As part of DEI’s four year investigation into the soft-shell clam decline in Casco Bay, DEI examined the effectiveness of different methods to protect shellfish from green crabs and other predators. As part of this we conducted systematic green crab trapping in 2014 and 2015, and combined that 2013 green crab catch data collected through clammer conservation time for the Freeport Shellfish Committee.
As part of the green crab trapping research DEI:
- Caught and weighed over 3,000 lbs. of green crabs.
- Measured 18,807 green crabs and analyzed data from 30,622 green crabs.
- Crab location (subtidal vs. intertidal, upper reach of the Harraseeket River vs lower reach of Harraseeket River)
- Time of year caught
- Sex and sex ratios through time and location
- Amount of egg bearing females
- Amount of “shedders” and time of year caught
- Water temperature corresponding to when crabs were caught
- Size frequency through location and time
- Catch rates per type of bait (herring vs clams)
- Crab trap soak times
Green Crab Trapping
DEI’s research found that it was not possible to reduce populations through trapping to save the clam fishery: DEI found that it is not possible to reduce crab populations locally through trapping in open areas of the coast.
DEI’s studies found that the numbers of crabs increased with water temperature, especially in the intertidal zone and crabs grow quite rapidly through late summer and early fall.
Timing of Recruitment: Like clams, green crabs have a larval stage where they are carried in and develop in the water column. DEI discovered that green crab recruitment peaks in early June but extends through mid-September.
Soft-shell clams are one of Maine’s most valuable fisheries. DEI’s research has found that:
- Trapping green crabs will not mitigate the damage crabs do to soft-shell clam populations, and it is a waste of our time.
- Sheds light on biological methods to swamp out green crab predation on clams.
- Provides methods to effectively protect clams from these voracious predators.
Read more about 2013’s green crab trapping, fencing and predator exclusion field trials.