The porpoise, sentinel of the ocean

Some years ago, Krishna Das, a Research Associate at the F.R.S. - FNRS in the Oceanography Laboratory, examined the health of porpoises, the most widespread cetacean in the North Sea (around 300,000 individuals). He explains, “The porpoise is also at the top of the food chain. It is a sentinel species. It constitutes a good indicator of the state of cleanliness of our oceans.” During his post-doctoral period at the FTZ (Forschung-und Technologie Zentrum) Krishna Das studied samples of thyroid glands taken from porpoises from the Atlantic, the North Sea and the Baltic Sea by microscope. “We have shown that the levels of PCB in the thyroid vary according to the place of origin of the porpoises. Those from the Baltic which is more polluted than the North Sea are more contaminated. Our study has also shown that thyroids of porpoises from the Baltic were in a poorer state than the others.” (1&2) The levels of PCB measured in the porpoises of the Arctic zones much further to the north and further from the sources of pollution were ten times lower than those measured in the North Sea.

Research on these large cetaceans is not easy however. It is a protected species so researchers must satisfy themselves with dead individuals that have stranded or have been accidentally caught in fishing nets. This is where the idea to work on another more accessible sea animal, namely the sea bass came from. The advantage of this fish is that it spends the two or three first years of its life in the estuaries of rivers before returning to the sea. Joseph Schnitzler explains. “And the estuary is a nursery: there are fewer predators, food is abundant etc. Yet it is a very polluted nursery!”

In order to accumulate thyroid samples from sea bass, Joseph Schnitzler has participated in several scientific initiatives organised by  IFREMER (the French Research Institute for Exploitation of the Sea) CEMAGREF (Research Institute for Agricultural Engineering and Environment, France), and INBO (Research Institute for Nature and Forest, Belgium). The initiatives were conducted to make an inventory of marine fauna with the intention of fixing fishing quotas. Joseph Schnitzler explains. “Once the net is hauled in, the contents of the catch is dropped onto a circular conveyor belt around which are gathered ten or so researchers. The fish pass before us and we sort them in terms of size and species. It is quite exhausting work. But after a day I could get on with my research: taking samples from the sea bass and, in the boat’s laboratory, extract the thyroids and muscle tissue and place them either in formaldehyde or in the freezer in order to be able to study them on my return to Liège.”

(1) Das  K., Vossen A., Tolley K. Vikingsson G., Thron K. Müller G., Baumgärtner W., Siebert U. , Interfollicular fibrosis in the thyroid of the harbour porpoise: An endocrine disruption?  Archives of Environmental Contamination & Toxicology, Springer Science & Business Media B.V., 2006  Consult the publication in ORBi
(2) Schnitzler J., Siebert U., Jepson P., Beineke A., Jauniaux T., Bouquegneau  JM, Das K., Harbour porpoise thyroids: Histological investigations and potential interactions with environmental factors Langue du document, Journal of Wildlife Diseases, 44, 0090-3558, 2008. Consult the publication in ORBi