Overview

Green crabs have been identified as an invasive predator of several marine types since the mid 1800s. In 2013, many clammers on the East coast were pointing to the green crab population as a major reason for a decline in the soft shell clam harvest.

In response to the perceived growing threat of the green crab to Maine soft shell clam flats, the ASCC decided to measure the spread of the green crabs in Arrowsic beginning in 2014. Since Arrowsic is in a transition zone from ocean water to briny river water, the initial purpose was to determine if green crabs could be found all the way up the island. We have now trapped green crabs in several locations since 2014. This has normally been a one-time event each year occurring in the June-July period. We have also sexed and measured the catch at each location in the event each year.

For 2014 & 2015 our results showed that we have green crabs in varying numbers up to Fisher Eddy on the Kennebec river and also to just below Mill Isle on the Back river. On the Kennebec river, we did not find any green crabs in 2 trap sites above Fisher Eddy which supported our initial conjectures. The results per trap location have been quite consistent between the 2 years of measurement in terms of the number of green crabs trapped.

In 2016, our results were very different from the previous 2 years. With a reduced number of traps set on the Kennebec river, we found that the green crabs had shifted up the river and now our greatest number of green crabs were found at Fisher Eddy. This was quite a drastic change in location so we initiated a 2nd trapping event in September and found that the data supported a general movement of green crabs up the river and we found them all the way up to Preble Point. Lots of conjecture as to why the shift but we really don&t know why the location shift occurred. We also were able to set more traps in the Sasanoa river in June for the first time in 2016. No conclusions on the Sasanoa data yet as it was our first year of measurement.

In 2017, we trapped in July versus June which was a change from the previous 3 years. That change in event date may help account for the large increase in green crabs trapped. As in previous years, we found no green crabs above Fisher Eddy on the Kennebec, and we found green crabs in all 3 traps on the Sasanoa. The Sasanoa green crabs have a tendency to be larger than the Kennebec green crabs. From additional monitoring efforts of the green crab, we know that the species can move up the shoreline of Arrowsic as the summer progresses. We suspect green crabs progress up the island as both the salinity and the water temperature increase. We had green crabs above the Doubling Point lighthouse as of mid August. An additional element this year was the identification of the Asian shore crab as another invader of our waters. It is a smaller crab and we only seem to have very tiny ones. We catch them coincidently as they fall thru the mesh of our green crab traps.

We are always looking for support in this annual endeavor for both trap set & pull plus the count-measure-gender identification efforts. Please contact a member of the ASCC if you have interest in aiding in the work.

Timeline