From 1-8-70 till 31-3-71 research was done on the cause of the phenomenon of fishing seagulls taking place in the Wadden Sea and on the systematic and population-dynamic aspects of the smelt from the Wadden Sea and the Ysel Lake. The phenomenon of fishing in groups of the black-headed gull, turned out to be caused by the smelts in the water surface. This is being caused by: turbulence in the scour stream by scour locks in the Afsluitdyke, causing the smelts sluiced from the Ysel Lake, are brought to the surface and through panic reactions of the smelts in the channels, which are caused by the strong changes in salinity, causing the smelts to rise to the surface. In aquaria smelts turned out to avoid every change in the salinity and panic reactions were observed. A method to determine the age and the speed of growth, according to the scales, is being described. The great majority of the captured smelts belonged to the 0-group, so not a year old yet. The population density in the Ysel Lake was high. In September the number of smelts was probably higher than 3 billion. In the Wadden Sea the population density varied enormously. The number was estimated at 10 to 30 million. The smelt from the Ysel Lake and the Wadden Sea was for 98 to 99% contaminated with cysts of meacercaria of the parasite Cotylurus erraticus (Rudolphi, 1809). Great smelts from the Texel stream were usually not contaminated. It turned out there were about four arguments to distinguish two varieties, namely, a fast-growing up to 30 cm long variety, and one that remained small, a mere 10 to 12 cm of length variety. |
