A map showing the three regions of the coast and the locations of the brushing vs predator protection study sites within each region. Map by Kaitlyn Raffier.

In May of 2019, DEI scientists deployed studies across the Maine coast that will determine if the brushing method works to increase clam populations. This study marks the first time ever that this popular clam enhancement method has been tested to determine its effectiveness. The results will provide valuable information to clam managers about tools they can use to increase clam harvests and adapt the fishery to the changing marine ecosystem.

The studies are taking place in each of three regions of the Maine coast – Downeast, Midcoast, and Southwest, in the towns of Gouldsboro, Bremen, and Harpswell, respectively. DEI scientists are working directly with the town’s Shellfish Conservation Committees and clammers to deploy the experiments, as well as collect and process the data.

Deploying brushing at the Harpswell location.

 

Brushing is the practice of inserting tree boughs into the mudflats. In concept, the brush reduces tidal and wind currents that help to create eddies and provide bottom zones of slow-moving water where recently settled soft-shell clam juveniles may congregate.

The study deployed three sets of brush in each town, and will measure the amount of clams that settle and survive around the brush. In addition, at each location, three nets that protect clams from predators were deployed. The number of clams that settle and survive in brushed areas will be compared to the number of clams that settle and survive in netted plots. This comparison will clarify the utility of the two methods and demonstrate to local shellfish managers the most effective method to increase clam populations.

Municipal officials and DEI scientists work together to secure one of the three predator protection nets in Harpswell.

Study Design

Field schematic of the experiment that was deployed in Gouldsboro, Bremen, and Harpswell. In each location there are 6 “blocks”: 3 with brushing in the middle and 3 with netting.

 

A. A close-up of a block. The center block receives the brush or the net. Eight control plots surround the central plot. B. The same block showing that recruitment boxes will be added to each of the nine plots in order to measure recruitment to the area.

Results from Baseline Density Clam Surveys, May 2019

During deployment DEI scientists took samples (n=108) of the mud to establish baseline densities of clams in each location. Clams were found in half the samples in Gouldsboro, 26% of the samples in Bremen, and 13% of the samples in Harpswell. The average density of clams in Gouldsboro was 3.4 clams per sq. ft., in Bremen the average density was 1.6 clams per sq. ft., and in Harpswell the average density was 0.6 clams per sq. ft.

The baseline survey results show that the amount of clams in the mud decreases the further south the location. This finding correlates with previous field experiments DEI has done across the coast of Maine. Read more about the baseline survey results here.

Clam harvests in Maine have declined dramatically over the last 30 years. Recent research by DEI has found that increased predation by invasive green crabs and other species are to blame for the decline. The Gulf of Maine’s warming ocean water temperatures drives the increase in predation.

Samples from each of the three sites will be collected in November. DEI scientists will then analyze the data. The results of the experiment should be known by the end of 2019.

Funding for this study came from the Maine Shellfish Restoration and Resilience Project, the Broad Reach Fund, and Maine Sea Grant.

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