Seasonal and diel utilisation of inshore microhabitats by larvae and juveniles of Leuciscus cephalus and Leuciscus leuciscus

From July 1995 to January 1996, we examined the seasonal variations of the diel dynamics of habitat use by young-of-the-year cyprinid fishes (chub Leu ciscus cephalus and dace L. leuciscus) in a lotic stream (River Ourthe, Sou thern Belgium). Inshore bays and neighbouring habitats (riparian shelters, entrance of the bay and shallow riffles) were sampled every three hours fro m 6:00 to 22:00 h, using DC electrofishing with prepositioned frames. In ea rly summer, chub larvae moved exclusively in between the middle of the bay and riparian shelters inside the bay. Juvenile dace and, later in the seaso n, juvenile chub showed diel dynamics of which the amplitude was dependent on temperature and illumination: they moved into the bay in the morning, ga thered in greater numbers at mid-day (up to 586 chub and 387 dace m(-2)), t hen progressively left the bay and entered neighbouring riffles in the late afternoon or evening. Small fish immigrated earlier into the bay and emigr ated later than fish of larger size. By late September, most dace had left the bays, but returned there when water temperatures were < 7 degrees C. Du ring autumn and winter, juvenile dace and chub of all sizes occupied exclus ively inshore shelters with submerged riparian macrophytes or fallen tree l eaves (corresponding densities of 0.7, 9.5 and 15.8 dace m(-)2, and of 6.8, 20.2 and 94.2 chub m(-)2). These results support the idea that young-of-th e-year dace and chub shift from a restricted use of inshore shallow bays to a diel dynamics with alternate inshore-offshore movements at the time when they become juveniles, although the precise timings of these movements are still influenced by fish size and water temperature afterwards. The signif icance of these dynamics is discussed within a context of trade-off between the use of food resources and avoidance of predators.