Fishers and scientists have been working together to understand why porpoises and dolphins become accidentally entangled in fishing gear. Authors of the recently published Royal Society Open Science article, '', tell us more.
Static fishing nets are widely used by fishers throughout the world, from small coastal canoes in developing nations to larger boats that set kilometres of nets in deep, offshore waters. They offer a cheap and effective way to catch fish, they are generally non-destructive to the seabed and they are relatively selective in the species that they catch. Static nets are, in many ways, a success story; providing a highly practical and low-cost means for many coastal communities to secure their livelihoods and ensure food security.
But these nets also have a significant downside: bycatch, the unintended capture of non-target species, which threatens the future of some of our most iconic marine fauna including turtles, seabirds, dolphins, seals, porpoises, sharks, rays and other fish. Bycatch is by far the largest cause of direct anthropogenic mortality in dolphins and porpoises, with around 300,000 animals per year estimated to be killed across the globe. It has contributed to the extinction of the Baji River dolphin, is the main threat to the critically endangered vaquita in Mexico, and is a major contributing factor in the decline of populations of other dolphin and porpoise species around the globe. Bycatch can also be a major issue for fishers, costing them time and money when they lose their gear or catch as a result, risking their safety if they try to release bycaught animals, and causing them distress.
However, simply banning static nets would, in many circumstances, be unworkable; it would force fishers to switch to other forms of fishing which would also come with impacts on the marine environment and could result in economic impacts on already struggling coastal communities.
For decades, scientists across the globe have been working with fishers to explore new bycatch reduction ideas, often conducting large-scale experiments to test an idea then repeating with an altered or different approach. There have been a few notable success stories in reducing marine mammal bycatch; for example, the development of pingers (acoustic devices that emit sounds to alert animals of nearby nets), underwater lights to illuminate netting, and acoustic reflectors which make it easier for animals to detect nets with their biosonar. However, there doesn’t seem to be one single solution that works across different species and areas, and large-scale trials can be slow and are often conducted with little understanding of how or why bycatch occurs. In other words, we are trying to solve an issue we do not fully understand.
In 2016, to address this knowledge gap we began exploring new ways to monitor porpoise and dolphin behaviour around nets. There was a good reason why nobody had managed to tackle this before; it is incredibly difficult to observe and track porpoises or dolphins underwater, especially in deep water where video cameras do not work very well. So, we approached the problem from another angle, instead of recording images, we focussed on listening for the tell-tale sounds of dolphins and porpoises; specifically, the loud clicks they use to communicate, hunt and sense their surroundings through echolocation. We developed compact, high-tech recording devices equipped with hydrophones (underwater microphones) and orientation sensors, capable of not only detecting echolocation clicks but also reconstructing an animals’ movements in three dimensions.
We then worked with fishers in Cornwall to make the system practical and rugged so it can easily attach to their nets, survive being thrown overboard, and did not become entangled in the nets when they were fishing. Without this collaboration, the experiment would have been impossible. After several failed attempts and lessons learnt, we managed to get the system working properly and began recording porpoise movements around nets in a proof-of-concept project funded by DEFRA between 2017 and 2019.
When the data started flowing in, the first thing we noticed was that porpoises were frequently present around the nets; we would often detect the sounds of porpoises and dolphins and the tracks showed them swimming near nets without becoming entangled We also noticed lots of tell-tale foraging sounds indicating that animals were actively feeding - sometimes very near the nets. This meant, as many had long suspected, that porpoises are not simply swimming into the nets whenever they encounter them, but instead that they are routinely occupying the same space as fishing nets.
Then, in November 2019, something unexpected happened: as we hauled the net, we found a porpoise entangled just 20m from one of the recording devices. When we looked at the acoustic data, we saw the tracks of two porpoises foraging around the net. One of the animals became entangled and, as it tried to escape, it lifted the net towards the surface several times before it died. We also noticed a remarkable change in acoustic behaviour: as soon as one of the animals became entangled, both animals switched to producing special sequences of clicks that we think are used for communication. After the bycaught animal died, the other porpoise remained close by and made communication calls for at least an hour (and possibly returned twice over the next five hours). We think this was a mother and mature calf (approximately one year old) pair, with the calf bycaught.
It is unfortunate when any wild animal unintentionally dies due to human activity, and this is a poignant reminder that bycatch is a welfare issue as well as a conservation one. However, the data from this event has huge potential to help us design more effective mitigation approaches to reduce porpoise and other small cetacean bycatch in the future. For example, the bycaught animal lifted the net several metres up into the water column which would take significant force - could we use this information to fine tune the strength of nets to allow larger animals to escape but still retain the fish the fisher wants to catch? We have learned that porpoises often come close to nets but only rarely become entangled – this probably means that porpoises are aware that nets are present, and so bycatch appears to occur due to temporary lapses in sensory perception or perhaps inexperience. Making the nets more reflective, for example by , might reduce these sensory lapses. We also now know that porpoises often forage near nets – is that just chance, or are the nets aggregating prey fish? If porpoises are motivated to forage near nets, then that might affect what mitigation approaches work best. Finally, the unusual calls detected around the time the bycatch occurred may be a distress response – could this be integrated into acoustic warning devices to keep porpoises away from nets?
This research continues, with fishers and researchers working together to deploy this equipment in the UK, Iceland and Sweden as part of the EU’s project, supported by DEFRA's Bycatch Monitoring Programme and the Scottish Government's Bycatch Monitoring Programme. We are also using the tracking system to test porpoise behavioural responses to new bycatch mitigation approaches, including pearl beads and interactive pingers that only activate when they detect porpoises or dolphins (reducing noise pollution and other unintended effects). We hope that this new approach will speed up the testing phase of new mitigation ideas by serving as a first step to provide insights into different approaches before moving to large scale field trials. As we continue to study the behaviours that lead to bycatch, we hope to design practical solutions for more sustainable fisheries that serve coastal communities, protect our top predators, and benefit the wider environment.
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Image captions
Figure 1: Harbour porpoises at the sea surface.
Figure 2: Lifting one of the recording devices attached to a net onto a fishing boat.
Figure 3: Acoustic recording devices are used to track porpoises around nets. Each device calculates a bearing to echolocation received clicks. With two devices, the intersection of their bearings indicates the location of the detected porpoise or dolphin. As the porpoise or dolphin clicks regularly, their 3D swim tracks around the net can be reconstructed.
Video: An animation showing the dive tracks and acoustic behaviour of the two porpoises along with net movement during the bycatch event.
