Comparative Field Trials to Examine the Efficacy of a Traditional Management Tool—Brushing—To Enhance Local Densities of 0-Y Class Recruits in the Soft Shell Clam Mya arenaria L. Fishery in Maine, USA

The intertidal soft-shell clam Mya arenaria fishery in Maine is comanaged under a cooperative agreement, or shellfish ordinance, between coastal communities and the state. The ordinance, among other things, defines conservation tools that encourage communities to engage in activities with a goal to enhance local densities of 0-y class individuals of this infaunal bivalve. One method used by communities for over 75 y is referred to as “brushing,” which involves forcing dozens of the recently cut white spruce Picea glauca boughs (approximately 70 cm in length) vertically into the soft sediments of a mudflat so that 40–45 cm protrudes into the water column. Typically, multiple rows of boughs are deployed on flats in the spring before clam spawning where they remain through the fall after annual settlement and recruitment have ended. Brushing purportedly results in slowing down tidal currents and creating eddies that allow recently settled clams that otherwise are susceptible to bedload transport away from the flat to establish themselves in the vicinity of the boughs. During spring–fall 2019, a comparative field experiment was conducted at three intertidal flats within a 160-km stretch of the Maine Coast to test, for the first time, the efficacy of this traditional management tool versus another approach used to enhance local densities of 0-y class clams—predator-exclusion netting. At each flat, neither treatment enhanced densities when compared with controls of recruits of Mya as well as recruits of another commercially important infaunal bivalve that occurred at a single site, the northern quahog Mercenaria mercenaria. That is, neither bivalve responded positively to experimental additions of spruce boughs to soft sediments at any study site. Passive bivalve collectors demonstrated that both species settled into plots, ruling out recruitment limitation. Comparison of recruit densities between collectors and core samples from both brushed and netted plots demonstrated losses greater than 97% for Mya and greater than 93% for Mercenaria. Collectors also provided data on densities of recruits of the invasive green crab Carcinus maenas, which ranged from 1.2 to 13.9 ind. m–² across sites, and were 3–6× more abundant in collectors within plots with brush or netting versus adjacent control plots, suggesting that this predator selects heterogeneous versus homogeneous environments during its early life history. As green crab populations in Maine vary directly with seawater temperatures, and the Gulf of Maine is warming relatively rapidly, adding potential habitat for green crabs, such as brush, to intertidal flats should be discontinued in favor of more effective methods to enhance local densities of 0-y class individuals.