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For artisanal fishers, fish fences are an easy, but problematic, option

  • The widespread use of fish fences by fishing communities in tropical countries leads to extensive economic, social and environmental damage, a new study finds.
  • The technique involves stringing a net along stakes typically set in an intertidal flat, where it traps fish as the tide goes out. But the practice results in the indiscriminate catch of juvenile fish, threatening the sustainability of fish stocks.
  • In the area studied, in eastern Indonesia, the fences are also a source of social tension, where they’re the exclusive domain of the island-based ethnic group and denied to the seafaring Bajo community.
  • The researchers have called for restrictions on the use of fish fences, but acknowledge that getting fishermen to start going out to sea to fish will be difficult, given the low risk and high convenience that fish fences afford.

JAKARTA — The widespread use of wooden fences to trap fish by coastal fishermen in tropical countries is causing extensive economic, social, and environmental damage, according to a new study.

Used across the Atlantic, Indian and Pacific oceans, artisanal fish fences are found to significantly disrupt the sustainability of coastal marine ecosystems and create socioeconomic conflicts among fishing communities, according to the study published May 21 in the journal Nature Communications.

This fish fence in Indonesia stretches more than 80 meters (260 feet) across an intertidal flat, where it traps fish as the tide recedes. The combined length of such fencing around some islands runs longer than the entire local coastline. Image courtesy of Dan Exton for Operation Wallacea.

The researchers presented a study case of the use of artisanal fish fences around Kaledupa, one of the islands that form the Wakatobi archipelagic district in Indonesia’s Southeast Sulawesi province.

“Fish fences might differ slightly between locations, but in general they follow a similar design and function,” Dan Exton, the head of research at UK-based NGO Operation Wallacea and the lead author of the report, told Mongabay in an email.

“This means that most, if not all, of our findings around Kaledupa should translate to fish fence use more widely, both within Indonesia and also throughout the tropics,” he added.

These semi-permanent fish fences are set up to follow the natural movements of fish. They form a barrier from which fish are unable to escape as the water recedes during low tide, which is when the fishermen come to collect the catch. They are typically built using wooden poles harvested in local mangrove forests, with a fine-meshed net strung in between to trap the fish. Fishermen usually move each fence around four times a year to exploit new fishing grounds.

In Indonesia, these fish fences, locally known as sero, are only allowed to be installed within 2 miles from shore.

Fish fence users in Indonesia will often clear the seabed, sometimes using poison, in the belief that their catches will be boosted. A halo of dead coral is visible here surrounding the fence structure. Image by Benjamin Jones for Project Seahorse.

While such equipment is often championed by fishermen as having a smaller environmental footprint than more industrialized or modern fishing techniques, the fences are highly non-selective in trapping fish for as long as they stand. In January, a whale shark was found trapped in one of these fences in Southeast Sulawesi. The 7-meter (23-foot) fish was eventually freed by fishermen and conservation officials.

“The fishermen collect just about everything, including rabbitfish, butterflyfish, stingrays, and pufferfish,” Exton said. “Fishermen usually sell the high value fish in the market, and will often keep the fish with less value for their consumption.”

Nothing is spared, he added. “[P]retty much everything caught will be eaten or sold, especially considering fences are positioned on intertidal flats and any fish that are trapped are unlikely to survive and be able to be released,” Exton said.

Given the lack of any technical barriers, access to fishing grounds, and required expertise, the report concludes that “fishing pressure is maintained in the face of diminishing returns and habitat damage.”

From their research on Kaledupa, the researchers found a drastic increase in the practice over a 15-year period, from just 37 fences in use in 2002 to more than 160 in 2016. The practice peaked in 2009, when 210 fences were recorded. The island’s fishermen appeared to have changed their preferred fishing technique, moving from widespread use of low-intensity methods such as lines and spears, to a greater reliance on net fishing and fish fences, the researchers said.

The area covered by these fences was also found to have expanded over the studied period. In 2005, the fences stretched a total of 10.6 kilometers (6.6 miles) in 2005; by 2015, they ran a combined 27.9 kilometers (17.3 miles).

The fences are typically laid out in an arrow shape, consisting of a central spine and multiple wings. If the wings are taken into consideration of the fence length, then the total span of physical barriers in place around Kaledupa in 2015 was 69.7 kilometers (43.3 miles) — greater than the island’s coastline, which runs 60 kilometers (37.3 miles).

