This study assessed the bycatch of harbour porpoise (Phocoena phocoena) in the Dutch commercial bottom-set gillnet fishery, which is one of the priorities defined in the conservation plan for the Harbour porpoise in The Netherlands. The results show that bycatches occur in both single-walled gillnets as well as trammel nets with an estimated annual number of 23 (95% C.I. 2-44) porpoises bycaught during the study period (1 June 2013 to 31 March 2017). Applying this value to the average number of harbour porpoises occurring in Dutch waters (about 40.000), the approximate annual mortality lies between 0.05% and 0.07%.
Target on harbour porpoise bycatch: reduce to below 1%
The harbour porpoise is protected under various national and international treaties and agreements, including the Habitats Directive, OSPAR, ASCOBANS and the conservation plan for the Harbour porpoise in The Netherlands. ASCOBANS recommends reducing bycatch to below 1%, based on the most reliable population estimate. This research was commissioned to assess the impact of the Dutch bottom-set gillnet fishing sector on porpoises occurring in Dutch waters. The results show that during the study period mortality caused by this fishery lies between 0.05 and 0.07% per year. While this is substantially less than the objective of 1%, it is important to bear in mind that this value does not include bycatch from other gillnet fleets, such as non-Dutch commercial or Dutch recreational fishery. Also, as the “Dutch” porpoises are part of a larger North Sea population, they might face bycatch in other areas of their range. To determine impacts on the North Sea population level similar studies need to be conducted by all range countries.
Bycatches in trammel nets and gill nets
Fourteen fishing vessels were equipped with a Remote Electronic Monitoring (REM) system for the research period. Cameras and sensors on board the vessels took video footage and recorded the locations of nets and the times at which they were hauled. Two different categories of bottom-set gillnets were investigated – single-walled gillnets (GNS) and trammel nets (GTR). Single-walled gillnets are mostly used in spring and summer and target sole, among other, while trammel nets are primarily used in winter and spring targeting mainly cod, brill and turbot. Based on previous research in the North Sea, the assumption was that most, if not all, bycatch would occur in trammel nets. However, this research found that bycatch occurred in both net types, with a bycatch rate higher for trammel nets (0.004 porpoises/net length km) than for single-walled gillnets (0.0006 porpoises/net length km). However, because the fishing effort using single-walled gillnets is tenfold higher than trammel net fishing effort, this fishery is estimated to have more porpoises bycaught than the trammel nets.
Recommended steps in conservation measures
The most important recommendation is to carry out research into the extent of bycatch on the Dutch continental shelf by other fisheries sectors, such as foreign commercial and Dutch recreational bottom-set gillnet fishing. The researchers also call for the development of a cost-effective mobile Remote Electronic Monitoring system that could also be applied in the larger context of the Data Collection Framework. For the future it is also important to legalise the landing of harbour porpoise bycatch for research (such as pathology). One key challenge of bycatch studies in general is the current poor quality of available fishing effort data, in particular for gillnets. Effort data should include reliable and complete information on the number of nets set, the soak time and length of nets, as well as their height and mesh size. It should be collected by all fleets operating in the North Sea, to allow reliable mortality estimates for the North Sea porpoise population. Finally, as bycatch levels will change in relation to fishing effort and changes in fishing gear, as well as changes in distribution and numbers of harbour porpoises, they should be assessed on a regular bases.
Successful collaboration after a difficult start
This project was commissioned by the Dutch Ministry of Agriculture, Nature and Food Quality and conducted in partnership between Wageningen Marine Research (former IMARES) and Marine Science & Communication (MS&C), working closely with the voluntarily participating fishermen. In spite of a difficult start of the project - due to concerns by the participating fishermen regarding their privacy data security, as well as technical problems - a successful cooperation between all project partners - the government, fishers and researchers –was achieved.
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