Diel drift behaviour of fish eggs and larvae, in particular barbel, Barbus barbus (L.), in an English chalk stream

To estimate the potential loss of fish larvae to downstream stretches, the downstream drift behaviour of fish eggs and larvae, in particular barbel, Barbus barbus (L.), was examined in the River Lee, a small, nutrient-rich chalk stream in England, using drift nets over nine consecutive 24-h periods in June 1993 at one location and over ten 24-h periods, once a week for 10 weeks from May to July 1995 at a location slightly more upstream. The density of drifting fish larvae was not correlated with river discharge in 1993 or 1995. A clear diel pattern was found in the drift of fish eggs and larvae, with barbel being a predominant species. Almost all fish larvae drifted at night both in 1993 and 1995, but the drift of eggs in 1995 occurred regardless of luminosity, although most eggs drifted during the day and at dawn. The highest densities of drifting fish larvae (in particular barbel) were found in the nets set in the highest water velocities, with the opposite pattern observed for fish eggs, suggesting either active response to the water current or shape-related differences in the drift behaviour of passive particles.