North & South Rivers Watershed Association

Green Crab Trapping and Burrow Surveys Continue

The invasive European Green Crab has been a destructive force around the Gulf of Maine for several years, clearing estuaries of eelgrass as they dig up clams, and destabilizing marsh banks with their burrows. In order to better understand how this invader may be impacting the North and South Rivers estuary, this past summer the NSRWA and MassBays repeated crab burrow surveys started in 2015 and also piloted a small crab trapping effort in partnership with a local student, Calli Bianchi.

Burrow Surveys

In 2016 we resurveyed six sites on the North and South Rivers that were established for burrow surveys in 2015. In 2015, 81 burrows were found over the six sites, but in 2016 only 3 burrows were found. We do not have an explanation for this, but hope to keep checking the sites over the next few years to determine if this year was an anomaly. That said, we have found green crab burrows in the subtidal mudflats (see photo) while snorkeling blue mussel beds for a separate project, so it may be worthwhile to document those going forward as well.

Trapping

Crab traps were deployed in two locations – the North River and an adjacent creek. Each trap was left in the water for approximately 24 hours on three different days. After retrieval the crabs inside were counted and categorized as larger (“large”) and smaller (“small”) than 4cm.  In a 2014 experiment performed by Ms. Bianchi with assistance from NSRWA and MassBays to test crab predation on blue mussels, the crabs used were 4.7 cm (4.5-4.9 cm range), all at the lower end of the large category from this survey.

In general, more crabs were caught in the creek traps (123-202) than the river traps (90-122), and more large crabs were caught than small crabs. More trapping may be done in 2017 at other burrow survey locations.

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