Science for Everyone

Understanding Green Crabs in Maine

Yesterday, December 10, 2014, I had the opportunity to travel down to Castine, Maine, to attend an Eastern Maine Skippers Program cohort gathering hosted at Maine Maritime Academy-- the first time the skippers have connected as a full group since their kickoff event on Hurricane Island. The focus of this day was to give students a chance to explore and discuss some of the more nuanced facets of "the green crab problem, " and start to explore project ideas that they can research or test this school year.  A variety of experts attended to share their expertise from the perspective of marketing, management, harvesting, and mitigation and ecology, including Les White, biologist at the Maine Department of Marine Resources (DMR), Dr. Brian Beal, professor of Marine Ecology at University of Maine at Machias, and Mark King from the Department of Environmental Protection

The session I sat in on was run by Dr. Brian Beal on Understanding green crab population dynamics and their effects of cultured and wild populations of soft shell clams. Dr. Beal is the director of research at the Downeast Institute on Great Wass Island, and has worked extensively with clams in a hatchery setting, but also expanded his work to look at the impacts green crabs have on clam populations.

The first part of his talk was focused on understanding the life history of green crabs (Carcinus maenas). It is important to have this background when brainstorming potential solutions to "the green crab problem," because the way that green crabs reproduce, the habitat they prefer, and their tolerance to a wide range of temperature and salinity conditions explain why they have been so successful at invading Maine's coastline, and can give us clues on the best strategies to mitigate (reduce or manage) green crabs to help maintain the coastal systems and commercial fisheries they impact. 

Green crabs originated from Ireland or England, and have since spread to different parts of the world including Japan, Australia, Sough Africa, and Patagonia. The first green crab sighting in Maine was in 1905 in Casco Bay, and, Dr. Beal noted that from that point they slowly spread north reaching Lubec in 1951. The net flow of Maine's tides is south, which means that green crabs likely traveled in ships ballast (the same strategy that brought them over from Europe in the first place) to spread up to northern Maine rather than migrating against the tides.

A female green crab carrying a large mass of eggs (orange), secured to her telson with an excreted glue-like substance

Dr. Beal describes green crabs as a "consummate invader of new ecosystems," because several aspects of their life history make them well-suited to quickly expand their populations when they reach a new area. Green crabs are highly fecund, which means that they produce a TON of eggs, and the number of eggs female crabs can produce increases exponentially as they get larger (a 2-inch carapace length female can hold an estimated 165,000 eggs). Females protect and carry their eggs under their telson ("tail" flap) until the eggs hatch into planktonic (floating) larvae.  Larvae are then carried long distances by the wind, tide, and ocean currents-- so by the time they settle out of the water column onto the ocean floor (after 50-80 days of floating) one female's offspring may have dispersed to locations miles away. Another aspect of their life history that makes green crabs really resilient is that the larvae can survive in a wide range of temperatures (a range from 8-25°C or 46.4-77°F) and salinities (10-30 ppt). Adult crabs can survive even more drastic temperature and salinity ranges, and researchers have seen green crabs survive completely out of water for nearly 10 days in typical Maine summer conditions. Finally, green crabs aren't picky: they thrive in mud, cobble, sandy, and rocky intertidal and sub-tidal environments, and they eat just about anything--clams, mussels, lobsters, marine worms, cord grass, and eel grass--so  they can be successful nearly anywhere along Maine's coastline.

One parameter that does seem to keep green crab populations in check is really cold temperatures (at least -1°C or 30.2°F) that are sustained over several winter months. If the average minimum monthly temperature increases by just a few degrees (2°C or 5°F) a large portion of the population survives the winter and continues to grow and reproduce. This type of change happened between 1940 and 1950 (which caused populations of green crabs to explode), and again on a smaller scale between winter conditions in 2013 to 2014. Dr. Beal's initial green crab data collected during his research in 2013 shows how much difference one cold winter can make: his team caught an average of 10 pounds of green crabs per research trap in 2013, and a pound represented an average of 12 crabs. In 2014, he only caught 1-1.5 pounds of crabs per trap, and the crabs caught were much smaller--it could take up to 56 crabs to make up a pound. Maine experienced a particularly warm winter in 2012-2013, allowing the green crab population to thrive and grow, but then a cold winter in 2013-2014 killed back a lot of the adult crabs, leaving Dr. Beal and his researchers with a catch of mostly small young crabs in 2014.

As we continue to see global changes in water temperatures (the Gulf of Maine Research Institute found that the Gulf of Maine is warming 99% faster than the rest of the worlds oceans), it appears that this "green crab problem" isn't going to just go away. This is where our Eastern Maine Skippers come into the picture. They are working hard this year to develop projects to figure out creative ways to mitigate the impacts of green crabs in Maine. Stay tuned!

Subscribe in a reader