Abstract
The first Unusual Mortality Event (UME) related to fishing activity along the Atlantic coast recorded by the French Stranding Network was in 1989: 697 small delphinids, mostly common dolphins, washed ashore, most of them with evidence of having been bycaught. Since then, UMEs of common dolphins have been observed nearly every year in the Bay of Biscay; unprecedented records were broken every year since 2016. The low and unequally distributed observation efforts aboard fishing vessels in the Bay of Biscay, as well as the lack of data on foreign fisheries necessitated the use of complementary data (such as stranding data) to elucidate the involvement of fisheries in dolphin bycatch. The aim of this work was to identify positive spatial and temporal correlations between the likely origins of bycaught stranded common dolphins (estimated from a mechanistic drift model) and fishing effort statistics inferred from Vessel Monitoring System (VMS) data on vessels >12 m long. Fisheries whose effort correlated positively with dolphin mortality areas after 2016 included French midwater trawlers, French Danish seiners, French gillnetters, French trammel netters, Spanish bottom trawlers, and Spanish gillnetters. For the French fleet only, logbook declarations, sales, and surveys carried out by Ifremer were integrated into fishing effort data. Six fleets were active in common dolphin bycatch areas at least twice between 2016 and 2019: gillnetters fishing hake, trammel netters fishing anglerfish, bottom pair trawlers fishing hake, midwater pair trawlers fishing sea bass and hake, and Danish seiners fishing whiting. Except for changes in hake landings in some fisheries, there were no notable changes in total fishing effort practice (gear or target species) based on the data required by the ICES and Council of the European Union that could explain the large increase in stranded common dolphins recorded along the French Atlantic coast after 2016. Small scale or unrecorded changes could have modified interactions between common dolphins and fisheries, but could not be detected through mandatory data-calls. The recent increase in strandings of bycaught common dolphins could have been caused by changes in their distribution and/or ecology, or changes in fishery practices that were undetectable through available data.
Highlights
The first multiple stranding event along the French Atlantic coast was recorded in winter 1989: 697 delphinids, mostly common dolphins, washed ashore, most of them presenting evidence of having been bycaught (Figure 1)
The average number of strandings with bycatch evidence was estimated at 205 ± SD 177 common dolphins per year for the entire study period (1990–2019), while the average in the last four years increased to 520 ± SD 172
No multiple stranding events were recorded in 2010, but one to three events were observed along the French Atlantic coast each year starting in 2011, with an average of 138 ± SD 104 common dolphins showing evidence of bycatch per event
Summary
The first multiple stranding event along the French Atlantic coast was recorded in winter 1989: 697 delphinids, mostly common dolphins, washed ashore, most of them presenting evidence of having been bycaught (Figure 1). French driftnet fisheries, fishing for albacore tuna, began operating in the eastern North Atlantic in the mid-eighties (Northridge, 1991) This fishery operated between June and late September, more than 200 NM off the coast of the Bay of Biscay. High levels of strandings reported in the Bay of Biscay in winter and the increase of driftnet fishing effort in summer were two independent processes that were erroneously confused in the media and public opinion. This likely influenced the Council of the European Union’s decision to reduce driftnet length in the Bay of Biscay to 2.5 km in 1991, and in 1997 to announce a ban of the fishery from January 1, 2002 (Council of the European Union, 1998)
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