Statistical analysis of the cetacean bycatch monitoring programme on board the Spanish set gillnet and bottom pair trawl fleets in the Bay of Biscay

Poster.-- ICES Annual Science Conference 2023, 11–14 September 2023, Bilbao, SpainThe first year of data, from October 2020 to September 2021, of the Spanish cetacean bycatch observer programme was analyzed to explore the following issues: a) representativeness of the sampling frame; b) estimation of bycatch per unit of effort (BPUE); and c) sampling coverage. Regarding the first issue, the probabilistic sampling design of the Spanish cetacean bycatch monitoring programme, with record of refusals, has allowed to address bias analysis, find the source of bias and correct it in the parameter estimation. Secondly, in relation to the estimation of nominal BPUE, several aspects have been decisive: to use an estimation process disaggregated by domains configured by the combination of fishing gear (métier) and fishing area (ICES Division), given the different bycatch rates between fleets and fishing grounds; as well as recording the direct scientific observation, which is essential in the pair trawl fleet when it is monitored by a single observer located in a single boat. Finally, in relation to the coverage rates necessary to achieve statistical robustness with rare events such as the bycatch of marine mammals, our results point to levels around 30% sampling coverage, depending on the métier and its bycatch rate. The results obtained allow to conclude that the current Spanish European DCF at-sea sampling programme (which include the sampling of by-catch of non-target protected species) and the Spanish cetacean bycatch monitoring programme provide similar results of cetacean bycatch for some of the ICES Divisions and métiers monitored. On the other hand, achieving the high levels of sampling coverage necessary for the proper estimation of the bycatch of rare events, which represents an economic and logistical challenge, it is necessary to explore the combination of monitoring based on scientific observers at sea with other alternative methodologies, such as the use of electronic monitoring, improvement of recording of bycatch in official control logbooks, etcN