THERMOREGULATORY BEHAVIOR OF BROWN TROUT, SALMO TRUTTA

William W. REYNOLDS & Martha E. CASTERLIN

Department of Biology, The Pennsylvania State University, Wilkes-Barre, Pennsylvania i 8708 U .S.A .

Received January 31, 1978

Keywords : thermoregulation, behavior, temperature preference, brown trout, Salmo trutta, diel rhythms

Abstract

Brown trout, Salmo trutta, were allowed to thermoregulate in-dividually in an electronic shuttlebox. Pooled data for 6 fishshowed a diel pattern of preferred temperature, with a diurnalminimum of 10.3°C, an early nocturnal maximum of 13 .7°C,a less pronounced mid-scotophase minimum of 11 .7°C, and asecondary dawn maximum of iz .8°C, in a somewhat crepus-cular pattern . The 24-hour mean preferendum was 12 .2°C .

Introduction

Several warm-water fish species have been found to ex-hibit diel rhythms of preferred temperature, includingthe goldfish Carassius auratus (Reynolds, 1977a ; Rey-nolds et al., 1978a), the bowfin Amia calva (Reynolds etal., 1978b), and the blackbasses Micropterus salmoidesand M. dolomieui (Reynolds & Casterlin, 1978) . Brett(1971) cited field data indicating a diel rhythm of pre-ferred temperature associated with diel vertical migra-tions across the thermocline by lacustrine sockeye sal-mon, Oncorhynchus nerka. However, there have been noprevious laboratory data regarding diel thermoregu-latory patterns in any salmonid, including the browntrout Salmo trutta which is the subject of the presentstudy .

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

Methods

Wild adult brown trout were captured by electroshockingfrom a stream in central Pennsylvania during December .They were maintained in the laboratory for at least a weekprior to testing under seasonal conditions of temperatureand photoperiod, i .e ., o-5°C, LD 9 : 16 (natural lightingthrough windows). The same photoperiod was main-tained during testing. The trout did not feed in captivity .

Six trout were tested individually in an electronicthermoregulatory shuttlebox or 'Ichthyotron' (Reynoldset al ., 1976 ; Reynolds, 1977b) which permitted them tocontrol water temperatures by their movements, over apotential range of 0 to 35+°C . Following an initial 24hour introductory period to allow gravitation to the finalpreferendum (Reynolds, 1977c, 1978), data were collectedover the ensuing 48 hours . One fish was tested for a longerperiod of over a week . Data for all six fish were pooled,and hourly preferred temperatures were plotted (Fig . 1)to show the diel pattern .

Results

The brown trout exhibited a diel rhythm of preferredtemperature (Fig . i) characterized by a diurnal mini-mum of 10 .3'C. There was a less pronounced nocturnalminimum of 11 .3°C, separated by crepuscular maximaof 13.7°C during the early portion of scotophase and12.8'C at dawn. The 24-hour mean was 12 .2'C. Thesingle fish tested for more than a week showed erraticthermoregulatory performance and a shift to lower tem-peratures after the first 4 days, a phenomenon noted withother species and as yet poorly understood (cf. Reynolds,1977c, 1978) .

Hydrobiologia vol. 62, 1, pag . 79-80, 1979

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24TIME (HOURS)

Fig. i . Hourly mean preferred temperatures in relation to photo-period from pooled data for 6 adult brown trout individuallythermoregulating in an electronic shuttlebox ('Ichthyotron') .Dark bars show scotophase (night), delimited by approximatelocal times of sunrise and sunset (Eastern Standard Time) fornortheastern Pennsylvania in December and January (naturallighting from windows) . The 24-hour mean was 12 .2'C .

Discussion and conclusions

The 24-hour mean of 12.2°C is nearly identical to the

12°C reported for this species by Jammes (1931), as cited

by Coutant (1977) . The diel pattern greatly resembles that

exhibited by Oncorhynchus nerka (Brett, 1971), another

salmonid whose preferred temperature shows a pro-

nounced diurnal minimum (5°C) associated with move-

ment to deeper water below the thermocline, crepuscularmaxima near 17'C associated with surface-feeding, and

a smaller nocturnal minimum . S. trutta similarly ex-

hibits a pronounced crepuscular pattern of locomotor

activity in laboratory tests (our own data, as yet un-

published) ; this pattern occurs even in the absence offeeding, and also when fish are held at a fixed temperature

(5°C) and not thermoregulating . We believe these data

reflect endogenous behavioral tendencies which exhibit

themselves under appropriate environmental conditions,

but which might be masked by exogenous modifying

factors (such as disturbance or times of food availability)under other circumstances, so that they might not be ap-

parent to casual field observers .

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Acknowledgements

We thank Mr Robert Bachman for providing the brown

trout. This study was aided by Scholarly Activity Fund

Grants SAF-77-113 and 1-74071 from PSU .

References

Brett, J . R . 1971 . Energetic responses of salmon to temperature .A study of some thermal relations in the physiology and fresh-water ecology of sockeye salmon (Oncorhynchus nerka) . Am .Zool . 11 : 99-113 .

Coutant, C . C. 1977. Compilation of temperature preferencedata . In : F . P . Richards, W . W. Reynolds & R . W. McCauley(eds .), Temperature preference studies in environmental im-pact assessments: an overview with procedural recommen-dations, pp . 739-745 . J . Fish . Res. Board Can . 34 : 728-761 .

Jammes, L . 1931 . Sur le comportement, au lac d'Oredon, de latruite commune, Salmo trutta L ., en fonction des agents ther-mique et nutritif. C . R . Soc. Biol . (Paris) 107 : 1482-1485 .

Reynolds, W. W . I977a. Circadian rhythms in the goldfish Caras-sius auratus L . : preliminary observations and possible impli-cations . Rev . Can . Biol . 36 : 42-43.

Reynolds, W . W . I977b . Fish orientation behavior: an electronicdevice for studying simultaneous responses to two variables .J . Fish . Res. Board Can . 34 : 300-304 .

Reynolds, W . W . 1977c . Temperature as a proximate factor inorientation behavior. In : F . P . Richards, W . W . Reynolds & R .W. McCauley (eds .), Temperature preference studies in envi-ronmental impact assessments : an overview with proceduralrecommendations, pp . 734-739 . J . Fish . Res . Board Can . 34 :728-761 .

Reynolds, W . W . 1978 . The final preferendum of fishes : shuttingbehaviour and acclimation overshoot. Hydrobiologia 57 :123-124 .

Reynolds, W . W. & M . E . Casterlin . 1978 . Complementarity ofthermoregulatory rhythms in Micropterus salmoides and M .dolomieui . Hydrobiologia (in press) .

Reynolds, W . W ., Casterlin, M . E., Matthey, J. K ., Millington,S . T . & Ostrowski, A . C . 1978 . Diel patterns of preferred tem-perature and locomotor activity in the goldfish Carassiusauratus. Comp . Biochem . Physiol. 59A: 225-227 .

Reynolds, W. W., Casterlin, M . E. & Millington, S . T. 1978 .Circadian rhythm of preferred temperature in the bowfinAmia calva, a primitive holostean fish . Comp. Biochem . Phy-siol. 6oA: 107-109 .

Reynolds, W . W., McCauley, R . W ., Casterlin, M . E . & Craw-shaw, L . I . 1976 . Body temperatures of behaviorally thermo-regulating largemouth blackbass (Micropterus salmoides) .Comp. Biochem . Physiol. 54A : 461-463 .

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