During scientific fishing campaigns researchers have cast their nets into five estuaries: those of the Gironde, the Charente, the Loire, the Seine and the Escaut. Joseph Schnitzler continues: “The results are clear; the fish taken from the Seine and the Escaut are much more contaminated by PCBs than the others.” (3) This is not surprising given that the river catchment areas that feed these two estuaries are much more populated and industrialized than the others. The river catchment area of the Seine, for example, covers an area of almost 80,000 km², a territory that is three times bigger than Belgium, with 16 million inhabitants and includes 6 large cities including Paris. That of the Gironde only covers a territory of 50,000 km², with fewer than 3 million inhabitants and only two large towns including Bordeaux.

The comparison with fish samples from the Mediterranean Sea is also enlightening. For his DEA dissertation in 2006, Joseph Schnitzler went to Greece to study the levels of contamination in both farmed and wild sea bass. “They are unquestionably higher in the North Sea than in the Mediterranean.” (4)

Fish that fight pollution

But do these higher or lower levels of PCB cause abnormalities of the thyroid? Is the health of the fish damaged? In order to be sure, the researcher from Liége resorted to his microscope. The thyroid gland of a fish resembles a bunch of grapes. It is made up of several follicles which, under the microscope, look like berries. When examined, the “berries” of the samples from fish taken from the Escaut estuary which is extremely polluted, are seen to be smaller on average than the others. This suggests greater thyroid activity compared with that of the individuals from less polluted estuaries.

More pollution = more hormones? How can this seemingly paradoxical equation be explained? Joseph Schnitzler offers an explanation: “This is a compensation effect, to the extent that pollution also causes a loss of hormone production in the organism. The pollutants, which have a very similar form to the hormones, tend to affix themselves to the proteins which circulate in the blood and to take their place. The hormones that do not find proteins are eliminated by the kidneys. It is to compensate this loss of production caused by the pollutants that the follicles of the thyroid prepare to synthesize more hormones”

The researcher from Liège also measured the interaction between enzymes produced by the liver and a hormone synthesized by the thyroid (T4). This enzyme activity which transforms a hormone of the thyroid to render it directly active in the organism (transformation of T4 into T3) is doubled in fish that come from the most polluted estuaries. “It’s a fresh clue, that the organism tries to compensate for the loss of normal hormone production caused by the pollutants,”continues Joseph Schnitzler.

(3) Schnitzler J., Thomé JP., Lepage M., Das K., Organochlorine pesticides, polychlorinated biphenyls and trace elements in wild European sea bass (Dicentrarchus labrax) off European estuaries Langue du document, Science of the Total Environment, Elsevier Science, 2011, 409, 19, 3680-3686. Consult the publication in ORBi
(4) Schnitzler J., Koutrarkis E., Siebert U., Thomé J-P., Das K. Effects of persistent organic pollutants on the thyroid function of the European sea bass (Dicentrarchus labrax) from the Aegean Sea, is it an endocrine disruption?, Marine Pollution Bulletin , 2008 :56, 10, 1755-1764. Consult the publication in ORBi