Fish fences act as a barrier to normal ecosystem function. These photos show both sides of the same fence, with the healthy reef community coming to an abrupt end at the seaward side, right, and a barren expanse on the landward side. Image by Benjamin Jones for Project Seahorse.

While the total installed fish fencing has increased, the catches have dwindled. Average daily fish catches per fence were 713 in 2005. By 2007, they had dropped by 75 percent to just 172, and by 2012 to 36 fish per net per day. The researchers also discovered that fish population densities almost halved between 2002 and 2012, driven by a loss in the most pristine reef areas.

Delving further into the catches, the researchers found a high rate of juveniles being caught from 575 different fish species. Keeping juvenile catches at a minimum is a core principle of sustainable fishing, to prevent the depletion of fish stocks.

“In our experience there is little or no separation of adults and juveniles,” Exton said. “Over the course of our study, we found smaller and smaller mesh sizes being used in the fences, as a response to declining catches, and this has led to increasing numbers of juveniles being caught.

“Caught juveniles are eaten or sold, with little or none being wasted,” he added.

The fences also contribute direct damage to seagrass ecosystems, with cascading impacts on connected coral reefs and mangroves. Natural connectivity enhances fish abundance and protects ecological processes, as well as preserving resilience, and so any activity that acts to disrupt this connectivity is counterproductive to conservation efforts, the paper said.

In addition, the researchers found that the use of artisanal fish fences has created social conflict by assuming unofficial and unregulated property rights.

In the case of Kaledupa, they noted a perceived social hierarchy between Pulo (islander) and Bajo (sea nomad) communities. Informal restrictions mean only Pulo fishers are socially permitted to own fences, therefore excluding not only based on economic status but also on ethnicity.

The fishermen will typically invest about $400 of capital to construct a new fence, and typically no further financial outlay is required for at least one year, after which there will be small costs associated with maintenance and repair.

“There are a lot of social complexities — often wealthier fishers have the capital to buy the materials for fishing gear,” Gabby Ahmadia, director of marine conservation science at WWF and a co-author of the report, said in an email to Mongabay.

“This is also a region where there are many Bajo communities which have historically been marginalized. There are sensitivities between cultural groups over access to fishing grounds,” she said.

Taken in Belize, this photo shows a local variation on fish fence design, where chicken wire is used instead of netting. Image by Gabby Ahmadia.

The researchers suggested a restriction on the use of fish fences, saying it would result in direct and immediate benefits to the health and extent of three distinct ecosystems: coral reefs, seagrass beds, and mangrove forests.

Banning these fences would also remove a significant sink for large numbers of juvenile fish across many hundreds of species, and would benefit the wider fishing community by removing an important source of social conflict while also reopening large areas of currently inaccessible fishing grounds and restoring natural fish movement or migrations.

Ahmadia suggested more sustainable fishing practices, including pole and line, and even nets.

“Many of the local fishing gear such as hook and line, or even net fishing — when they are not dragging it over coral reefs — can be more sustainable, particularly if smaller fish are released,” she said.

But making such a switch may prove difficult for fishing communities long accustomed to low risk and high convenience of using fish fences, Ahmadia said.

“These fishers know the ocean better than anyone,” Ahmadia said. “They understand the impacts and have seen the decline in fish catch following the increase of fish fences. But once a fisherman has the capital for the materials to build the fish fence, it take relatively little time to maintain and check the fish fence at low tide — compared to more active fishing methods. They don’t have to spend long hours on the water, which is desirable. When faced with the option of spending all day at sea, or just a couple of hours to go check on your fish fence, there is an obvious choice.”

One of the challenges might be scaling up the more sustainable approaches and providing communities with the tools, skills and resources required to manage their resources.

“Through long-standing relationships and trust building with communities, local people are often open to alternatives,” Ahmadia said. “They often have the will, but at the end of the day, they need to feed their families, and we must provide opportunities that can help ensure they have stable incomes.

“Thinking into the future is challenging when you have mouths to feed that day,” she said.

Fish fences are typically positioned on tropical seagrass meadows, which are important ecosystems for fish communities and the health of neighboring habitats. Image by Benjamin Jones for Project Seahorse.


Exton, D. A., Ahmadia, G. N., Cullen-Unsworth, L. C., Jompa, J., May, D., Rice, J., . . . Smith, D. J. (2019). Artisanal fish fences pose broad and unexpected threats to the tropical coastal seascape. Nature Communications, 10(1). doi:10.1038/s41467-019-10051-0

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