WP1 : Common dolphins in the Bay of Biscay

The challenge of this WP is to provide new knowledge on dolphins aspects that can lead to accidental captures, and essential knowledge for management of common dolphin population(s) affected by captures in the Bay of Biscay. The first step will be to characterize the population(s) of common dolphins frequenting the Bay of Biscay and the coastal-offshore structuring of this (these) population(s), and to (re)define the possible management units of the species in the northeast Atlantic. In parallel, several aspects of the ecology of dolphins that can have a major influence on the risk of capture will be caracterized: the state of health of individuals (mainly addressed via their levels of contamination), their spatial distribution and their movements at fine scales, and their food ecology. To perform the various analyzes (genetic and ecological tracers, contaminants, trophic analyses, etc.), samples from stranded dolphins and corresponding to individuals captured on the continental shelf will be selected, and biopsies carried out on common dolphins during the DelGOST campaign to provide samples of dolphins from the oceanic zone, a priori unaffected by captures. New genetic tools such as environmental DNA will be used to obtain additional information on population structure from non-invasive samples. This WP1 is divided into 4 tasks:

Populations tracers and management units

For the populations management and the set up of appropriate conservation measures, it is important to know the populations structure and to consider the different pressures (interactions with fisheries or pollution, for example) impacting them. The populations stucture can be assessed at different time scales:

genetic tracers reflect gene flows and genetic differentiation across generations;
ecological tracers (stable isotopy, certain contaminants, etc.) reflect food/habitat preferences on shorter time scales.

Coupling both tracers allows the study of the population structure dynamics (is the same structure observed at different time scales?) and of the management units.

This first task aims to answer the following questions:

Can we identify a (new) structuring of populations using the new (genetic) tools available?
Are there one or more management units for common dolphins frequenting the Bay of Biscay?
Can we use new methods (eDNA) to characterize intraspecific diversity of common dolphin?

Contamination of captured individuals

The marine environment is the final sink for many contaminants (metallic or organic). Some of these contaminants have the ability to accumulate in fatty tissues and/or to biomagnify along food chains. Cetaceans can be particularly impregnated.

Analyzes of historical contaminants (metals and organic contaminants) and emerging contaminants (certain brominated and chlorinated flame retardants, per- and polyfluoroalkylated metals or metals from the rare earth family) will be carried out on individuals whose cause of death is proven (from the Pelagis collection).

This second task aims to answer the following questions:

Do common dolphins with obvious capture marks present different contamination levels and profiles from the rest of the individuals found stranded without capture marks?
Among common dolphins with obvious capture marks, do individuals stranded in recent years show different contamination levels and profiles than those stranded one to two decades ago?

Spatial distribution and fine scale movement

Aerial surveys will be carried out to monitor the variations on a fine temporal scale in the common dolphins distribution on the Bay of Biscay plateau during winter 2023, the major capture season. A similar overflight campaign in 2020 has shown that common dolphins sometimes congregate massively inside the continental shelf for very short periods. This aerial campaign in winter 2023 will be carried out in close collaboration with the underwater drone deployment campaign (WP2) for the monitoring of small pelagic fish, to identify potential links between dolphin movements and changes in preys distribution/behaviour.

This third task aims to answer the following questions:

What are the movements at fine spatial and temporal scales of common dolphins?
What are the distribution variations of common dolphins in winter in the Bay of Biscay, with what dynamics?
Can we link these movements and changes in distribution to other ecological factors?

Food ecology

Common dolphins feed mainly on small fatty pelagic fish such as sardines, anchovies, sprats, horse mackerel, mackerel on the shelf. Also, captured dolphins often show full stomachs with fresh remains, which suggests capture in the feeding phase and raises questions about the role of feeding behavior in the risk of capture. Within the framework of Delmoges, an update of knowledge on the diet and food ecology of common dolphins in the Bay of Biscay will be carried out, and possible temporal changes apprehended, through the study of stomach contents (direct method of study diet) and analysis of stable isotopes such as carbon and nitrogen (indirect trophic tracers).

Finally, in connection with WP2 and in order to partially fill the current lack of data concerning the distribution of small pelagic fish in winter (due to the absence of fishing campaigns in this season), the dolphins whose stomach contents will be studied will also be used as potential biological samplers of these prey fish. A spatialization of the distribution of prey predated by common dolphins will thus be carried out using the reverse drift model MOTHY.

This last task aims to answer the following questions:

What are the preferred prey species of common dolphins during captures? Have they changed over time?
Do isotopic signatures reveal a temporal evolution of feeding areas in common dolphins?
Can dolphins be biological samplers to provide information on the distribution of fish in winter in the absence of a fishing campaign?