Underwater sensors caught a rare event off Cornwall: one porpoise became trapped in a net while two animals exchanged possible distress calls

Bycatch, the accidental capture of animals in fishing gear is considered the world’s greatest direct human-caused threat to dolphins and porpoises.

Off the coast of Cornwall, a set of underwater sensors was listening to the ocean when it detected something researchers rarely get to see. According to an online report by Phys Org, two harbor porpoises swam around a fishing net, seemingly foraging for food and avoiding the gear, until one got caught up in it. What happened next was recorded in unusual detail, both acoustically and in terms of movement, providing researchers a rare insight into how these animals might react to a dangerous encounter. Scientists from the University of St Andrews’ Sea Mammal Research Unit and Scottish Oceans Institute found that their passive acoustic monitoring systems had inadvertently detected the bycatch event. Their results were published on August 26 in Royal Society Open Science. The devices had been deployed on a static gill net off Cornwall to detect marine mammal activity. Two harbor porpoises were observed foraging for several minutes around the gear and were recorded. The animals seemed to know how to avoid the net. Perhaps while chasing prey, one may have lost sight of the obstacle for a moment and became entangled, researchers speculate.Once caught, the porpoise could exert enough force to snap the net, but never enough to get away.Potential distress callsThese recordings offered more than evidence of the entanglement. The acoustic data showed that the two porpoises exchanged different types of calls during the incident. Researchers say some of the sounds may have been distress calls, but more study is needed to determine exactly what the calls conveyed. Together, the sound and movement data give scientists an unusually detailed view of an event that is normally difficult, if not impossible, to see in real time.The role of By-catchBycatch, the accidental capture of animals in fishing gear is considered the world’s greatest direct human-caused threat to dolphins and porpoises. It is believed that around 1,000 porpoises die each year after being caught in static fishing nets in the UK. However, recording such encounters is difficult due to the thousands of kilometers of fishing gear deployed across UK waters, only a fraction of which can be monitored. “The chances of sensors recording a bycatch event at the exact moment are very slim,” said lead researcher Dr Jamie Macaulay. “Finding out how and why entanglements happen could lead to practical solutions for researchers and the fishing industry,” he said.For safer fishing equipmentThe findings could help lead to new methods of reducing accidental captures. Researchers are looking at options such as more acoustically reflective nets so that marine mammals can detect fishing gear more easily, nets that allow stronger animals to escape, and biomimetic acoustic deterrents designed to warn animals away. The researchers stress that the goal is not necessarily to stop static-net fishing, which is still important to the fishing industry, but to make it safer. Scientists are hoping partnerships with fishing communities, such as the UK Bycatch Monitoring Program and CIBBRiNA, will allow them to turn rare glimpses beneath the waves into solutions that can save marine mammals.