Helpt Baars Perca fluviatilus Zwarte Stern Chlidonias niger?

In autumn Black Terns make use of Lake IJsselmeer during their wing moult. In this period, yearling Smelt Osmerus eperlanus is their most important prey. The availability of Smelt is determined by several abiotic factors such as turbidity and light conditions. Smelt show a distinct diurnal pattern in vertical distribution, with concentrations at greater depths during the day and a more evenly distribution during twilight and night. In addition, seasonal variation can be large and horizontal distribution is very unpredictable. Since Black Terns can only feed in the upper centimetres of the water column, they depend on Smelt that are present in this layer. This makes it difficult to predict which proportion of the Smelt stock is actually available to Black Terns and other waterbirds. In a pilot project in which we aim to quantify the proportion of Smelt that is available as food for birds, we sampled fish in August 2002 in combined surface and bottom hauls. Most Smelt were found in the hauls that held many Perch, the main predator of Smelt, in the corresponding bottom hauls. The abundance of Black Terns in the surroundings of the sampling sites showed a positive correlation with the number of Smelt caught. Smelt seem to be chased towards the surface by hunting Perch, and are driven into reach of Black Terns. Therefore from the 'food for bird' perspective an optimum density of Perch is likely to exist. As a predator of Smelt, Perch is a direct competitor of waterbirds and high densities can result in a reduction of the Smelt stock. On the other hand a minimum density of predator fish is necessary to chase Smelt within reach of aerial fishing waterbirds. Using long term datasets on the relationship between Smelt stocks and their predators and detailed measurements on vertical distribution of Smelt, we aim to better understand this complex of factors, including the role of Perch/Pikeperch Stizostedion lucioperca fishery. This may lead to ecologically sensible recommendations for fish stock management.