where these can be shownto differ significantly. The relation between the two for agiven species will depend on the shape of the preference-acclimation curve, relative rates of upward and down-ward acclimation, and factors affecting the extent of be-havioural cycling above and below the preferendum (i.e .,between upper and lower avoidance temperatures) .

References

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Reynolds, W. W. & Casterlin, M . E . 1976. Thermal preferendaand behavioral thermoregulation in three centrarchid fishes .In : Thermal Ecology II (Ed. by G. W. Esch & R. W. McFar-lane), pp . 185-190. Springfield, Va . : Natl . Tech. Info . Serv .

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