Les échouages des mammifères marins en France depuis 1990

• Nearly 27,000 stranded marine mammals have been recorded since 1990 in metropolitan France. The increase in strandings on the metropolitan coast has been significant since then. With nearly 2,600 strandings, the 2019 result is the largest ever recorded over the 1990-2019 period. • The common dolphin (Delphinus delphis) is by far the most represented species in these strandings, followed by the harbour porpoise (Phocoena phocoena) and the striped dolphin (Stenella coeruleoalba).

The Atlantic arc accounts for three-quarters of strandings recorded since 1990. These are mainly concentrated in the Landes, Finistère, Gironde, Vendée and Charente-Maritime departments. In the Eastern Channel – North Sea, the Manche, Pas-de-Calais and Nord departments are most affected. The Var is the Mediterranean department most affected by strandings, while remaining well below Atlantic departments.

In overseas territories, 1,280 strandings have been recorded since 1990 with significant year-to-year variations mainly due to mass stranding events. 2006 and 2010 were record years with 168 and 166 individuals respectively.

• From 1990 to 2019, nearly 27,000 stranded individuals were recorded in metropolitan France. The number of strandings varies greatly from year to year. However, a significant increase in their number is observed (Mann-Kendall statistical trend test). The annual average number of strandings rose from 514 individuals in the 1990s to 789 in the 2000s and then to 1,429 over the 2010-2019 period. 2019 is the record year of the historical series with 2,590 stranded individuals. • Generally, strandings are more numerous from February to April, a winter period of high mortality. Most strandings involve animals that died at sea. Causes of death may be linked to direct human-caused pressures such as accidental bycatch from fishing activities, ship collisions, or disturbance; or indirect pressures such as chemical or noise pollution and overfishing of their food resources. • The number of individuals killed by accidental bycatch is variable but can sometimes reach significant proportions. For example, in 2018 the PELAGIS Observatory estimated that 60% of apparent common dolphin bycatch rates along the Atlantic coast (Dars et al., 2018). Dead animals are washed ashore through marine currents and winds. Only a small percentage reach land. According to carcass drift studies, only 20 to 25% of animals float and drift; the others appear to sink and cannot be found stranded (Peltier et al., 2019). • Living individuals are also found stranded on the coast. In mass stranding events (relatively rare on the metropolitan coast), the reasons can be multiple. A leader individual may lead its group onto the beach. It also happens that individuals are trapped by falling tide. Many dolphins and whales use sound waves to navigate at sea. Underwater acoustic disturbances could disorient animals and cause them to strand.

