The accidental entanglement of cetaceans and other protected species such as seabirds and seals in fishing gear across European waters is cause for concern. Trawl nets, gillnets, and many more methods employed by vessels during commercial fishing can lead to this undesirable byproduct, which can harm protected species populations and also potentially the dynamics and interactions of foodwebs.
The recently-issued advice, the product of a request to ICES by the European Union (EU), focuses on quantifying as far as possible the impact of fisheries on cetaceans (whales, dolphins and porpoises), with estimates given on the numbers of common dolphin and harbour porpoises that die a result of becoming trapped in fishing gear.
As well presenting the findings of data analysis, a key message of the advice is that 'any move to integrate the monitoring of the bycatch of protected species in EU waters under the Data Collection Framework will require the very careful consideration of sampling regimes and, as such, monitoring will require significant adjustments from that used for commercial fish bycatch.'
The advice also states the importance of dedicated, specifically-designed cetacean bycatch monitoring procedures that cover all relevant fisheries if good estimates of protected species bycatch are required.
The conclusion was reached as the result of a comparison of data gathered by means of three different observation techniques practised in 2014: observers on board fishing vessels, charting in log books (done under the Data Collection Framework, DCF), and the use of remote electronic monitoring (REM) where video equipment is mounted on the boats. This comparison was done separately for both towed fishing gears – such as pelagic trawls – and static nets.
Despite the fact that monitoring under the three schemes was not always conducted in the same fisheries, exactly the same areas, or at the same times, it was found that the differences between each technique were so large, i.e. bycatch estimates derived from DCF were considerably lower than those gathered from dedicated observers or REM, that the advice suggests the use of specifically-designed schemes are best for good estimates of cetacean bycatch.
The advice also sees an update of bycatch rates of harbour porpoise in the Kattegat and Belt Seas (Danish Straits connecting the Kattegat with the Baltic) with 2014 fishing effort and observer coverage. Mortality due to bycatch was estimated, with results suggesting that less than one percent of the harbour porpoise population in the area is being taken. This comes with the caveat, however, that total fishing effort is likely underestimated due to underestimation of effort from smaller vessels.
The issue of unrepresentative sampling was true in the case of the common dolphin, as bycatch of the species was assessed across three ICES subareas. It was found that annual removals of common dolphins in European fisheries may exceed a 1.7% limit established by ASCOBANS (Agreement on the Conservation of Small Cetaceans in the Baltic, North East Atlantic, Irish and North Seas) and used in previous advice. However, these estimates of mortality are based upon incomplete information.
It appears that common dolphin bycatch rates are highest in pelagic trawls. Although observed rates are lower in static net fisheries, they might be as significant as pelagic rates. This is because the observed sample, once raised, might produce a similar level of total bycatch due to the effort in static net fisheries being larger than in pelagic ones.
The data behind the advice is mainly derived from reports submitted for 2014 by EU Member States, required under the EU Regulation 812/2004, as well as the stated number of cetaceans observed as taken as bycatch. Although the total mortality was not assessed due to uncertainties in fishing effort data, total common dolphin bycatch estimates were provided by two member states in two different fisheries in the same year.
Common dolphins; photo - Mike Breen, Marine Scotland