Seasonal occurrence of the invasive ctenophore <i>Mnemiopsis leidyi</i> in the western Dutch Wadden Sea

The ctenophore Mnemiopsis leidyi is an invasive species in northern European waters since 2006. This paper presents the first quantitative data for the western Dutch Wadden Sea based on weekly measurements year-round in 2009 of abundance and size distribution. Due to the short residence time of the water, the seasonal occurrence of this species in the western Dutch Wadden Sea is a reflection of its occurrence in the Dutch coastal zone of the North Sea. M. leidyi was present the whole year round with multiple peaks. Spawning started in May and was followed by an increase in density of 3 orders of magnitude, leading to the first peak in mid-June (highest mean density 360 ind m(-3)). After a decrease in July numbers increased again and a second peak occurred in mid-August (highest mean density 342 ind m(-3)) during which the highest density of 912 ind m(-3) in a single haul was observed. At peak densities the population consisted almost entirely of small (