LEEUW JJ DE (2001) Competition between fish-eating birds and fishery: matter of density dependence. LIMOSA 74 (2): 69-72.

If fish-eating birds and fishery utilize the same species of prey fish, it is often assumed that there is strong competition tor their common resource. The implicit assumption that the removal of any prey fish results in a loss of benefit for the other consumer may be untrue when density-dependent growth and/or mortality regulate the number of prey fish. This phenomenon is demonstrated for a population of Smelt Osmerus eperlanus which is utilized by different groups of birds and a commercial fishery in Lake Ijsselmeer, The Netherlands (Tab. 1). A time series of 30 years reveals that Smelt growth is density dependent (Fig. 1), due to food limitation (large zooplankton). A reconstruction of the seasonal pattern of Smelt production and consumption shows that Black-headed Gulls Larus ridibundus, Black Terns Cliidonias niger. and Great Crested Grebes Podiceps cristatus utilize Smelt in summer when Smelt abundance and density-dependent growth and mortality are highest (Fig . 2). Sawbills Mergus sp. are the main avian predators in winter, a period of density-dependent mortality. An intensive Smelt fishery is confined to the spawning period in early spring (March) and probably does not affect density-dependent recruitment. Cormorants Phalacrocorax carbo feed their young with the remaining 1 + population of Smelt and may potentially suffer from Smelt fishery, although alternative prey may be available. This seasonal pattern of utilization during periods of major density-dependent processes greatly reduces potential competition between avian predators and fishery.Black-headed Gull Chroicocephalus ridibundus = Larus ridibundus Black Tern Chlidonias niger Great Crested Grebe Podiceps cristatus Goosander Mergus merganser Red-breasted Merganser Mergus serrator Smew Mergellus albellus Great Cormorant Phalacrocorax carbo

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