Incredible Numbers of Juvenile Clams Found Inside Clam Impoundments That Were Protected from Predators

Discovery is Further Proof that Green Crab Predation is the Primary Cause of the Massive Declines in Commercially Valuable Soft-shell Clam Populations in Maine

High amount of clam seed found inside the zippered impoundments.

Freeport Researchers were amazed when they found hundreds of thousands of juvenile soft-shell “seed” clams inside mesh-protected clam impoundments. The impoundments were being tested for their ability to protect and store adult clams so that clammers could sell their previously harvested clams in the late summer when prices are as much as 50% higher. While survival of the impounded adult shellfish was somewhat disappointing, zippered impoundments provided enough protection from predators to allow over 2,500 soft shell clams per square ft. to settle and survive.

An impoundment after sampling.

In fact, the density of juvenile clams under the protected impoundments were the highest ever recorded by scientists. Analysis of 6-in diameter sediment cores showed that the zippered impoundments had 2,562 clams per square foot, or 27,583 per square meter. This density is almost double the previous record of 1,377 per square ft. found on the east side of the Harraseeket River (on flat called Across the River) in 2014.

Outside the protected areas – in the open, unprotected areas – clam densities were drastically smaller. Sediment cores taken in this area showed a density of only 8.5 juvenile clams per square foot, which are unlikely to survive as predation rates continue to intensify during September and October as water temperatures will remain high.

Results Adds to Growing Mountain of Evidence that is Redefining the Idea of “Unproductive Mudflats”

This experiment, along with others conducted as part of the Freeport Clam Experiments, has begun to reveal the true productivity of the mudflats. For example, the location of the recent discovery had been deemed “unproductive” as it had not been commercially productive for over 30 years (meaning that it has been without any clams for local clammers to dig). However, if we could ask the green crabs that inhabit the area they would tell us that the flats have and continue to provide an incredible amount of food – these mudflats are actually hugely productive. The experiment shows that the settling clams need to be protected from predators in order to survive.

A sled and onion bags with some of the recovered protected seed.

What’s more, the cove had been tested for pH levels to try to determine if acidity from ocean or coastal acidification, plays a role in shellfish mortality. Results of that testing, conducted by the Friends of Casco Bay, found that pH levels were among the lowest (more acidic) in Freeport, leading some to be concerned that ocean acidification could be playing a role in why the area remained commercially unproductive. However, three years of sediment buffering studies as well as this 2017 impoundment study shows that juvenile soft-shell clams actually survive and grow in the cove when they are protected by predators. These results show clearly that predation is the leading cause of shellfish mortality, not ocean acidification, and lead to the conclusion that large-scale predator protection projects should be swiftly implemented throughout Maine to save the industry.

This clam impoundment experiment is part of the Freeport Clam Field Research, a project of the non-profit Downeast Institute for Applied Marine Research & Education (DEI). Since 2013, reserchers from DEI and clammers from the Maine Clammers Association have been conducting applied marine research to test the effectiveness of different methods to protect shellfish from green crabs and other predators with the goal of enhancing soft-shell clam populations. Since 2014, DEI has conducted 24 distinct experiments at 100 different field sites and produced a huge amount of data about Maine’s second most valuable fishery.