The Workshop on estimation of MOrtality of Marine MAmmals due to Bycatch (WKMOMA) addressed a special request from OSPAR regarding the bycatch mortality of marine mammals (harbour porpoise Phocoena phocoena; common dolphin Delphinus delphis; and grey seal Halicho-erus grypus) within the OSPAR maritime area. The objective of the workshop was to generate bycatch rates and associated confidence intervals for static and towed gears for relevant species within the three species assessment areas defined by OSPAR. Subsequently, the species-specific bycatch mortality estimates in the defined assessment area were requested. OSPAR provided thresholds for the relevant species/assessment units and ICES were tasked to compare the mor-tality estimates to the provided thresholds and identify any critical issues relevant for the com-parison.

ICES issued an official data call requesting 18 of the 20 ICES countries with fisheries operating in the OSPAR area to provide data. Norway, the Faroes, and Russia did not submit bycatch mon-itoring and effort in response to the data call, and it was therefore not possible to estimate bycatch in these waters. The data call aimed to collect data describing total bycatch monitoring/sampling effort and grey seal, harbour porpoise and common dolphin bycatch incidents from the years 2005 until 2020 from fisheries operating in the OSPAR Region. Most of the contacted countries submitted data, but the quality and quantity of the data provided varied widely among nations. Regarding data on fishing effort, ICES asked all EU member states for permission to use fishing effort data held in the ICES Regional Database (RDB) which contains data on fishing effort data in various metrics by métier level, country, vessel size and ICES rectangle. When permission was granted, a data extraction was undertaken by the ICES data centre providing effort data from 2015 to 2020.

All submitted monitored effort data from 2005 until 2021 was summarized and resulted in a total of 884 common dolphins, 1221 harbour porpoises and 574 grey seals were observed bycaught from 2005 to 2021.

As recommended in ICES WGBYC 2020, a modelling procedure was carried out to generate by-catch rates. Before bycatch modelling occurred, statistical tests were run on the datasets of the three species to test how bycatch rates were affected by year, month, vessel size, ICES sub-area, and métier (level 4). Results varied between the three species, with all three species having higher bycatch rates the more recent years (2015-2020) and significant effects of sub-areas and métiers. Vessel size was significant factor for harbour porpoise with larger vessels (12-15m or larger) hav-ing higher bycatch rates, while the opposite was true for grey seals for which smaller vessels (up to 12 m) had higher bycatch rates. Thereby the monitored effort data sets were pooled from 2015 to 2020 for further analyses and for harbour porpoise and grey seal data was stratified by vessel size.

A Gamma Hurdle model was used to estimate bycatch rates per day at sea. This two-step process first estimates the probability of a bycatch occurring, and then their intensity (number of animals being caught). Multiplying those values together results in an overall bycatch rate for the ob-served days at sea.

For common dolphin, the highest frequencies bycatch events over 2015-2020 were recorded in PTM and OTM in ICES area 27.8 and OTM in ICES area 27.6. In ICES Subarea 27.7, highest fre-quencies were estimated in GTR, OTB and OTT. Bycatch event frequencies were also estimated for GNS and PS gears in ICES area 27.9 however these rates were below 0.01 events per day at sea.

The average number of common dolphins/bycatch event was close to one individual in most gears operating in ICES area 27.7 and static gears in 27.8. It ranged from 1.5 to 2 individuals in PS and GNS in ICES area 27.9. The numbers of common dolphins bycaught per haul was highest in PTM in 27.8 and OTM in 27.8 and 27.6 (3.58 common dolphins/bycatch event), and in PTB operating in 27.8 (4.09 common dolphins/bycatch event).

The bycatch rates extrapolated to the fishing effort gave the total number of common dolphins bycaught estimated to 6,404 individuals (95% CI 3,051-9,414) in 2020 for the entire assessment area. The highest bycatch estimate was calculated for PTM followed by GNS/GND and GTR. Bycatch estimates in 2020 are consistent with previous understanding of common dolphin by-catch and remain in the same order of magnitude as previous ICES bycatch estimates based on observer programs and strandings (ICES, 2020a). However, the 2020-point estimate is higher than that of the mean annual bycatch estimate across all métiers of 3973 (95% CI 1998–6599) dol-phins for 2016-2018 for the Bay of Biscay and Iberian Coast.

Highest frequencies of harbour porpoise bycatch events were recorded in large vessels using GNS in ICES areas 27.3 and 27.4, when all data (2015-2020) are considered. However, sampling by one country in that area was unrepresentative due to non-random sampling and constitutes a very high proportion of the observed effort. Without that country, the rates are much lower for ICES Subarea 27.4. Small vessels using GNS also had relatively high rates in subareas 27.4 and in 27.3. Rate of bycatch events was also high in GNS within ICES subarea 27.5 and in Subarea 27.7 the highest bycatch rates were found in GNS/GTR/ for large vessels. In OTT and OTB there were few bycatch observations and therefore bycatch in all areas were grouped together to cal-culate the rate.

The average number of porpoises/bycatch event over 2015-2020 was generally between 1 and 1.5 individuals in most métiers and areas, apart from large vessels using GNS in ICES Subarea 27.4 where 2.5 individuals were observed on average per bycatch event if the non-random sampling observer effort is included. Removing that sampling lowers that estimate down to 1.33 individ-uals/bycatch event.

The mortality for harbour porpoise was estimated for all requested assessment areas except the Belt, the Faroes and the Iberian Peninsula. WKMOMA estimates that the bycatch in 2020 in the West Scotland and Ireland assessment unit to be 305 (134-686) harbour porpoises. In the Irish Sea’s assessment unit 12 (6-27) porpoises were bycaught, of which 2 individuals were estimated to be caught in GNS/GND while 10 individuals were estimated to be caught in OTB/OTT. WKMOMA estimates bycatch in the Icelandic assessment unit to be 1712 (1123-1973) harbour porpoises, all caught in GNS. In the North Sea two estimates are presented, one higher estimate including submitted data from all countries, but heavily skewed due to very frequent bycatch observations from targeted few vessels and one estimate where the monitoring effort data from this country has been taken out. The two estimates for the North Sea are 5929 (95% CI 3176-10739) and 1627 (95% CI 922-3325; not including the unrepresentative data). Majority of the by-catch is estimated to be from GNS/GND in both cases (1306/5327 individuals), followed by GTR (198/479 individuals) and to lesser extent from OTB/OTT (123/123 individuals).

Highest frequencies of grey seal bycatch events were recorded in small vessels using GNS in ICES Subarea 27.7 and in small vessels using GNS in Subarea 27.5. Bycatch frequency in GND and GNS in ICES subareas 27.3 and 27.7 were around 50% lower than in the areas mentioned above and even slightly lower in GTR in subareas 27.7 and 27.8. Bycatch events were observed also in OTM in these areas, however fewer than in GNS/GTR. The average number of grey seals caught per bycatch event was between 1 and 1.5 individuals in most métiers and areas, besides small vessels using GNS in ICES area 27.5 where 3.5 individuals were observed on average.

The overall bycatch estimates for grey seals in the three assessment units were 3096* individuals (95% CI 2019–5042) based on bycatch events/frequency from 2015-2020 and raised with effort data from 2020. Broken up by assessment unit, WKMOMA estimates that 2229 individuals (95% CI 1598-3199) are caught annually in the Great North Sea assessment unit, 761 individuals (95% CI 333-1715) in the Iceland assessment unit and 108 individuals (95% CI 89-129) in the Ireland as-sessment unit. Gillnet métiers were the main gears with observed bycatch in all assessment units, but a small amount was also estimated to be caught in OTM in the Greater North Sea assessment unit.