Observational study of the relationship between pelagic prey and top predators

To complete the study on food webs, the members of the Éolien en Mer SIG asked the consortium in charge of the study to develop a specific section on the relationships between pelagic prey and top predators.

Context of the study

The aim of the food web study is to characterise and quantify the functioning of the local food web prior to the construction of the Dieppe Le Tréport offshore wind farm. The approach is based on the creation of a “zero” state using a BACI (Before After Control Impact) approach. The project is being carried out in parallel with the regulatory monitoring carried out for the reference state of the wind farm, and relies as much as possible on the data acquired in this context.

Carbon and nitrogen isotope analyses will be carried out on all the identified compartments in order to document the trophic levels and relationships of the various species present in the study area. Ultimately, the aim is to feed a numerical ecosystem model as part of a thesis to be completed by 2024.

Taking into account the upper compartments (birds and marine mammals) using data other than bibliographic data is a major advance in this type of modelling approach. Isotope analyses have been carried out on samples of stranded animals (marine mammals and birds) via the Réseau National Echouage (RNE) and Réseau National Oiseaux Marins (RESOM), as well as on chicks sampled from colonies in the sector as part of the DCSMM. These results, currently being analysed, will provide important information on the feeding ecology and trophic levels of the main species in the eastern Channel.

In order to obtain as complete a picture as possible of how the ecosystem functions, a gap was identified among the compartments sampled in the area: pelagic fish (Figure 1).

Figure 1 : Compartments sampled as part of the food web study

Initially, data from the literature was used to fill this gap, but the initial results of isotope analyses of megafauna showed little correspondence with the literature data.

With a view to completing the information on this essential link and in order to better document the relationships between prey and predators, an additional component of the food web project, focusing on these compartments, has been set up.


Although the aim of this project is not to investigate the distribution of prey and predators (which is covered in the regulatory monitoring), it does seem appropriate to use the pelagic fish sampling campaigns to observe the behaviour of the marine megafauna and deepen our understanding of the trophic relationships in the area.

The aim of this complementary phase is therefore to extend the studies carried out in the Trophic Networks project by focusing on pelagic fish and marine megafauna. The aim will be to carry out sampling and observation campaigns at sea on prey-predators, with a twofold objective:

The aim will be to (1) complete the information on local food webs by integrating isotopic data on pelagic fish and carrying out stomach content analyses in order to quantify flows; (2) verify and extend the information obtained in the literature and the initial results of isotopic analyses on the species hunted by the marine megafauna in the area.

To achieve this, pelagic trawl fishing campaigns lasting 3 days each will be carried out in and around the future wind farm zone. The target species will be pelagic fish such as garfish, herring, sprat, mackerel, anchovy, sardine, horse mackerel and sand eel (species identified during the regulatory monitoring fishing campaigns).

Figure 2 : A mini pelagic trawl measuring 1.7×1.7 metres will be used. The end mesh of the trawl will be 6mm square allowing smaller species such as sandeels, sprats or anchovies to be caught.

Specialists from CSLN will be on board to identify the species caught and take samples for isotope and stomach content analyses.

Specialists from Cohabys will be on board to observe the marine megafauna and identify the behaviour of birds and mammals in the hunting areas.

Pelagic fishing will be carried out on the hunting grounds of the marine megafauna in the sector, in particular through the scouting carried out by the dedicated observers, and observations of behaviour.

Figure 3 : Hunting common dolphins (picture: Ludivine Martinez)

The data collected (observations, isotope ratios and stomach contents) will be cross-referenced with the results already obtained for the various compartments in the food web project (isotope data), as well as with other existing data on the area: scientific literature, national censuses, distribution and abundance results from initial and reference surveys, etc.