Predicting the ‘peaks’: a delicate exercise

All these discoveries still tell us nothing about the reasons for the cyclical proliferations of one of the most well-known jellyfish on the beaches of France: Pelagia (Pelagia noctiluca). The same applies to Aurelia (Aurelia aurita) which, in Belgium too, from Panne to Knokke-le-Zoute, sometimes spoils the summer for bathers. Amandine Collignon reminds us that this phenomenon of cyclical proliferation has been reported for over two hundred years! Thus it is not only a recent problem. “Due to a lack of precise data, it would certainly be premature to put it down to global warming. My research only focused on Calvi Bay and I cannot offer an exact explanation for the cyclical proliferation of Pelagia (around every ten years). On the other hand, we can confirm the great annual and interannual variability of the species. In Calvi Bay, for example, the years 2004 and 2005 were marked by a gradual increase in populations followed by a net increase in 2006. Then, during the summers of 2007 and 2008, we observed an important decrease in populations. The summer of 2014 seems to have seen a new ‘peak’, but we do not yet have all the data to confirm these empirical observations. What is clear, moreover, is that the active swarms of Pelagia generally occur at the end of the spring just below the surface of the water. This phenomenon is broadly spread over a few hours. It happens around terrestrial points where the surface of the water is protected from wind, where there is less turbulence and where zooplankton accumulates. These swarms do not remain for long, barely a few days. They migrate vertically at close of day but they are also influenced by wind fluctuations. In the end, only dying individuals that join the passive swarms carried by the North or North-East winds remain on the surface near the coasts”.

Will we one day be able to predict the appearance of masses of jellyfish on coastlines, both in the Mediterranean and the North Sea? “At the present time, it is an extremely hazardous exercise”, comments the oceanographer. “We cannot affirm that only one factor – temperature, the presence of plankton, etc. – plays a role, but rather a conjunction of factors is involved. In addition, local conditions vary greatly according to whether you are in an oligotrophic environment like the Mediterranean Sea (where there are not many nutrients available for plant and animal plankton), or eutrophic like the North Sea (rich in nutrients). Public authorities often ask scientists for this kind of prediction but they must understand that our studies are necessarily long-term commitments. Providing funding just because there is some kind of public outcry following a proliferation of jellyfish is ill-advised. We need continuity, especially when nothing spectacular is seems to be occurring”.