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Northcott, Dorabella Susan (1981) The role of aquatic macrophytes in the availability of food for young fish. Doctoral thesis, City of London Polytechnic.
Quantitative samples from a gravel-pit lake in 1977 (April-December) showed higher geometric mean densities and biomasses of microcrustacea in the marginal weedbeds (907/1 and 3107ug/l) than in the open water (225/1 and 245ug/l). The weedbeds were dominated by Cyclops vernalis americanus and Ceriodaphnia pulchella and the open water by Bosmina longirostris and C. vernalis americanus. C. pulchella and B. longirostris seemed mutually exclusive . Evidence from a second gravel-pit lake (lower fish stock) indicated that this was partly caused by fish predation pressure in the first lake (higher fish stock). No microcrustacea longer than 1mm occurred in the open water. The size range in the weedbeds was 0 .1 -2 .0mm. Diversity and abundance were highest amongst Potamogeton natans where C. pulchella was most abundant, but few microcrustacea/plant associations were found. The O+ roach diet contained microcrustacea from the open water and the weedbeds (50% numerically and 68% by weight). B. longirostris (open) was the preferred food but if not abundant the roach switched to pulchella (weed). Feeding was determined by prey mobility and abundance rather than by prey size. In contrast the O+ perch diet was mainly microcrustacea of weedbed origin (63% numerically and 78% by weight). Feeding was possibly size-selective. Cyclops was the main food item. Competition for food between roach and perch seemed to be minimised; diet overlaps were most common for weedbed microcrustacea. Growth of O+ roach was average in 1977 and 1978 and good in 1979. That of O+ perch was consistently average. Perch exhibited large fluctuations in first year survival. Field caging experiments provided evidence that macrophytes may be beneficial to O+ perch growth but O+ roach grew as well without them. Improved growth rates of O+- perch in the presence of macrophytes were attributed to a greater availability of macro-invertebrates and a typical feeding strategies in the absence of macrophytes.
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