Testing the use of molecular analysis to study the diet of seals

This study will assess the contribution of molecular tools (environmental DNA) to seal diet monitoring. These new analyses will complement the traditional method of studying hard matter in seal faeces.

A protocol will be set up for routine application of these analyses to long-term monitoring, such as that carried out with the construction of the future Dieppe Le Tréport offshore wind farm.

Context: seal diet monitoring in relation to the future construction of the Dieppe Le Tréport offshore wind farm

When the baseline is established for the Dieppe Le Tréport wind farm, the diet of grey seals and common seals will be analysed in the Somme, Authie and Canche bay colonies. This study will show whether the diet of the seals will change in relation to a change in available prey during the construction and operation of the wind farm.

Grey seal ingesting a mullet at the surface (photo: J. Mestre)

The traditional method of studying the diet of seals is to identify the hard pieces found in the faeces. These are mainly fish bones and otoliths and cephalopod beaks. This technique provides the most detailed data for a quantitative description of the diet of these species.

Hard, indigestible parts: fish otoliths and bones, cephalopod beaks (photo: J. Spitz)

However, it has limitations as it requires the presence of hard pieces in the faeces. These will not be present if the predator only eats the flesh of the prey or if the prey does not have any indigestible parts (cartilaginous or gelatinous species).

Objectives of molecular testing

In view of the limitations of the conventional seal diet monitoring method, the GIS Éolien en Mer has proposed to test the use of molecular tools during the baseline ascertainment phase of the Dieppe Le Tréport offshore wind farm.

Using DNA sequencing technologies, it is now possible to list the prey species present in a sample of a predator’s faeces. However, the reliability and routine application of these methods for marine mammals remains to be improved.

The CEBC (Chizé Biological Studies Centre at La Rochelle University – CNRS), which monitors the diet of seals as part of the regulatory environmental monitoring of the Dieppe Le Tréport wind farm, was asked by the GIS to carry out these tests on the use of molecular tools in addition to the conventional method.

Grey seal resting on the beach

First of all, the literature will be analysed and discussions held with specialists in these molecular tools in order to determine the parameters of the analysis protocol.

A series of tests will then be carried out to assess the reliability of the protocol, particularly in relation to sub-sampling of the faeces and the representativeness of the sub-samples in relation to the full sample.

Finally, in order to assess the contribution of this new method, the list and occurrence of the species identified by the conventional method will be compared with those obtained from DNA analysis.

This test phase will establish an optimised protocol for routine molecular analysis during the subsequent phases of the offshore wind farm.