Proliferations that cause major headaches

“These episodes are dreaded throughout the world today”, comments Amandine Collignon, the author of a doctoral thesis (1) which she defended last spring at the Oceanology Laboratory (Department of Biology, Ecology and Evolution) of the University of Liege under the supervision of professors Jean-Henri Hecq and Anne Goffart. “But we still don’t understand why these proliferations occur and we also need to distinguish between purely subjective theories and real scientific data. If, for example, the fishing industry decides to frequent a new fishing zone, the fishermen might be tempted to interpret an abundance of jellyfish as an abnormal phenomenon whereas these animals might have been present in this area for a long time”.  

A prudent approach also needs to be applied to phenomena occurring in tourist resorts: “the recorded jellyfish ‘invasions’ of beaches do not necessarily mean that they are abnormally numerous in the sea. Sometimes a conjunction of relatively harmless events can create a false impression. This is the case, for example, when the vertical migration of jellyfish (a phenomenon that is frequent at the end of the day in temperate waters) is followed by an encounter with a shoal of predatory fish. This swarm of jellyfish that were initially harmless and healthy can then become transformed, under the effect of other factors such as the turbulence of currents, into a ‘passive’ swarm made up of exhausted and wounded individuals. They drift in strong currents and are washed up on beaches”. These somewhat benign ‘invasions’ are interpreted as aggressive by tourists who are disturbed by the presence of the creatures…

Corsica: an ideal study location

In order to study such phenomena, and particularly to study the development of jellyfish populations, the University of Liege has access to an invaluable tool: the Stareso oceanography station. Built at the beginning of the 1970s and having the particularity of being located far from densely populated areas, the station is literally ‘immersed’ in the terrestrial and maritime environment of Calvi Bay in the North-West of Corsica and is thus protected from human influences such as pollution. In addition, the station has amassed an impressive volume of temporal data (meteorological, physical, chemical and biological) relative to the marine ecosystem. Once a week, a marine expert combs a transect (an established route that is regularly followed) of the bay in a boat equipped with a 200 micrometer steel mesh fishing net for a period of twenty minutes. The expert also gathers precious zooplankton, a fundamental research ‘material’ for the investigations carried out by the oceanographers from Liege: fish larvae, prawns, eggs, sea-urchin eggs, appendicularians, gastropods, etc.

A group of jellyfish that are particularly represented in Calvi Bay also feature in this daily fishing expedition: Siphonophores and Calycophores, are of particular interest to Amandine Collignon “I chose this particular group of medium-sized jellyfish for my thesis because it is an abundant lesser-known species in the Mediterranean and also because our samples have revealed that these organisms, which are made up of several individual categories with special functions, are present all year round in the marine ecosystem, particularly the polymorphous species Chelophyes appendiculata. In fact, I could not run the risk of studying only jellyfish whose interannual cycles are known to be spread over six to ten years for my four-year doctoral thesis. This is why I concentrated on the Calycophores while also studying one of the best-known scyphozoan jellyfish (Editor’s note: the ‘real’ large jellyfish) in Europe: Pelagia (Pelagia noctiluca).  My objective was to determine the variability of these jellyfish and the influence that biotic factors (presence of phytoplankton and zooplankton) and abiotic factors (wind, temperature, ocean currents, pollution, etc.) have on these”.

(1) Abondance et variabilité des méduses en baie de Calvi (Abundance and variability of jellyfish in Calvi Bay), doctoral thesis, Amandine Collignon, University of Liege, 2014.