Reciprocal interactions between roach, Rutilus rutilus, and zooplankton in a small lake: Prey dynamics and fish growth and recruitment1

Recruitment success of roach varied dramatically between 1979 and 1982 in Alderfen Broad, a small lake in eastern England. When fry were abundant (in 1979 and 1981, but not in 1980 or 1982) the summer zooplankton became sparse and was dominated by copepods and rotifers. In years of good recruitment, as each of the preferred cladoceran prey species entered the diet of underyearling roach, its density dropped dramatically. Mean size of Daphnia hyalina and Ceriodaphnia quadrangula also declined significantly during these seasons. The results of enclosure experiments indicate that the link between roach recruitment and zooplankton dynamics is causal. Older roach also feed to a significant extent on zooplankton, but the 0+ age group exerted the greatest influence. The abundant underyearling roach in years of good recruitment showed poor growth, as a result of depression of their prey populations. Older fish also grew poorly in these years and were then less fecund in the following season. The evidence indicates that there may be a 2‐year cycle in roach recruitment, and we describe the unusual circumstances in Alderfen which may be responsible.