SEASONAL ACTIVITIES OF BARBUS-BARBUS - EFFECT OF TEMPERATURE ON TIME-BUDGETING

Surgically implanted activity-circuit radio transmitters (40 MHz) were used to study the seasonal activities of 21 adult (males: 23 to 35 cm F.L. and females 38 to 55 cm F.L.) Barbus barbus (Pisces, Cyprinidae) in the River Ourthe (Southern Belgium) in 1989-1991. During the autum nal thermal transition (water temperature 9 to 10-degrees-C), the typi cal dusk and dawn pattern observed in summer turned to a trimodal patt ern with the emergence of a diurnal phase. The auroral then crepuscula r and finally diurnal activity periods progressively vanished as water temperature decreased down to the thermal limit for activity (4.0-deg rees-C), when barbel entered a dormancy period. An opposite progressiv e shift was observed during the spring thermal transition. Daily activ ity budgets ranged from 0 to 720 min-on the annual cycle and were sign ificantly (r2=0.686, P<0.05, d.f.=36) dependent on water temperature a nd on morpho-dynamic unit size, while fish size was non-significant. A lthough the dusk and dawn rhythm pattern was consistent throughout sum mer, water temperature significantly (P<0.05) interfered with the resp ective duration of crepuscular and auroral activities (r2=0.586, d.f.= 57 and r2=0.692, d.f.=55). The precise timing of activities was also t hermal-related and the activities of small male barbel were proportion ally more nocturnal than those of large female barbel (ANCOVA, F=80.61 , d.f.=31 and F=4.5, d.f.=23, at dusk and dawn respectively), possibly due to predation pressure on small fish. It is concluded that the sea sonal variations of activity budgets, rhythm patterns and timings in B . barbus correspond to a form of time-budgeting partly to achieve ther mal homeostasis in a variable environment.