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

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 198 1, 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. It is now widely accepted that predation plays a critical role in determining zooplankton community structure in ponds and lakes. Highly selective predation by fish and zooplankton together determine which species of primary consumer can persist in limnetic regions and which are excluded or confined to refuges (Macan 1977; Zaret 1980). Phenomena frequently attributed to heavy predation by fish include depressed zooplankton biomass, small individual size of plankters, and reduced representation of the vulnerable Cladocera (e.g. Brooks and Dodson 1965; Wells 1970; Hall et al. 1970; Hutchinson 197 1; Van Densen and Vijverberg 1982). Investigators have commonly compared zooplankton communities in a series of water bodies naturally subject to different intensities of fish predation or have studied individual water bodies in which predation by fish has been manipulated experimentally or has changed dramatically after invasion by an exotic species. One aim of our study was to investigate the dynamics i This work was supported by grants from NERC (to M.C.) and SERC (to G.P.). 2 Present address: Tangariro Hatchery, Private Bag, Turangi, New Zealand. 3 Present address: Anglian Water, Yare House, Thorpe Rd., Norwich, U.K. of zooplankton and fish simultaneously to discover the extent to which fish recruitment, varying naturally from year to year, influences parameters of the zooplankton community in predictable ways. A second aim was to assess whether variations in the zooplankton feed back on growth and fecundity of the fish. Alderfen Broad is a small (4.7 ha), shallow (mean depth 0.8 m, max 1.3 m), isolated lake in Norfolk, U.K. (National grid ref. TG 354196). It has an even, semifluid sediment and was devoid of submerged macrophytes until near the end of our study. Roach, Rutilus rutilus (L.), and bream, Abramis brama (L.), are both important consumers of microcrustaceans in the broad. Rudd, Scardinius erythropthalmus (L.), tenth, Tinca tinca (L.), ruffe, Gymnocephalus cernua (L.), perch, Perca Jluviatilis (L.), and pike, Esox Lucius (L.), are also present and do take microcrustaceans, at least in their early life stages. However, all tend to include larger proportions of other invertebrate prey, or fish, in their diet than do roach or bream and most occur only at low densities. Our analysis here will concern only roach, the major predator of pelagic zooplankton in the broad. Both under-yearling and older bream tend to forage nearer the bottom than roach, and frequently at night,