A unique collaboration between marine researchers and net fishermen over 10 years has now made it possible to set quite precise figures on the bycatch of porpoises – figures that are significantly smaller than assumed just over 20 years ago. At the same time, the 10 years of research provide new, solid knowledge on how the current regulation affects bycatch.
Bycatch is a very important area to look at when, in a world that is under pressure from both climate change and the biodiversity crisis, we have to work for a more sustainable food production, such as fishing.
And bycatch of the small whale porpoise has become of increasingly concern around the world, not least in the EU.
"There are no fishermen who want to catch porpoises, so the fishermen themselves want solutions to avoid catching them as bycatch."
Lotte Kindt-Larsen, Senior Researcher, DTU Aqua
Marine researchers from DTU Aqua and Sweden's University of Agriculture have just published the results of 10 years of close collaboration with gillnet fishermen, where the researchers, using monitoring equipment on the fishermen's boats, have investigated the extent and conditions surrounding bycatch of porpoise in commercial gillnet fishing.
The research results are published in the scientific article Knowing the fishery to know the bycatch: bias-corrected estimates of harbor porpoise bycatch in gillnet fisheries in the scientific journal Proceedings of the Royal Society B
Through the extensive collaboration, the researchers have developed a new mathematical model which, for the first time, can put very precise figures on the bycatch of porpoises.
The new model estimates that 2,000 porpoises perish each year in Danish and Swedish net fisheries. A figure that is lower than the 5,500 animals estimated at the end of the 1990s in the North Sea alone.
"Our new model is much more accurate than previous ways of estimating bycatch because it takes into account a lot of different factors such as how long the nets are and how long they have been standing, seasons and the species being fished for,” says Lotte Kindt-Larsen, senior researcher and head of the research project.
The rules do not prevent bycatch
When asked what the most important new knowledge gained from the long study is, Lotte Kindt-Larsen answers:
"There is really a lot of important information to be found. We now know both where porpoises are caught, but also where they are not. Likewise, based on the model, we can see how the current regulation works.”
Globally, the porpoise is not listed as threatened on the IUCN Red List. But in Europe, the small whale is listed as 'vulnerable' because it is exposed to a high degree of bycatch in the coastal net fishery. According to the research, the status of the porpoise populations in the Baltic Sea and the Black Sea is more worrying, as it is listed as 'critically endangered' and 'threatened' respectively.
"Our figures with the new model for bycatch indicate that the management rules reduce bycatch, but they do not prevent bycatch. This is because the acoustic alarms that scare away porpoises, the so-called 'pingers', are not required to be used in all net fisheries and on all boat lengths. Eg. fishing boats less than 12 meters must not use pingers," says Lotte Kindt-Larsen.
17 Danish net vessels during 10 years
The model that the researchers have developed for estimating bycatch of porpoise includes a number of parameters for time and place, size and types of net etc., which overall have not been included in previous models for estimation. In addition, the investigations over the many years have relied on camera surveillance.
The camera systems consisted of a control unit, connected to a position sensor (GPS) and a set of at least two waterproof CCTV (closed circuit television) cameras that record the fishing activity.
The researchers then marked each bycatch event with a time stamp and a recording location.
The camera systems were installed on board 17 Danish gillnet vessels between 2010 and 2020. The monitoring of individual vessels varied from several months to many years.
Geographically, the camera surveillance covered many important commercial net fishing grounds, both in the Øresund, the Belt Sea, the Skagerrak and the North Sea.
In addition to the camera surveillance, the researchers conducted a census of Danish and Swedish fishermen's reported data, including all official log books, monthly statements, sales slips and landing statements for the commercial vessels that registered nets as their primary or secondary gear between 2010 and 2020.
The balance between biodiversity and socioeconomics
Nets, when compared to other fishing gear, are generally considered a good solution to minimize the environmental impact associated with fishing.
Net fishing is also an important part of small-scale fishing globally, including in Europe, and net fishing therefore has an important socio-economic significance as part of the livelihood of coastal communities.
Unfortunately, gillnet fishing is also, as this research project has investigated, a type of fishing that involves the bycatch of marine mammals.
Considering the two sides of net fishing, it is, according to the researchers, crucially important to find ways to reduce the biodiversity footprint of fishing nets, as it is called, in order to ensure food security, while at the same time that the fishery meets the increasingly strict environmental requirements.
Environmental requirements and regulations, which aim to save biodiversity in the sea - in this case for the conservation of marine mammals such as the porpoise.
Translating knowledge into management
The researchers' studies contribute to the important knowledge base that politicians and legislators need to be able to manage the ocean's resources effectively.
"There are no fishermen who want to catch porpoises, so the fishermen themselves want solutions to avoid catching them, but there is currently no easy solution. At present, only the acoustic alarms, the pings, work, but it can also be problematic to just set alarms on all our gillnets and create a lot of noise pollution," says Lotte Kindt-Larsen and continues:
"But with the new data, we can show our authorities where the biggest problems are, so they have the opportunity to make the best possible management."
Precise assessments of the extent of bycatch of porpoises are therefore important in order to translate knowledge into management. In-depth knowledge of fishing and bycatch is also necessary to be able to develop new fishing technology that can help fishermen prevent bycatch of, among other things, porpoises - where are the researchers in that area?
"We work in many different ways to avoid bycatch. We look at which types of pingers work best, but also at whether there may be other methods, so that we may can completely avoid sending noise into the water," says Lotte Kindt-Larsen.
Fortunately, 10 years of collaboration between the fishermen and the researchers from DTU Aqua and Sweden's Agricultural University is not over. Collections with cameras continue, and the hope is to be able to continue to provide the model with new data, so that the research can be constantly updated and thereby contribute to the best possible management of nature and biodiversity in the sea, including the porpoise.
The project is financed by EHFF funds.
Photo: Porpoise. By Dmitry Travnikov/COLOURBOX