• The common dolphin is by far the most represented species over the 1990-2019 period with 38% of all strandings. It is followed by the harbour porpoise and striped dolphin with 15% and 9% of strandings respectively. • Common dolphin strandings show significant annual variations with a significant increasing trend over the entire time series (Mann-Kendall statistical trend test). Thus, the annual average of strandings rose from 173 individuals in the 1990s to 318 in the 2000s. Over the 2010-2019 period, the annual average is 544 with nearly 1,144 strandings in 2019, a record year for common dolphin strandings. • While common dolphin strandings are regular on the Atlantic arc, they are occasional in the Mediterranean (only 0.2% of the species strandings since 1990). In the Atlantic, certain years saw high numbers of strandings due to accidental bycatch (notably in 1997, 1999, 2000, 2001, 2016, 2017, 2018 and 2019) linked to many fishing practices, including pair trawling and gillnets and trammel nets. • Since 2016, the common dolphin mortality rate from bycatch in professional fishing gear has exceeded the threshold of 0.78% beyond which the 100-year survival of the Northeast Atlantic population is threatened (threshold defined in 2020 by the International Council for the Exploration of the Sea). In 2019, this mortality rate reached 2% (the indicator 'Accidental bycatch of common dolphins in fishing gear' of the National Biodiversity Observatory - ONB). • Furthermore, the 2018 assessment of good environmental status (GES) under descriptor 1 'Marine Mammals' of the Marine Strategy Framework Directive (MSFD) showed that for the bycatch indicator, accidental bycatch rates in common dolphins exceed the GES threshold values set at national level (greater than 1.7% in one year or more of the evaluation cycle; less than 1% for all years of the evaluation period) in all marine sub-regions of the Atlantic. • To significantly reduce dolphin strandings, France made it mandatory, starting January 1, 2019, for all French fishing professionals to report accidental bycatch of marine mammals. Furthermore, starting January 1, 2020, by ministerial decree, all French pelagic trawlers over 12 meters operating in the Bay of Biscay during the winter period are required to be equipped with acoustic deterrent systems 'pingers'. • Over the study period, strandings of harbour porpoises, grey seals (Halichoerus grypus) and harbour seals (Phoca vitulina) show an increasing trend. For the harbour porpoise (second most affected species), this increase can be partly explained by the increase in accidental bycatch particularly between 2000 and 2010 as well as by population displacement which returned from the British Isles and the North Sea to the Bay of Biscay. • For grey seals and harbour seals, the increase in strandings may likely be related to population growth. Thus, according to the indicator 'Evolution of harbour seal populations in metropolitan France' of the National Biodiversity Observatory (ONB), harbour seal abundance increased by 8% over the 2014-2019 period. Strandings of short-finned pilot whales (Globicephala melas) and Risso's dolphins (Grampus griseus) vary by year without a clear trend. Over the entire series, a large number of individuals could not be identified to species level either due to lack of distinctive features: skull degradation or absence, advanced state of decomposition, or due to carcass removal before examination. Thus, species could not be confirmed for nearly 3,500 small delphinids (most often common dolphins or striped dolphins), 1,200 odontocetes (small delphinids or harbour porpoise) and 560 seals. • During the third assessment of marine mammal conservation status, conducted under the Habitats Directive for the 2013-2018 period, conservation status could not be determined for most evaluated species, namely 20 species out of 25 in the Atlantic and 13 out of 13 in the Mediterranean. For most of these species, knowledge in 2018 is much higher than in previous assessments (2001-2006 and 2007-2012). Nevertheless, unknown conservation status in the 2006 and 2012 assessments prevents establishing trends and tends toward unknown conservation statuses. Only the harbour seal and grey seal benefit from 'favourable' conservation status in the Atlantic. The status of common dolphin, bottlenose dolphin (Tursiops truncatus) and harbour porpoise is considered 'unfavourable-inadequate' in the Atlantic. • Regarding the IUCN Red List of Threatened Species (updated in 2020), the sperm whale (Physeter macrocephalus) and fin whale (Balaenoptera physalus) are considered vulnerable globally. The grey seal, harbour seal, harbour porpoise, fin whale, Risso's dolphin and white-beaked dolphin (Lagenorhynchus albirostris) are considered 'near threatened' in France and 'least concern' globally. The common dolphin, minke whale (Balaenoptera acutorostrata), short-finned pilot whale, striped dolphin and bottlenose dolphin are currently classified as 'least concern' in France and globally.

• The number of strandings varies greatly by maritime region. Between 1990 and 2019, the South Atlantic coast accounted for 37% of strandings, followed by the North Atlantic - Western Channel (36%), Eastern Channel - North Sea (18%) and Mediterranean (8%). • The number of recorded strandings has increased substantially on most maritime coasts. Considering 1990-1995 and 2014-2019 averages, strandings increased 19-fold in the Eastern Channel - North Sea, 4-fold in the North Atlantic - Western Channel and 2-fold in the South Atlantic.

• The number of stranded marine mammals varies greatly from department to department. The distribution of dominant species, fishing fleets and the interaction between them largely explains this distribution, marked primarily by common dolphin strandings in the Atlantic, harbour porpoise strandings in the Eastern Channel - North Sea and striped dolphin strandings in the Mediterranean. • Six coastal departments averaged more than 80 strandings per year over the study period. All are located on the Atlantic arc. These are Landes (148 strandings/year), Finistère (122 strandings/year), Vendée (103 strandings/year), Gironde (92 strandings/year) and Charente-Maritime (82 strandings/year). • In the Eastern Channel – North Sea, the Manche, Nord and Pas-de-Calais departments are most affected. The Var is the most impacted in the Mediterranean, while remaining well below Atlantic departments. • When annual strandings are related to each department's coastal length, the number of strandings per linear kilometer is very high in Landes. These are followed by Nord, Pas-de-Calais, Vendée, Somme, Pyrénées-Atlantiques and Gironde departments.

