GOULDSBORO — A local scientist says clam brushing isn’t any more effective in increasing soft-shell clam numbers than doing nothing.
Clam brushing is the practice of standing cut spruce boughs in tidal areas to encourage clams to settle around the brush rather than being dispersed more widely by water currents. While clams do settle more densely around the brush, so do green crabs, and therein lies the rub, according to Brian Beal, who spoke Jan. 8 to members of the Gouldsboro Shellfish Conservation Committee.
“As far as I can see, sticking out brush on the flats right now is akin to asking the crabs to come in,” said Beal, a professor of marine biology at the University of Maine at Machias.
From May 23 through Oct. 31, 2019, Beal conducted the first-ever study on the effectiveness of clam brushing. Working with both his students and with staff from the Downeast Institute, Beal set up the experiment in three locations — Gouldsboro, Harpswell and Bremen. Each area consisted of 54 plots measuring 22-by-14 feet into which recruitment boxes were anchored in place.
“If you put these boxes out, the ones that settle in the box are somewhat protected from predators,” Beal said, referring to juvenile softshell clam spat. “The ones that settle outside the box aren’t.”
At each site, brushed plots were set up and surrounded by control plots without brush. Additional control plots were set up at a distance.
At the start and the end of the experiment, Beal and his crew took core samples in each of the plots to determine how many clams were there. Core samples taken at the beginning showed an average density of seven clams per square foot. Of those, 77 percent were less than a half inch. The remaining ones were larger but none measured the 1 inch required to be legal for harvesting.
The core samples taken at the end of the experiment were considerably different.
“Of all the samples that were taken in the brush plot itself, we didn’t find a single soft-shell clam,” he said. “Not one.”
The core samples revealed 49 green crabs in the boxes in the brushed areas, and these ranged in size from 2 millimeters to more than 1 1/4 inches.
Beal said the study results indicate brushing would have been effective in 1968 when the colder water deterred green crabs.
“Unfortunately, we’re not in 1968. We’re in 2020,” he said. “We look at the seawater temperature trend through time. It’s going up. And it’s been going up since the ’80s.”
Attempting to trap green crabs is ineffective because the smallest crabs can escape.
Beal’s study also examined whether netting could protect clams from predators, setting up netted plots surrounded by control plots, in a fashion similar to that of the brushed plots. By the end of the experiment, netting remained on only one of the three plots.
On the plot where netting remained, Beal found 25 times more clams had survived than in the other areas.
Netting won’t work in practice because it protects clams that are already there, like the ones seeded by the experiment. Netting doesn’t cause clams to settle around or under them.
“If you set out nets to protect clams that you think are going to settle, then you’re flipping a coin,” Beal said.