Researchers in American Sāmoa were surprised to find that giant clams are faring much better than expected. These huge molluscs are at risk from many human threats – including overharvesting, warming waters and pollution – but community-led management has proven successful in protecting them.
Scientists from UH Mānoa Hawaiʻi Institute of Marine Biology (HIMB) ToBo Lab studied data collected over several decades to determine the size and density of giant clams in American Sāmoa, a group of volcanic islands and coral atolls in the South Pacific Ocean.
Areas managed by local villages had more clams and the individuals were bigger compared with no-take reserves where the federal government does not allow extractive activities, such as fishing.
The findings are published in the journal PeerJ.
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Capable of growing to more than one metre long, giant clams are, arguably, one of the world’s most iconic molluscs. These huge creatures are immediately recognisable thanks to their bright mantle – which makes them look like they’re wearing lurid lipstick – and play an important part in keeping reef ecosystems healthy.
Giant clams are vulnerable to several human threats, in particular overharvesting – they can be collected for their meat and shells (for souvenirs) as well as to be sold to aquaria. In many regions, they are being removed from the environment more quickly than populations can replenish themselves.
Increasing water temperatures and ocean acidification also pose a problem and, as filter-feeders, giant clams can also be negatively impacted by water pollution. So, the researchers were surprised to find some giant clam populations thriving.
“Many expected that giant clam populations would be in sharp decline, especially near populated islands,” says the study’s lead author Paolo Marra-Biggs in a statement. “Instead, we found that clam abundances have remained relatively stable over the past 30 years, and in some areas have maintained high abundances.”
The data illustrates how successful Indigenous management can be. “The biggest surprise was that village-managed closures outperformed federally protected no-take areas, highlighting the efficacy of cultural stewardship,” says Marra-Biggs who is a PhD student at HIMB.
Learning which conservation methods have the best results is important if we are to find the most effective ways of protecting these vulnerable animals. There are currently proposals to list several types of giant clam under the U.S. Endangered Species Act (ESA) but the scientists warn against blanket bans in areas where native community protections are already working.
“The push for ESA listing is supposed to protect giant clams, but our data show that in areas where federal no-take protection already exists, clams populations were among the lowest,” says the study’s senior author Robert Toonen, Principal Investigator of the ToBo Lab.
He worries that federal restrictions could have a negative impact in areas where traditional Indigenous management is already resulting in high densities of healthy clams. “If giant clams were listed as endangered species, these traditional practices that have maintained high clam densities in the region would become illegal,” he says.
The research team highlights the importance of working directly with local communities – like they did for this study – and hope their results will help to better protect vulnerable species such as magnificent giant clams.
“It is an honour to contribute to the stewardship of the absolutely stunning reefs of American Sāmoa,” says Marra-Biggs.
Top image: American Sāmoa. Credit: Getty
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