Bycatch is considered one of the most significant threats affecting cetaceans worldwide. In the North Sea, bottom-set gillnets are a specific risk for harbor porpoises ( Phocoena phocoena). Methods to estimate bycatch rates include on-board observers, remote electronic monitoring, and fishermen voluntarily reporting; none of these are systematically conducted. Additionally, necropsies of stranded animals can provide insights into bycatch occurrence and health status of individuals. There are, however, uncertainties when it comes to the assessment of bycatch in stranded animals, mainly due to the lack of diagnostic tools specific for underwater entrapment. We conducted a literature review to establish criteria that aid in the assessment of bycatch in small cetaceans, and we tested which of these criteria applied to harbor porpoises retrieved from gillnets in the Netherlands ( n = 12). Twenty-five criteria were gathered from literature. Of these, “superficial incisions,” “encircling imprints,” and “recent ingestion of prey” were observed in the vast majority of our confirmed bycatch cases. Criteria like “pulmonary edema,” “pulmonary emphysema,” and “organ congestion” were also frequently observed, although considered unspecific as an indicator of bycatch. Notably, previously mentioned criteria as “favorable health status,” “absence of disease,” or “good nutritional condition” did not apply to the majority of our bycaught porpoises. This may reflect an overall reduced fitness of harbor porpoises inhabiting the southern North Sea or a higher chance of a debilitated porpoise being bycaught, and could result in an underestimation of bycatch rates when assessing stranded animals.

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Borges, L., van Keeken, O. A., van Helmond, A. T. M., Couperus, B., and Dickey-Collas, M. 2008. What do pelagic freezer-trawlers discard? – ICES Journal of Marine Science, 65: 605–611. Estimates of discards by the Dutch pelagic freezer-trawler fleet were derived based on data from observers on board commercial vessels. In all, 27 fishing trips of duration 2–5 weeks each were sampled between 2002 and 2005, covering the North Sea and the western waters of the British Isles. Different methods to estimate discards were compared, and raising these estimates by number of trips or by total landings did not greatly influence the annual estimates of total discards. Some 30 000 t of fish were discarded annually by the fleet, the commercial target species mackerel, herring, and horse mackerel being discarded most, with 41%, 19%, and 15% of the total discards by weight, respectively. The most commonly discarded non-commercial species was boarfish, accounting for 5% of total discards. Slippage accounts for 10% of all discards, and the most common species slipped is herring. There is extensive discarding of juvenile mackerel in the horse mackerel fishery, and to a lesser extent highgrading of mackerel in the same fishery.

This study assessed the bycatch of harbour porpoise (Phocoena phocoena) in the Dutch commercial bottomset gillnet fishery, which is one of the priorities defined in the Dutch "Harbour porpoise conservation plan". The results showed that bycatches occur in both single-walled gillnets (GNS) as well as trammel nets (GTR) and that the average annual bycatch of harbour porpoise for this fleet was 23 (95% C.I. 2-44) during the study period. This translates to an annual mortality of between 0.05 and 0.07% of the Dutch harbour porpoise population (with a maximum worst case value of 0.3%). ASCOBANS (resolutions 3.3 and 5.5) advises to follow the "precautionary objective to reduce by-catches to less than 1% of the best available population estimate". This study did not include mortality caused by other fishing fleets operating in Dutch waters. From 1 June 2013 to 31 March 2017 fourteen fishing vessels were equipped with remote electronic monitoring (REM) systems. Closed-circuit television cameras (CCTV) in combination with sensors were used to obtain video footage, time and position of all net hauls. Video footage was analysed for porpoise bycatch events and correction factors were applied to account for video quality. Bycatch rate was defined as takes per unit effort (using "fishing days", "landings in tons" & "km net length"). A non-parametric bootstrap was used to estimate confidence intervals and coefficients of variation. Bycatch numbers were estimated by applying the bycatch rate to the effort of the complete fleet. In total 900 fishing days of the bottom-set gillnet fleet (of a total of 8133 fishing days) were analysed (11%). Net types were categorized as single-walled gillnets and trammel nets. 760 fishing days of a total of 7756 fishing days (9.8%) were monitored for single-walled gillnets and 140 fishing days of a total of 377 fishing days for trammel nets (37.1%). 13 porpoises were registered during the study period, with one additional animal after the project had officially stopped. For all fishing days for which REM videos were analysed, 6 animals were registered by either the fishermen or the REM video analyses, or both. In addition 7 porpoises were reported by the fishermen during the study period when the REM was either not running, not installed or for REM days that were not analysed. A total of 8 animals were necropsied by the University of Utrecht. The results obtained through bycatch per effort value "net length km" were considered the most suitable, as the results based on this metric had a lower variability than "ton landed" and were a more realistic reflection of effort than "fishing days". Bycatch rate (porpoises/net length km) was different between net types, with 0.004 for trammel nets and 0.0006 for single-walled gillnets. The bycatch rate for all net types combined (0.0011) was applied to calculate bycatch numbers, resulting in an estimate of 88 animals for the complete study period (95% C.I. 6-170; C.V. 14.54) and an annual average of 23 animals (95% C.I. 2-44). The scale of the average annual...

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