Strategies to Minimize Bycatch in Fisheries and Protect Marine Life
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Credit: This article draws from the scientific work “A systematic review of sensory deterrents for bycatch mitigation of marine megafauna” by Sol Lucas and Per Berggren (full citation and link provided at the end).
As the global population continues to grow, food security becomes an increasingly pressing concern, despite the United Nations' objective to eradicate hunger by 2030. Consuming fish, including its byproducts, plays a crucial role in addressing this issue, as fish are rich in vital nutrients like vitamins, minerals, and omega-3 fatty acids (for further reading, see: How Fish Waste Can Be Turned Into A Valuable Resource).
However, fishing practices also pose significant challenges. One major issue is the abandonment of fishing nets in oceans, which entraps and endangers various marine species. These "ghost nets" account for approximately 10% of all marine debris, with an estimated addition of 500,000 to one million tons each year.
Additionally, fishing activities jeopardize ecosystems, as captured marine animals can no longer contribute to the food web, which consists of all interlinked food chains in the ocean. Unfortunately, fishers often catch non-target species as bycatch—unwanted marine creatures that become entangled in nets or traps while targeting different species. This often occurs due to sensory attraction, such as visual or olfactory cues from potential prey.
The repercussions of fishing practices and bycatch are significant; they can disrupt food chains, alter ecosystems, and threaten both commercial and subsistence fishing communities. For instance, the fishing of knife fish in the Yangtze River has been banned to allow for population recovery.
Given the gravity of these consequences, it's crucial to minimize unnecessary animal deaths, necessitating a reduction in bycatch. Numerous strategies have been proposed and implemented to achieve this goal.
One approach has been to outlaw particularly harmful fishing methods that result in high levels of bycatch. For instance, dynamite fishing—also known as blast fishing—has been banned. This method employs explosives to stun or kill fish, making them easier to capture as they float to the surface or sink. However, even traditional fishing nets can lead to significant bycatch, and the extent of this issue is often unclear due to illegal, unregulated, and unreported catches.
Other tactics include modifying fishing gear, releasing bycatch unharmed, reducing fishing efforts, setting catch limits, and designating no-fishing zones for certain periods. These measures can be unpopular among fishers as they can diminish the target catch or cause harm to animals that might otherwise escape. Thus, preventing fish from encountering gear in the first place is essential.
To achieve this, various solutions leveraging sensory cues have been developed. The following sensory deterrents can help mitigate bycatch:
Acoustic Deterrents
Acoustic deterrents utilize sound to provoke negative reactions in marine animals:
- Artificial sounds ranging from 20 Hz to 20,000 Hz can deter sharks from pursuing prey. While these frequencies fall within the human hearing range, they must often be combined with other sensory cues for effectiveness.
- Recorded sounds of killer whales can repel white sharks, but this is most effective when played back in the same region where the original recording was made.
Acoustic deterrents can also deter seabirds, such as common murres (Uria aalge), by using pingers emitting 1,500 Hz sounds, effectively halving their bycatch. However, this method has not proven effective for rhinoceros auklets (Cerorhinca monocerata).
Furthermore, marine mammals can also be deterred by acoustic signals:
- Alarms with frequencies between 40,000 and 160,000 Hz and pingers at 10,000 to 160,000 Hz can significantly reduce harbor porpoise (Phocoena phocoena) bycatch when affixed to gillnets.
- Pingers at 10,000 Hz have shown an impressive 86% reduction in dolphin bycatch. However, caution is needed as they might also exclude some species from their habitats or inadvertently attract others, thereby increasing bycatch.
Olfactory Deterrents
Olfactory deterrents utilize smell to trigger adverse responses in marine animals:
- Chemicals released by deceased sharks, known as necromones, can repel live sharks. These semiochemicals signal other organisms to alter their behavior.
- The type of bait used can also impact shark bycatch; for instance, substituting squid with fish may yield different results based on species preferences.
Seabirds can also be influenced by olfactory cues:
- Decomposing animal remains can deter certain species, like albatrosses, from scavenging bait on longlines.
- Shark liver oil may prevent flesh-footed shearwaters (Ardenna carneipes) from diving after bait.
The effectiveness of these olfactory deterrents can vary significantly by species.
For turtles, using fish bait instead of squid can significantly reduce bycatch rates, particularly for loggerhead (Caretta caretta) and leatherback (Dermochelys coriacea) turtles.
Visual Deterrents
Visual deterrents use sight to provoke negative reactions in marine life:
- Green LEDs have shown promise in significantly reducing shark bycatch, although further research is needed to confirm these findings.
- The SharkSafe barrier serves as a visible deterrent for sharks underwater.
Visual deterrents can also deter seabirds:
- Turning off lights on longline vessels decreases seabird bycatch, as fewer birds are attracted in the dark.
- Scaring lines, or tori lines, can effectively reduce seabird bycatch, although their effectiveness can be influenced by environmental factors.
Using white meshes on the upper portion of drift gillnets can alert animals and reduce common murre bycatch by 40–45%. Coloring bait blue may also help, though it could impact target species catch rates. The efficacy of laser beams for this purpose remains uncertain and requires further investigation.
Looming eye buoys, featuring rotating panels with drawn eyes, have been found to substantially deter long-tailed ducks within proximity.
Visual deterrents can also impact marine mammals:
- Green LEDs have been shown to reduce bycatch of small cetaceans by as much as 67% in bottom-set nets and 71% in driftnets.
Visual deterrents can be effective for turtles as well:
- LEDs emitting ultraviolet light on gillnets can decrease green turtle bycatch by at least 40% without affecting target catch levels.
- Three-dimensional models resembling sharks or spheres can dissuade loggerhead turtles from pursuing bait.
Electrosensory Deterrents
Electrosensory deterrents utilize electrical impulses to trigger responses in marine animals:
- Ferrite magnets can repel various species, decreasing the bycatch of smaller shark species while potentially increasing target catch.
- Neodymium-based magnets may also deter elasmobranch species from feeding in fishing areas, depending on the species involved.
- Pulsing electrical signals can deter sandbar sharks (Carcharhinus plumbeus) and largetooth sawfish (Pristis pristis), although these results are primarily based on laboratory studies and need validation in real-world scenarios.
Echolocation Reflection Deterrents
Echolocation reflection deterrents involve reflecting echo signals to affect dolphins:
Bottlenose dolphins and harbor porpoises can detect fishing nets from greater distances. However, changes in net flexibility due to added materials may influence results, and the behavior of captive dolphins may differ from those in the wild, necessitating additional research.
Conclusion
By employing acoustic, olfactory, visual, and electrosensory cues, as well as reflecting echolocation signals, bycatch can be significantly reduced. While some deterrents have demonstrated effectiveness, others require further evaluation. Moreover, certain methods may not be suitable for commercial fishing practices. Thus, combining various sensory strategies may be the best approach to minimize bycatch across different scenarios.
How We Can Take Action
Here are some practical steps individuals can take to mitigate bycatch:
- Immediately returning bycatch to the water
- Utilizing fishing gear designed for specific target species to minimize unintended catches
- Avoiding fishing in over-exploited areas
- Steering clear of regions with high bycatch likelihood
Which of these actions can you incorporate into your routine? Do you have additional suggestions for reducing bycatch? We welcome your thoughts and encourage you to share them to inspire others.
Credit
This article is based on:
Lucas, S., & Berggren, P. (2022). A systematic review of sensory deterrents for bycatch mitigation of marine megafauna. Reviews in Fish Biology and Fisheries, 1–33.