Elements of a trophic model for IJsselmeer (The Netherlands), a shallow eutrophic lake

摘要: IJsselmeer in the central part of the Netherlands is a 182,000 ha shallow eutrophic freshwater body with an average depth of 4 m. Commercially important fish species are the eel (Anguilla anguilla), two predators, pikeperch (Stizostedion lucioperca) and perch (Perca fluviatilis) and the short-lived smelt (Osmerus eperlanus). Other important fish species in the ecosystem are ruffe (Gymnocephalus cernua), bream (Abramis brama) and roach (Rutilus rutilus). The fishery consists mainly of a fyke net fishery for eel and spawning smelt and a gill net fishery for pikeperch and perch. Important fish-eating birds are the cormorants (Phalacrocorax carbo), grebes (Podiceps cristatus), mergansers, gulls and terns. The trophic relations between phytoplankton, zooplankton, fish, birds and the fishery were estimated over the period 1983-1987. The zooplanktivorous smelt, producing 130 kg· ww. ha¹· year‍¹, appeared to be a key species within the food web. Smelt was mainly consumed by perch (59 kg· ha‍¹year‍¹). Birds consumed 48 kg· ha‍¹· year¹ of fish, of which 42% was smelt. The impact of the fishery consisted of overexploitation of eel and pikeperch stock and of catching large amounts of 0-and 1-group fish as by-catch in the fyke net fishery. The discarded by-catch was utilized by gulls and terns.