Advice explainer: Mitigation measures to reduce bycatches of common dolphin in the Bay of Biscay

​ICES released advice on mitigation measures to reduce bycatches of common dolphin (Delphinus delphis) in the Bay of Biscay (ICES Subarea 8) on 29 June 2023. Here, we take a look at some of the key points contained within.
Published: 4 April 2024

​What is the advice about?
​​​The advice, issued in response to a request from the European Commission (EC), estimates annual bycatch mortalities of common dolphins in ICES Bay of Biscay and Iberian Coast ecoregion (ICES Subarea 8 and Division 9.a) and evaluates mitigation measures to reduce these bycatches in the Bay of Biscay (Subarea 8). 

Which method was used to estimate bycatch?

Two different methods and data sources were used to estimate bycatch in 2019–2021. The same approach was followed in a similar advice release in 2020 (based on data for 2016–2018):

  1. At-sea bycatch monitoring data and total fishing effort data was collected from ICES Subarea 8 and Division 9.a
  2. Reverse-drift modelling was applied to data on stranded dolphins recovered from divisions 8.a–b and 7e along the French coast. 

Why use two methods?

The two bycatch mortality estimates, obtained by applying the two methods, were seen as two views of the same phenomenon and were considered within their uncertainty range to contain the true bycatch levels.

Which mitigation scenarios were tested?

Fifteen different bycatch mitigation scenarios were tested. These included the use of pingers in pair trawlers and temporal fisheries closures of different duration for fishing gears of concern (those with recorded common dolphin bycatch in ICES databases in Subarea 8 and Division 9.a).

As per the EC request, and for consistency with the previous 2020 advice, in this advice the mitigation scenarios (temporal fishing closures) were applied to Subarea 8 only.

What does the advice say?

Taking into account bycatch estimates based on strandings data, none of the fifteen proposed scenarios applied to Subarea 8 can reduce bycatch of common dolphins in Subarea 8 and Division 9.a below the potential biological removal (PBR) limit for the species' population in the Northeast Atlantic (i.e. 4927 animals per year).

Based on bycatch estimates derived from at-sea monitoring data, six of the fifteen scenarios applied to Subarea 8 are likely to reduce bycatch in the Bay of Biscay and Iberian Coast (Subarea 8 and Division 9.a) below the PBR limit.

What is PBR?

The potential biological removal (PBR) was used as a quantitative measure of the limit above which bycatch mortality might threaten the conservation status of the species. The PBR used was calculated based on the US Marine Mammal Protection Act. ICES has considered different proportions of PBR as potential thresholds corresponding to different levels of precaution and to adjust to regional agreements.​

Is there bycatch of common dolphin outside Subarea 8? 

Yes, there is. The advice included data on bycatch events from divisions 9.a and 7.e. In addition, ICES working group reports and previous advice have also documented bycatch of common dolphins in the Celtic Seas, Greater North Sea, and Azores ecoregions.

Is there a separate common dolphin population in the Bay of Biscay?

No, common dolphins in the Northeast Atlantic are considered a single panmictic population that ranges from Northwest Africa to Norway and west to at least the Mid-Atlantic ridge.

Why was advice on this issue released in January 2023 and also in June 2023?

The advice released in January 2023 applied the mitigation scenarios for Subarea 8 and Division 9.a. However, as per the European Commission's request, and for consistency with similar advice released in 2020, the scenarios were re-run for the advice released in June 2023, applying them for Subarea 8 only. 

What happens next?

The advice was delivered to the EC's Directorate-General for Maritime Affairs and Fisheries (DG MARE), which may use it as a basis for management decisions.

Find out more about ICES advisory process

The latest advice is always available to view and download from our library.​​​​​

Print this pagePrint it Request newsletterSend to Post to Facebook Post to Twitter Post to LinkedIn Share it
c FollowFollow Focus on ContentFocus on Content
HelpGive Feedback

Advice explainer: Mitigation measures to reduce bycatches of common dolphin in the Bay of Biscay

International Council for the Exploration of the Sea (ICES) · Conseil International pour l'Exploration de la Mer (CIEM)
ICES Secretariat · H. C. Andersens Boulevard 44-46, DK 1553 Copenhagen V, Denmark · Tel: +45 3338 6700 · Fax: +45 3393 4215 ·
Disclaimer Privacy policy · © ICES - All Rights Reserved