• In overseas territories, the marine mammal stranding observation network was established in the early 1990s and is gradually strengthening. With 1,280 individuals recorded from 1990 to 2019, the number of strandings remains low relative to the length of coast involved. • Significant year-to-year variations have been observed since 1990. These are mainly due to mass stranding events. Thus, 2006 and 2010, marked by mass strandings, remain the periods with the highest numbers recorded, with 168 and 166 stranded individuals respectively.

• The territories with the highest number of strandings since 1990 are the French Southern and Antarctic Lands (TAAF), with 27% of overseas strandings. These are followed by New Caledonia (22%), Saint Pierre and Miquelon (14%) and French Guiana (9%). For TAAF and New Caledonia, the high number of stranded individuals is explained by mass stranding episodes on these territories (101 individuals stranded in mass in New Caledonia since 1990, 259 individuals in TAAF). For Saint Pierre and Miquelon and French Guiana, the high numbers can be attributed to local network establishment and improved reporting systems through training and awareness activities. • The Scattered Islands, Wallis and Futuna and Saint Barthélemy had fewer than 6 strandings over the study period. This is explained by the recent network establishment, difficulty accessing coasts and the presence of predators (such as sharks) that cause dead animals to disappear.

• Short-finned pilot whales and tropical pilot whales (Globicephala melas and Globicephala macrorhynchus) are the most recorded species and most affected by mass strandings on overseas coasts. These two species account for 32% of strandings reported over 1990–2019. The largest mass stranding episodes of pilot whales were reported at Kerguelen in TAAF in 2006 (120 short-finned pilot whales stranded) and 2010 (139 short-finned pilot whales stranded). • Among other most recorded species are: the sperm whale (Physeter macrocephalus), Guiana dolphin (Sotalia fluviatilis), dugong (Dugong dugon) and spinner dolphin (Stenella longirostris) with over 50 individuals stranded for each. These are followed by the harp seal (Pagophilus groenlandicus), humpback whale (Megaptera novaeangliae), melon-headed whale (Peponocephala electra) and false killer whale (Pseudorca crassidens) with over 25 individuals stranded.

• Marine mammal strandings on French coasts are monitored by the National Stranding Network (RNE), consisting of over 565 correspondents coordinated by the PELAGIS Observatory, Joint Research Unit 3462, University of La Rochelle and CNRS. This network has been active on the metropolitan coast since the early 1970s. However, the observer network is considered relatively dense and stable since the 1980s and reliable figures since the early 1990s. It is therefore considered that fluctuations or trends observed since 1990 reflect biological or physical parameters such as abundance, mortality or drift conditions. • For overseas territories (Guadeloupe, Martinique, French Guiana, Mayotte, New Caledonia, French Polynesia, Réunion, Saint Pierre and Miquelon, Saint Martin, Saint Barthélemy, French Southern and Antarctic Lands, Scattered Islands, Wallis and Futuna), the network has been active since the 1990s. The time series stabilizes from the early 2000s but significant effort remains to strengthen local networks.

• In France, all marine mammal species are protected. Marine mammal stranding observations provide: • better understanding of populations living along French coasts; • better knowledge of their ecology; • better assessment of threats to them, as maritime uses become increasingly numerous. • Monitoring populations at sea is indeed complex to conduct and sampling even more difficult. Analysis of strandings is therefore an effective way to complement observational data. With each stranding, many parameters are analyzed, such as species, age, reproductive status, genetics, diet study, heavy metal or persistent organic pollutant concentrations in tissues, pathology assessment, cause of death, etc.

Three major families make up marine mammals: Cetaceans (dolphins, porpoises and whales), Pinnipeds (seals) and Sirenians in overseas territories (manatees and dugongs).