Do the long-term changes in zooplankton biomass indicate changes in fish stock?

The narrow, 53.5 m deep and 42 km long Slapy reservoir was sampled at 3-week intervals at a standard sampling station 9 km from the dam during the years 1964–2001. The chlorophyll-a increased relatively more than the total phosphorus concentration. The relative increase in the biomass of crustaceoplankton larger than 0.2 mm and also that of its cladoceran and copepod component was in-between. The biomass of copepods nearly equaled that of cladocerans. The seasonal maximum of copepods preceded that of cladocerans. The increase of the means of these maxims in subsequent decennia did not fully follow the general increase. In the last decennium compared to previous decennia there has been a clear increase of both taxonomical groups in the second half of summer and in the first half of autumn. The percentage of cladocerans larger than 0.71 mm determined in the period 1985–2001 decreased. In connection with the increase of the biomass of zooplankton this indicates an increase in the fishstock, especially of the fraction of small fish. The decrease in large cladocerans was distributed nearly uniformly over the season and is larger between the means of the 3-week intervals in the 1985–1990 period and 1991–1995 period than between the last one and the 1996–2001 period. The broad spring maximum is not uniform in individuals years. There are two subsequent high values the relation of which differs in different years in height and date. The two spring values for the impact of fish on the zooplankton are probably due to the spawning activities of two groups of fish, probably percids and cyprinids. The minimum percentage of the large cladocerans in autumn can be connected with the increasing size and therefore increased feeding activity of the yearlings in the period of decreasing biomass of zooplankton. The later mild increase can be connected to the decreased activity of cyprinids in the period of decreasing low temperature.