Loss of Eelgrass in Casco Bay, Maine, Linked to Green Crab Disturbance

Abstract

EXECUTIVE SUMMARYIn 2013, the Freeport Town Council approved funding a "Shellfish Restoration Project."The overarching goals of this field-based effort were to: 1) understand how trapping, netting, and fencing can reduce green crab predation on young-of-the-year clams (i.e., spat, "recruits," or 0-year class individuals); and, 2) understand how reduced crab predation correlates with increased spat survival.Planned activities focused on three areas of study: 1) green crab trapping in the Harraseeket River at areas adjacent to and south of Weston Point (near Collins Cove) vs. areas north of Weston Point (at Porter Landing, Pettengill flat, and Sandy Beach) to collect information on the relationship between trap immersion time and catch-per-unit-effort (CPUE), how CPUE varied through time and by location, and how size-frequencies of male and female green crabs as well as sex ratios varied spatially and temporally; 2) the role of fenced vs. control vs. netted plots in enhancing 0-year class soft-shell clam individuals (Little River Flat); and 3) the effect of large-scale green crab fencing across an entire cove on enhancing wild softshell clam recruits (Recompence Flat).Green crab trapping (using 18-inch diameter x 36-inches long wire traps) occurred from 27May to 5 November, and involved a total of 15 clammers, seven of whom fished five to ten traps regularly over at least some of that period. A total of ca. 300 hauls (1 to 10 traps per haul) were recorded over the 162 days between May and November, with a total of 13,065 lbs. (ca. 6 metric tons) of green crabs harvested. Of that total, 11,715 crabs were measured and the sex of each determined. The average size (carapace width -CW) of crabs was 59.5 mm, and overall sex ratio (M:F) was 72% : 28%. Approximately 1% of females measured were ovigerous, and the last date than an egg-bearing female was trapped was 22 August. At three of four locations south of Weston Point (upper and lower intertidal near CollinsCove, and the subtidal channel off Weston Point), catch-per-unit-effort (CPUE) was independent of time, averaging 8.8 lbs/trap. At the fourth location (lowest intertidal near Collins Cove), CPUE increased 100% from an average of 6.1 lbs/trap in June to 12.2 2 lbs/trap in August/September. At each of the three areas north of Weston Point, CPUE increased from June to September by an average of nearly 50% from 9.9 lbs/trap to 14.7 lbs/trap. Sex ratios were heavily skewed toward males during the earliest hauls (late May to mid-June) and, in six of the seven locations examined closely, the ratios fell significantly over time, with the greatest difference occurring at Pettengill flat where initial ratios were 89% : 11% that fell to 57% : 43% by late October/early November. The average CW of both male and female crabs in all locations decreased by about 4% over time.A concerted effort over a 6-day period between 25-30 July showed that there was no relationship between CPUE and trap immersion (soak) time (1-day vs. 2-days vs. 3-days) at two intertidal areas and one subtid...

Linking eelgrass decline and impacts on associated fish communities to European green crab Carcinus maenas invasion

“…In Placentia Bay, we observed a 25% or greater decline in eelgrass coverage at 12 of 16 eelgrass sites and eelgrass presence at 5 of 16 eelgrass sites, suggesting a dramatic decline in eelgrass cover and clear evidence of localized disappearance of eelgrass at sites now occupied by green crabs. Recent laboratory and cage experiments demonstrate that green crabs can reduce eelgrass biomass by burrowing for shelter, digging for prey, and directly cutting basal meristems (Malyshev & Quijón 2011, Garbary et al 2014, Neckles 2015. Further mesocosm experiments also correlate eelgrass losses with crab density (Davis et al 1998 Table 4.…”

“…soft-shelled clams Mya arenaria Linnaeus, 1758) requires digging within the sediment (Cohen et al 1995). Small-scale laboratory and field experiments demonstrate that such feeding activities, along with burrowing in surficial sediments and grazing by juvenile green crabs, damage eelgrass roots and rhizomes and reduce eelgrass biomass (Malyshev & Quijón 2011, Garbary et al 2014, Neckles 2015. Garbary et al (2014) linked these destructive green crab behaviours with increased eelgrass wrack comprised almost entirely of whole shoots (with rhizomes) instead of individual blades and the decline of eelgrass at their Nova Scotia study site.…”

PopulationDynamics of Green Crabs (Carcinus maenas) -- A Review

“…Negative impacts have been far greater in the invaded ranges, however. In the northwestern Atlantic, C. maenas has caused considerable damage to blue mussels [202], American oysters Crassostrea virginica [219], rock crabs Cancer irroratus and lobsters H. americanus [204] and eelgrass Zostera marina along the Atlantic Canada and New England coasts [204,[270][271][272][273][274]. As of now, there has been no decline in C. irroratus as a result of the emergence of green crabs in the Upper Bay of Fundy, Canada [88], but the situation merits monitoring.…”

“…Green crabs generally do not feed on eelgrass Zostera marina but their foraging activity digging in the soft substrate dislodges the grasses and has contributed to considerable loss of eelgrass beds along the New England coast [270][271][272][273][274].…”

Revisiting recent history: records of occurrence and expansion of the European green crab across Prince Edward Island, Atlantic Canada

“…Abundant nursery habitat and multiple anthropogenic vectors were expected to facilitate a further expansion for green crabs across PEI, and created concern among biologists and shellfish harvesters and aquaculturists. Green crabs are well known disruptors of eelgrass habitats (Malyshev and Quijón 2011;Garbary et al 2014;Neckles 2015;Matheson et al 2016) and predators of shellfish species in the region (e.g. Floyd and Williams 2004;Pickering and Quijón 2011;Pickering et al 2017;Poirier et al 2017) and elsewhere (e.g.…”