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 autumnal thermal transition (water temperature 9 to 10-degrees-C), the typical dusk and dawn pattern observed in summer turned to a trimodal pattern with the emergence of a diurnal phase. The auroral then crepuscular and finally diurnal activity periods progressively vanished as water temperature decreased down to the thermal limit for activity (4.0-degrees-C), when barbel entered a dormancy period. An opposite progressive shift was observed during the spring thermal transition. Daily activity budgets ranged from 0 to 720 min-on the annual cycle and were significantly (r2=0.686, P<0.05, d.f.=36) dependent on water temperature and on morpho-dynamic unit size, while fish size was non-significant. Although the dusk and dawn rhythm pattern was consistent throughout summer, water temperature significantly (P<0.05) interfered with the respective 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 thermal-related and the activities of small male barbel were proportionally 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 seasonal variations of activity budgets, rhythm patterns and timings in B. barbus correspond to a form of time-budgeting partly to achieve thermal homeostasis in a variable environment.