In May of 2019, DEI scientists deployed studies across the Maine coast to determine if the traditional practice of brushing works to increase soft-shell clam populations, the first time that this popular conservation method had been scientifically tested to ascertain its effectiveness. The results provide valuable information to clam managers regarding tools they can use to increase clam harvests by adapting the fishery to the changing marine ecosystem.
The studies took place in each of three regions of the Maine coast (Downeast, Midcoast, and Southwest) in the towns of Gouldsboro, Bremen, and Harpswell. There was also a brushing experiment in the downeast town of Milbridge that had a slightly different design. DEI scientists worked with the towns’ Shellfish Conservation Committees and clammers to deploy the experiments, as well as to collect and analyze the data.
Brushing is the practice of inserting tree boughs into the mudflats. Theoretically, the brush reduces tidal and wind currents, helping create eddies and providing bottom zones of slow-moving water where recently settled soft-shell clam juveniles may congregate.
Three sets of brush trials were deployed in each town, and the number of clams that settled and survived around the brush was quantified by collecting baseline mudflat samples, placing recruitment boxes in the study locations, and collecting end-of-year mudflat samples. In addition to brush and recruitment boxes, each location included three nets that protected clams from predators. Data was collected in November, at the end of the clam growing season. The number of clams that settled and survived in brushed areas was compared to the number of clams that settled and survived in netted plots. This comparison between the two treatments helped to compare the utility of the two methods and demonstrated to local shellfish managers the most effective method to increase clam populations.
Baseline Density Clam Surveys, May 2019
During deployment DEI scientists took 108 samples of the mud to establish baseline densities of soft-shell 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 was 3.4 clams per sq. ft. in Gouldsboro, 1.6 clams per sq. ft. in Bremen, 0.6 clams per sq. ft. in Harpswell. The baseline survey results show that the number of naturally-occurring soft-shell clams decreases the more southerly the location. This finding correlates with previous field experiments DEI has completed across the coast of Maine. Read more about the baseline survey results here.
Samples from each of the three sites were collected in November in collaboration with clammers from each community. (Read about the sampling effort in Bremen in this Lincoln County News article.) DEI scientists then processed the samples, and recorded and analyzed the data to determine the results.
Results and Final Report
DEI’s investigation determined that brushing is not an effective method for increasing soft-shell clam settlement because the brush also provides habitat for predators. Clams that settle in the brushed areas are quickly eaten by predators that are using the brush as habitat. Warming seawater enables predators (such as green crabs) to thrive, increasing predation rates on clams. Brushing may have been effective in the past when seawater temperatures were cooler but most areas of the coast will likely see no benefit from using this technique in the current warming marine environment.
The results were published in the Journal of Shellfish Research in December 2020. View the article.
Milbridge Brushing Experiment
Additional experiments were conducted from May 4 to October 26, 2019 in Milbridge in Pigeon Hill Bay. This experiment had a slightly different design. Eight treatments were deployed in 12-ft x 12-ft plots. Half were brushed (brush was placed along all four edges of the plot) and half were not brushed. Half were netted to protect against predators. Half were seeded with 2,500 seed clams per plot. Each treatment was replicated 4 times and there were 32 plots in total. At the end of the field season the plots were sampled, contents of each sample run through a 1mm mesh sieve, and all live clams counted and measured.
Most clams in the seeded plots were found dead with crushed valves from crab predation. However, it was clear that brushing did not bring the desired effect of increasing wild clam recruitment. Netting did have the effect of increasing wild clam recruitment, but the effect differed depending on whether or not the plots were seeded.
Learn more about the Milbridge study HERE. PDF is an extract from Dr. Brian Beal’s presentation at the 2020 Maine Fishermen’s Forum.
Presentations were held in each of the three communities to share the findings:
- Results from the Gouldsboro site were presented to the Gouldsboro Shellfish Conservation Committee on Jan. 7, 2020. Read about the presentation in this Ellsworth American article.
- Results from the Bremen site were presented to the Bremen Shellfish Committee on Jan. 8, 2020. Read about the Bremen presentation in this Lincoln County News article.
- Results from the Harpswell site were presented to the Harpswell Marine Resources Committee on Jan. 28, 2020. Read about the Harpswell presentation in this The Times Record article.
The coastwide results were shared on March 5, 2020 at the Maine Fisherman’s Forum. View the presentation below.
Funding and Acknowledgements
Thank you to the Broad Reach Fund, Maine Shellfish Restoration and Resilience Fund, and Maine Sea Grant for funding this research. We are indebted to the town shellfish committees and clammers of Gouldsboro, Bremen and Harpswell for their work in site planning and coordination, as well as their work in the field deploying the research and conducting final sampling. DEI also thanks Quahog Bay Conservancy for their assistance in sampling the Harpswell site.