EU request on mitigation measures to reduce bycatches of common dolphin (Delphinus delphis) in the Bay of Biscay (ICES Subarea 8)

This advice concerns the evaluation of mitigation measures to reduce bycatch of the common dolphin only in the Bay of Biscay (ICES Subarea 8), in order to facilitate direct comparison with previous advice. Advice released earlier this year evaluated the same mitigation measures in scenarios applied to both the Bay of Biscay and Iberian waters.

ICES concludes that, taking into account data from 2019–2021 and bycatch mortality estimates based on strandings data, none of the fifteen proposed mitigation scenarios can reduce bycatch of the common dolphin (Delphinus delphis) in the Bay of Biscay below the potential biological removal (PBR) limit.

ICES reaches the further conclusion that, based on data from 2019–2021 and bycatch estimates derived from at-sea monitoring data, six of the fifteen proposed mitigation scenarios are are likely to reduce bycatch of the common dolphin below the PBR limit. Two mitigation scenarios made it possible to reduce estimated bycatch to less than 75% of the PBR, while none could reduce bycatch to less than 50% of the PBR. This is in contrast with previous results based on 2016–2018 data, and reflects greater bycatch estimates in the recent period; these are partly due to improvements in the knowledge base (including métiers covered by at-sea sampling and more accurate assignment of effort to different métiers), and changes in the distribution of common dolphin within the Bay of Biscay.

ICES notes that uncertainty surrounding the extent to which the species is distributed, and of its abundance dynamics, are of concern. Availability of bycatch estimates for the species’ entire distribution range, and evaluation of mitigation measures at the same scale, would also increase confidence in the evaluation of the mitigation measures performance.

Given the low observation rates in several métiers of concern, ICES reiterates persistent issues in data quality, representativity, and coverage to inform bycatch assessments. ICES recommends enhanced monitoring to assess the effectiveness of management measures (including pinger use) and to augment precision in bycatch mortality estimates of common dolphin.