Study: Non-native Manila clam has established in Mission Bay, San Diego

A new study confirms that the non-native Manila clam has been taking hold in Mission Bay of San Diego. Published in the journal PLoS One by Sea Grant Extension Specialist Dr. Theresa Sinicrope Talley, Dr. Drew Talley (USD) and Alexander Blanco (USD), the study identifies that the clam is associated with man-made hard structures placed on the otherwise soft-bottomed bay, such as rip-rap.

The study does not identify how the clams arrived in San Diego, a question that typically requires genetic analysis to answer. Native to Japan, the Manila clam is a favorite shellfish served in many restaurants. It is frequently sold live in San Diego markets and may have been intentionally or accidentally released. It is also a common aquaculture species along the west coast of North America; its larvae may have dispersed from farms in the region or leap-frogged down the coast from the Northwest, where it has been commercially raised for many years.

The Manila clam can grow in dense beds in places where it has established, including in the canals of Venice, Italy and shores of British Columbia. The clam has crowded out native species in these areas, but their ecosystem impacts in Southern California have not yet been researched.

Sinicrope Talley says this means Manila clam is not a high priority for removal compared to known pests. When it comes to non-native species, she says, “We are often reactive, waiting until something starts interfering or costing us money before taking action.”

By that point, populations are usually well established and therefore costly and difficult to remove. For a new arrival like the Manila clam, “The time to discuss management strategies would be now,” she says.

Instead, Sinicrope Talley hopes that their arrival might spur discussion about the tradeoffs between non-native fisheries and their potential ecosystem impacts. In the Northwest, aquafarms actively seed the economically valuable Manila clam on tidal flats. It is found outside of these farms all over the region and is considered “naturalized”, an acceptable part of the ecosystem.  

CA Sea Grant Extension Director Rick Starr notes that such decisions are not new. California supports several non-native, economically important fisheries such as striped bass and largemouth bass.

The Manila clam is unlikely to achieve that status here. While Sinicrope Talley’s study reveals that conditions in San Diego appear favorable for wild populations of Manila clam, the poor water quality of local bays would prohibit recreational harvest.

“We could allow the Manila clam to continue to disperse in our bays,” says Sinicrope Talley, “but we couldn’t harvest and eat those.  Only clams from commercial farms are harvested from certified growing waters and tested for safety as food. So, one question is whether our demand and the economic benefits of the farmed clams are an acceptable trade-off for potential environmental impacts and management costs?”

Sinicrope Talley hopes that the conversation will also turn to other solutions such as the best use of native versus non-native species in aquaculture. Selling and cultivating a native species like Pacific littleneck clam might actually help ocean wildlife should their larvae settle offsite, as Sinicrope Talley explained to Voice of San Diego.

“If local natives escape, so what?” she says.