According to researchers at Newcastle University, attaching empty plastic bottles to bottom-set fishing nets reduced accidental dolphin bycatch by 88% during recent field trials published in Marine Mammal Science and Fisheries Research. The low-cost mitigation strategy uses plastic to reflect animal echolocation clicks, addressing a critical conservation challenge in global fisheries.
How Plastic Bottle Reflectors Prevent Dolphin Entanglements
Gillnets hang vertically in the water column to trap target fish like tuna, but their nearly invisible nylon filaments often spell disaster for marine mammals. According to data from Whale and Dolphin Conservation (WDC), static gillnets account for more marine mammal deaths than any other gear type in UK waters alone, claiming around 1,000 porpoises and hundreds of dolphins annually, including 250 common dolphins.
Dolphins navigate and hunt using echolocation, bouncing sound waves off surrounding objects to build a spatial picture of their environment. WDC notes that thin mesh nets often fail to produce an echo, causing animals to swim blindly into them without perceiving the danger. To counter this, Professor Per Berggren of Newcastle University theorized that attaching empty plastic bottles to the gear would create reliable acoustic reflectors using the trapped air inside.
Did you know? In addition to plastic bottles, researchers also tested glass bottles containing metal bolts during their trials to generate a distinct clinking sound as water moved the gear.
Global Field Trials in Zanzibar, Peru, and Brazil
The research team tested the bottle-reflector theory by attaching glass and plastic bottles to more than 1,600 fishing nets across Zanzibar, Peru, and Brazil, comparing the catches against control nets without bottles. The results varied dramatically depending on how the nets were deployed in the water column.
In Peru and Zanzibar, where fishers hang nets near the surface of the water, the bottles failed to reduce dolphin, porpoise, or turtle bycatch. According to Berggren, surface waters create a noisier acoustic environment that diminishes the effectiveness of the bottle reflectors. However, the surface trials in Peru did increase catches of target fish and sharks, while Zanzibar trials boosted tuna yields without providing enough dolphin interactions to yield a clear conservation conclusion.
Bottom-set nets yielded entirely different outcomes. In Brazil, where nets anchor directly to the seafloor, plastic bottles showed immediate promise by potentially reducing dolphin bycatch while maintaining target fish catch rates, according to the study authors. Building on this success, subsequent trials on bottom-set gear demonstrated an 88% reduction in dolphin bycatch without affecting the volume of target fish harvested.
Scaling Up Conservation Efforts in Global Fisheries
Following the success of the bottom-set trials, researchers are expanding their field testing to other vulnerable regions. According to the Marine Mammal Science paper, ongoing trials are currently taking place in Cambodia and Congo to verify whether the method protects coastal dolphin species worldwide.
Berggren emphasizes the accessibility of the solution for local fishing communities. Attaching plastic bottles to fishing nets can reduce dolphin bycatch globally and represents an intervention that every fisher can afford, according to Berggren, who notes that securely fastened bottles remained attached throughout the trials without any losses. This approach repurposes plastic waste to directly protect vulnerable wildlife populations.
Frequently Asked Questions
How do plastic bottles stop dolphins from getting caught in nets?
Empty plastic bottles trap air inside, which reflects the echolocation clicks emitted by dolphins. This acoustic reflection alerts the animals to the presence of the net so they can avoid entanglement.
Why didn’t the bottles work on nets placed near the water surface?
According to researchers, surface waters have a higher level of ambient background noise that interferes with the acoustic reflection of the bottles, making them less effective than nets anchored to the seafloor.
Did the bottles reduce the amount of fish caught by fishers?
Where are these net modifications currently being tested?
Following successful trials in Brazil, Peru, and Zanzibar, researchers have expanded their field testing to fisheries in Cambodia and Congo.
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