Managing an evolving relationship between trawling and marine life 

By Alice de Bernardy 

Overfishing and ocean pollution are only a portion of the results of today’s fishing activity. Although fishing can be a great and sustainable source of food and income under appropriate scales, the reality remains that a quarter of global fishing comes from trawling activity,  in which immense nets are dragged through the ocean.¹ The extent of its effects depends on the trawler type used and the sea floor, but, overall, the loud engines and nets associated with such vehicles still tend to disrupt marine life and the integrity of the seabed.¹ While providing an efficient source of fish, and accordingly of money income in the short term, it gives rise to multiple environmental problems, polluting oceans with nets, depleting marine life populations and destroying ecosystems.²  

One example illustrating the deleterious effects of trawling is the rapidly changing relationship between fishing boats and dolphins. Such is the case since the presence of fishing boats in shallow waters near the shore increases the odds of marine life and trawlers interacting. As fish eaters, competing with trawlers for their food, an increasing number of dolphins have been reported to follow fishing nets and to eat off them.³ It has even been suggested that they are able to recognize trawlers actively fishing by the sound they make, with 19 species of odontocete reported to have modified their behavior, associating foraging and trawlers.^{4, 5}  

Although this can be seen as a positive adaptation for dolphins, providing them with an easier food source requiring less energy expenditure, it also means that the number of bycatch- and fishing-related injuries are bound to increase. Indeed, while cetaceans use their perception of sounds very accurately, it has been hypothesized that they can’t distinguish nets and therefore are likely to get entangled in them.⁶ For example, a recent study estimated that the rate of observed cetacean bycatch was actually 7 times more frequent than recorded by trawl fishers.⁵ This is consistent with the recognition of bycatch and incidents with fishing boats and nets as the main threat to odontocete populations around the world.³ 

On another note, the Indo-Pacific humpback dolphin population found in Thailand is frequently seen foraging on fishes that fall out of nets when being pulled out of the water. The same population has been identified to bear a surprisingly high number of scars, resulting from a high rate of incidents.⁷ 

Several methods have been developed to limit the interaction between trawlers and marine mammals. The first and most drastic method – also regarded as the most efficient way to protect ecosystems – is the creation of marine parks to limit encounters and leave marine life to prosper and recover.^{4, 5} Indeed, restricting fishing in an area is known to effectively restore its ecosystem, and correlates with a spillover effect, meaning that healthy fishes from the protected area will also populate outside of the zone. In this scenario, fishermen are provided with a consistent fish supply.⁸ Albeit, finding a compromise to protect dolphin populations and enable locals to make a living can sometimes be very tricky. 

As a case study, we can look at the population of Irrawaddy dolphins in the Kep archipelago of Cambodia, which is closely monitored by researchers from Marine Conservation Cambodia (MCC).⁹ While being listed as endangered by the ICUN, incidents leading to the death of Irrawaddy dolphins due to bycatch and entangling in nets are still reported.⁹ To approach the matter, a non-trawling zone was created by the MCC, as it was thought to be the most sensible option to protect the animals.  In reality, however, this decision was found to be very difficult to implement in real life, as many fishermen live off the area’s fishes. Accordingly, the restriction to fish seemed unreasonably punitive to many, with the concept of spillover having trouble persuading most. 

Other methods to enable dolphins and trawlers to share the same zone while discouraging encounters have emerged with technological advances and can provide alternative solutions .⁵ For example, the development of acoustic deterrent devices, commonly called pingers, has allowed small cetaceans to detect gillnets. They have been developed based on the knowledge that toothed whales are able to swim around and visualize float lines. In this way, attaching a pinger to a net creates a float line sound that enables dolphins to swim around it, thereby reducing bycatch. The use of this method is now highly appreciated by fishermen, as it prevents their nets from being torn by dolphins, which would otherwise lead to the loss of all captured fishes.  Already, pingers have been successfully used in a trial in the Turkish Black Sea to protect a population of porpoises. ⁶ That said, while pingers might reduce bycatch, the technology is not perfect and requires further research to prevent harm to ecosystems. 

For this reason, the danger of trawlers and the behavioral change they inflict on marine species cannot be resolved in one day. This is a long-term process that includes environmental and marine research, as well as communication and outreach to local fishing communities. With sufficient support and perseverance, this should eventually create a sustainable relationship between the coastal waters and their inhabitants. 

References:

1. McConnaughey RA, Hiddink JG, Jennings S, Pitcher CR, Kaiser MJ, Suuronen P, et al. Choosing best practices for managing impacts of trawl fishing on seabed habitats and biota. Fish Fish (Oxf). 2020;21(2):319–37. DOI: 10.1111/faf.12431
2. Strong JA, Wardell C, Haïssoune A, Jones AL, Coals L. Marine habitat mapping to support the use of conservation and anti-trawl structures in Kep Province, Cambodia. ICES J Mar Sci. 2022;fsac001. DOI: 10.1093/icesjms/fsac001
3. Mahmud AI, Jaaman SA, Muda AM, Muhamad HM, Zhang X, Scapini F. Factors influencing the behaviour of Irrawaddy dolphins Orcaella brevirostris (Owen in Gray, 1866) in Brunei Bay, Malaysia. J Ethol. 2018;36(2):169–80. DOI: 10.1007/s10164-018-0549-9
4. Bot K. Edu.my [En ligne]. Boat Effects on the Behaviour of Indo-Pacific Humpback (Sousa chinensis) and Irrawaddy Dolphins (Orcaella brevirostris) in Cowie Bay, Sabah, Malaysia. Available from: http://www.ukm.edu.my/jsm/pdf_files/SM-PDF-40-12-2011/08%20Nur%20Azeyanti.pdf
5. Bonizzoni S, Hamilton S, Reeves RR, Genov T, Bearzi G. Odontocete cetaceans foraging behind trawlers, worldwide. Rev Fish Biol Fish. 2022;32(3):827–77. DOI: 10.1007/s11160-022-09712-z
6. Kratzer IMF, Brooks ME, Bilgin S, Özdemir S, Kindt-Larsen L, Larsen F, et al. Using acoustically visible gillnets to reduce bycatch of a small cetacean: first pilot trials in a commercial fishery. Fish Res. 2021;243(106088):106088. DOI: 10.1016/j.fishres.2021.106088
7. Jutapruet S, Huang S-L, Li S, Lin M, Kittiwattanawong K, Pradit S. Population size and habitat characteristics of the Indo-pacific humpback dolphin (Sousa chinensis) off donsak, Surat Thani, Thailand. Aquat Mamm. 2015;41(2):129–42. DOI: 10.1578/am.41.2.2015.129
8. Halpern BS, Lester SE, Kellner JB. Spillover from marine reserves and the replenishment of fished stocks. Environ Conserv. 2009;36(4):268–76. DOI: 10.1017/s0376892910000032
9. Tubbs SE, Baş AA, Côté G, Jones AL, Notman G. Sighting and Stranding Reports of Irrawaddy Dolphins (Orcaella brevirostris) and Dugongs (Dugong dugon) in Kep and Kampot, Cambodia. Aquat Mamm. 2019;45(5):563–8. DOI: 10.1578/am.45.5.2019.563