Feeding habits outside of the poisoning issue

More globally, another question the researchers are interested in is the position of seals in the food chain. “The question is important”, says the oceanographer, “Particularly in terms of the management of stock levels. Seals are animals that eat enormous quantities of food and their food, which is mainly made up of fish, varies according to age, sex and the place they live.  The evolution of their demographic can have important consequences for the profitability of commercial fishing”. But this is not the only interest of better understanding their dietary behaviour. “Several scientists have recently observed new behaviour in grey seals which could be the result of emerging competition from other marine mammals. Several studies point to the possibility that grey seals are likely to attack harbour seals and porpoises” 

The violent death of seals had already been observed, but it was presumed that boat propellers were the cause. For two years, there has been growing proof of the fact that their main rivals are behind these deaths. “Film footage showing grey seals attacking harbour seals exists. A subsequent autopsy on the victims showed that they had been peeled like apples (see article: When grey seals become killers). Importantly, some seals particularly like the skin and blubber of their prey, the parts which are richest in energy”. Traces of grey seal DNA have been identified in wounds by Thierry Jauniaux. A doctoral project begun in October 2015 and financed by the FNRS (France Damseaux) aims to use stable isotopes and trace elements to understand the role of grey seals and other species of marine mammals in the trophic food webs of the North Sea. “Analysis of isotopic ratios is one of the specialities of the Oceanology Laboratory of the University of Liege. In accordance with what is eaten by the organism, the variation of stable carbon isotopes and nitrogen can be observed in the skin or hair of the animals studied. This means that certain species can be identified by means of this study alone, but also the evolution of their dietary habits can be observed, without having to kill an individual to see what is contained in its stomach”. According to the tissues in question, we can now go further back in terms of analysing the diet of the specimen. Comparing the isotopic ratios of a maximum number of grey seals will make it possible to determine if the attack on harbour seals is becoming a collective habit or whether it remains the speciality of a few more aggressive and adventurous individuals.

The phenomenon seems new in the North Sea but had already been identified in Canada, “Was this down to lack of attention on our part or is it a recent development in our waters? Is it linked to an increase in the population which would mean a reduction in levels of prey and which is driving them into competition? We can also ask the question as to whether it is isolated behaviour or if it is more common. In summary, there are enough unanswered questions to merit our full attention”. This story, which seems so far removed from poisoning by trace elements and persistent organic pollutants has what it takes to arouse the curiosity of Krishna Das and his collaborators. “Up to now, we know that the dietary habits of seals are confined mainly to fish. If it emerges that grey seals are specialising in the capture of marine mammals, this would mean that they are by-passing a trophic level and will be even more contaminated by lipophilic pollutants. In these areas of research, everything is connected”, says the researcher. “There are still so many aspects that we don’t understand such as certain aspects of the mercury cycle or the cocktail effect of pollutants. For the moment, we are studying them separately. But they can act in synergy in the body of the animal. It is one of the many aspects that we wish to research more fully”.