Persistence of Hydraulic Dredge Tracks Following Surfclam Harvesting in Shallow Water

Abstract

Hydraulic dredges used for bivalve harvest are known to create immediate biological and physical changes to the substrate, but the rate of recovery is often site specific and habitat specific. Hydraulic dredging for the Atlantic surfclam Spisula solidissima (Dillwyn, 1817) occurred off Provincetown, MA, from November 2014 through April 2015. Acoustic surveys following the dredge activity were conducted in 2015 (June–August) with a phase-measuring sidescan sonar, yielding colocated sidescan backscatter and swath bathymetry. Dredge tracks were documented in 7.9% (402,309 m2) of the 5.1 km2 surveyed. The survey area was divided into 10 × 10 m grid cells, and track density was calculated within each grid by the aggregated count of dredge tracks within each grid cell. The resulting track densities ranged between 0% and 53%. On October 23, 2017, 875 days after the 2015 hydraulic clamming season, a subset (1.05 km2 ∼20%) of the same area was acoustically surveyed. The 2017 survey (1.05 km2) revealed that dredge tracks covered a total area of 9,784 m2 at a density of 0%–24%. Initial areas (2015) with dredge track density greater than 10% had not fully recovered in 2017, and the rate of dredge tracks remaining was proportional to disturbance (linear regression; r2 = 0.95; F = 110.6; df = 1,7; P < 0.001; SE = 0.206). The adjacent dynamic shoreline and the persistence of dredge tracks reinforce the idea that nearshore energy regimes should not be assumed based solely on more visible shoreline processes.