Light-induced swimming behaviour of Daphnia can lead to diel vertical migration. When this occurs, Daphnia may escape from predation by juvenile 0+ fish. For this to happen, swimming in response to the change in light intensity at dawn and dusk must be enhanced. This enhanced swimming reaction can be elicited by fish-associated kairomone in the laboratory. We studied the effect of these fish-associated kairomones in Lake Maarsseveen, The Netherlands. A bioassay was conducted in which light-induced swimming reactions of Daphnia were used to quantify the kairomone signal strength. In two successive years, 1998 and 1999, water from Lake Maarsseveen was tested weekly in early summer. Samples were taken from the epilimnion at 3 m depth and from the hypolimnion at 15 m depth. Simultaneously, we caught 0+ perch (Perca fluviatilis) with bongo nets to determine its abundance. The length and weight of individuals caught in 1998 were determined to establish a length–weight relationship. Using this relationship and the information on perch density from the trawls, the relative fish biomass was calculated for these two years. It was shown that water from the epilimnion layer increasingly enhanced light-induced swimming reactions until the second week of June, then the effect gradually disappeared. Water from the hypolimnion had no such effect. In 1998, these changes in signal strength correlated with the relative biomass of the 0+ perch, but in 1999, the maximum of the enhancement lagged 2 weeks behind the maximum of the biomass of the 0+ perch. This lag may be due to a different development of the thermocline. We conclude that kairomone concentration may well correlate with 0+ perch biomass and thereby might inform Daphnia not only about the presence, but also about the abundance, of juvenile perch.