Nets are set up at razor clam seedling farms on Yueqing Bay in Wenzhou, Zhejiang province. LIANG DAN/FOR CHINA DAILY
Species' populations declining due in part to nets set up by clam farmers, study finds
Many species of shorebirds along the East Asian-Australasian Flyway have experienced rapid population declines over the years. Habitat loss and degradation in key stopover sites in the Yellow Sea regions are seen as the primary reasons.
However, many of the species seem to be disappearing faster than their critical stopover habitats, said Liang Dan, an associate research scholar at Princeton University in the United States.
"There are certainly other factors behind the declines," he said.
To gain a deeper understanding of the factors causing the population decline of the shorebirds and to take protective measures to mitigate or even reverse the declining trend, the Chinese conservation scientist and his team have been conducting a research project since August 2019 to assess the direct effect humans are having on the species.
"While designing the project during the start of my post-doctoral work at Princeton, I planned to focus the study on such direct threats as hunting or bycatch (fishing nets accidentally catching shorebirds) across China's coasts, which are only a part of the full flyway," Liang said, using the term to describe the flight path of the migratory shorebirds.
Liang and his team randomly chose 29 important stopover sites along the coasts to investigate in the autumn of 2019. They returned to all those sites in the spring of 2021 and made an alarming discovery.
In addition to hunting and bycatch, he and his team found a potentially serious threat previously unknown to scientists and conservationists — nets used to prevent shorebirds from foraging on young razor clams that are raised on the intertidal mud flats in southeastern China.
In April and May 2021, they estimated, the bird nets set up in Yueqing Bay in Zhejiang province and Xinghua Bay in Fujian province, covering an area of approximately 9 square kilometers, caused the deaths of about 14,000 migratory shorebirds.
The team also estimated the deaths of five species of shorebirds that were caught by the nets in large numbers — black-tailed godwit, terek sandpiper, common redshank, curlew sandpiper and sharp-tailed sandpiper.
The deaths of these birds made up close to or even exceeded 1 percent of their flyway populations, posing a significant threat to their global populations.
The study was conducted by Liang and a group of researchers and conservationists from China and the United States and was published recently in Conservation Biology, a journal issued by the Society for Conservation Biology.
A terek sandpiper is entangled in a net at Yueqing Bay. LIANG DAN/FOR CHINA DAILY
Clam cultivation
Razor clams have long been considered a delicacy in China and are popular with diners.
China's coastal areas are the most important razor clam producing regions in the world. In 2015, the country produced 794,000 metric tons of the clams, with a total value exceeding 22 billion yuan ($3.24 billion). Razor clam farming is a significant source of income for coastal aquaculture farmers. While the practice is widespread in several coastal provinces, Liang said, the clam seedlings primarily come from the southeastern provinces of Zhejiang and Fujian.
Razor clam seedlings are mainly cultivated on intertidal flats, which are divided into small rectangular fields. Every November, clam farmers scatter these seedlings across the flats. The farmers nurture them until the end of March or early April of the following year, usually before Qingming Festival (also known as Tomb Sweeping Festival), which marks the end of the harvest season.
The mature razor clam seedlings are sold to clam farmers in other provincial-level regions, including Jiangsu, Shandong and even as far away as Liaoning, for further cultivation. The seedlings are grown into adult razor clams and then sold on the market.
Bird net problem
The tidal flats along the southeastern coast are not only used for clam cultivation but also serve as foraging grounds for many migratory birds in the East Asia-Australasian Flyway, especially for shorebirds, Liang said.
Some of them spend their winters there, while others that winter further south pass through these areas during their migrations in the spring and fall.
In the eyes of razor clam farmers, the birds are natural enemies of their cultivated clam seedlings. So starting around November, they begin to install horizontally placed nets to reduce the economic losses that may be caused by birds feeding on the seedlings.
"The bird-proof nets abruptly halt the journeys of many of the shorebirds, preventing them from reaching their breeding grounds and reproducing," Liang said.
Regarding the deaths of about 14,000 migratory shorebirds at Yueqing Bay and Xinghua Bay in April and May 2021, Liang said he and his team were not surprised by the figure, and they actually believe the number is a conservative estimate.
"The netting area in Yueqing Bay alone equated to more than 1,000 soccer fields," he explained. "We only calculated the mortality of shorebirds in the peak period of spring from April to May, while some species migrate earlier, such as in mid — to late March, and they can also be caught by these bird nets.
"Also, if the nets are indeed connected to razor clam farming, similar nets could have been used at other (razor clam seedling) farming sites and would certainly have caused more bird deaths."
Subsequent investigations by the research team have confirmed that similar bird nets exist in other seedling farms along the coasts of the two provinces.
The team used satellite imagery to examine the structure of tidal flats and identified 32 locations with similar tidal flat structures that may shelter clam seedling farms. Li Jing, one of the co-authors of the study published in Conservation Biology and founder of the Spoon-billed Sandpiper in China — a nonprofit organization dedicated to the conservation of migratory waterbirds in China — visited all 32 sites with her colleagues last winter and found that 17 had similar bird nets.
"The sites are all areas for razor clam seedling cultivation," Li said. "Our previous surveys usually focused on important shorebird stopover sites. Systematic waterbird surveys had never been conducted in most of the clam cultivation areas, but some of these sites can actually support a considerable population of migratory shorebirds. These are certainly areas that demand more attention."
Most of them have rarely been visited by bird-watchers, she said, and one location is so remote that a ferry is needed to reach it.
With the farming of razor clam seedlings and the presence of the nets, the farms have become "ecological traps" for migratory shorebirds, Liang said, and the nets in the region act as filters, reducing the bird populations as they pass through during migration.
The study also pointed out that most razor clam farmers do not remove the nets even after harvest season in late March and early April; instead, they rely on storms that occur during the typhoon season to blow the nets down in mid-July. These leftover nets can cause the deaths of many migratory birds, Liang said, adding that some of the nets are eventually swept into the ocean, posing a long-term threat to marine organisms, a scenario known as "ghost fishing".
"The phenomenon revealed in the article actually reflects the human-bird conflict that frequently occurs along the coast of China," said Liu Yang, a professor from Sun Yat-sen University in Guangzhou, Guangdong province, who is also one of the study's co-authors.
"The study is the first to understand the bird nets' distribution along the coast and quantify the harm they cause to migratory shorebirds. The focus of the next steps will be to find better solutions."
Removal not enough
According to Li Jing, a local conservation organization called the Wenzhou Birdwatching Society brought the issue to the attention of the relevant authorities in Wenzhou, Zhejiang province. The authorities organized large-scale net dismantling operations several times in recent years.
Liang said, "A simple action — removing all the nets right after the young razor clam harvest in late March and early April — can prevent most shorebird deaths in April and May, as well as the loss of marine life due to ghost fishing, without significantly affecting the incomes of these razor clam farmers."
However, removing the nets right after the razor clam harvest doesn't fully resolve the problem. In fact, many migratory birds depend on this region as wintering grounds during the period when razor clam farmers start cultivating clam seedlings in the mud flats.
"We hope that the results of our study will generate greater interest among local authorities and conservation organizations concerned about the human-bird conflict," Liang said. "To solve the conflict from the root, it is essential to foster collaboration among multiple stakeholders, including local authorities, scientists, conservationists and local communities. This collaboration is essential to safeguard the livelihood of the aquaculture communities while protecting the migratory shorebirds already under threat."
