Spatial and interannual variability of the Schyphozoan medusae, Pelagia noctiluca (Forksål), in the Bay of Calvi (Corsica)

The temporal distribution of the Schyphozoan medusae Pelagia noctiluca (Forksål) has been examined weekly in the oligotrophic coastal waters of the Bay of Calvi (Corsica) from 2003 to 2008. The observations show a gradual increase of jellyfish number in 2004 to 2005, followed by a dramatic increase in the 2006 and a significant reduction in summers 2007 and 2008. The numeration of ephyra larvae in plankton and ovarian histology show a higher reproduction in June 2006.
An analysis of the spatial distribution of the Pelagia noctiluca in the area of the Bay of Calvi emphasises that during the 2006 bloom, most of the organisms are present at a distance of the coastline where they are forming actives swarms of 200 m of length with 5 to 10 individual per m3. In the swarms, the individuals are healthful and swim actively in the upper layer (10 t 60 m) with tentacles extended, probably catching the food. The swarms are generally located around wind-protected area where the surface turbulence is reduced and where zooplankton is accumulating. Sometimes, the swarms migrate vertically probably in relation with changes of wind conditions.
In some cases, moribund individuals partially destroyed or at the end of their biological cycle escape from active swarms and drift passively, at the surface, under the action of water movements, into coastal waters and beaches where they accumulate and reach 200 individual per m3. In these passive swarms the individuals continue to swim slowly but are actively fed by some Sparid fish.