It’s a fishing version of the old “If life gives you lemons…” adage. Instead of passively watching invasive green crabs devour native species, fishermen and scientists in Nova Scotia have acted, turning the invader into an asset.

At the Maine Green Crab Summit in Orono in December, Chris McCarthy, ecologist with the province’s Kejimkujik National Park and National Historic Site, explained how the commercial green crab fishery was established, and how it helped sustain local ecosystems. By turning the green crabs into an alternative bait for the local lobster fishery, the invasive species was reduced and the existing fishery boosted.

Maine clammers and fishermen have been raising the alarm for the last couple of years about the invasion of European Green crabs and the damage they are causing to shellfish beds and near-shore ecosystems. The crabs seem to eat almost anything, including soft shell clams, juveniles of other crabs and lobsters, and eelgrass.

The new commercial fishery in Nova Scotia is a bright spot in an otherwise bleak picture. Fishermen there are now targeting the species, bringing down the numbers (and size) of green crabs left in the water, and making money in the process. The going rate for green crabs is $100/crate ($.83/lb.). A crate is about 120 pounds, and on average five crabs make up a pound, so about 600 crabs makes a fisherman $100. Considering that up to 1,187 crabs were caught in a trap one night, we thought this was worth further investigation.  

McCarthy reported that there were two challenges to getting the fishery started—building an effective trap and developing a market for crabs as bait.

“The challenge,” as McCarthy noted, “is that bait is like a religion to the fishermen.”

A local fishermen, Russell Nickerson, took the lead. He started by leaving a crate of crabs on each wharf in his area with a sign: “Free—an invasive species—try in every second trap”, along with his phone number. And people started calling.

At the same time, Nickerson worked on building the best trap he could for the job. He ended up with a modified shrimp trap and then worked with the local fishery management body to get it approved and set up a licensing system. Nova Scotia now has 53 Department of Fish and Oceans approved commercial green crab harvesters using the “Russell” trap.

McCarthy reported hearing that lobstermen had good luck baiting their traps with the green crabs, many using two large crabs on a spike. Spiking the crabs live seems most effective, more so than using frozen crabs.
With herring at $1.30/lb. and green crabs at $0.83/lb., the alternative bait is cost-effective, too.

In Maine, concerned fishermen and coastal residents hope that a couple of cold winters, like this one, would knock back the population of green crabs which have exploded with the warmer sea water temperatures. But McCarthy notes that the most recent wave of invasion in Nova Scotia appears to be a different genotype of green crab than those that first arrived on the east coast of North America in the early 1800s. This genotype, which showed up in Nova Scotia sometime in the 1980s, appears to be more cold tolerant and aggressive in nature and is likely the type that has spread into Maine in recent years. Nova Scotia fishermen are catching these crabs throughout the winter, even through the ice.

In Maine, green crabs are turning out to be hardier than previously thought.  A fisherman on Long Island in Casco Bay recently photographed a green crab exposed in the upper intertidal during a 10-degree day, so cold winters alone may not rid us of these crabs.

McCarthy also shared promising results from a scientific trapping study he has conducted with students for three years, systematically removing green crabs in a single estuary. Comparison with an unfished neighboring estuary has shown substantial ecosystem benefits of trapping green crabs. They documented a 30 percent decline in the number of green crabs each year for two years in a row. The average size of crabs remaining in the water also decreased, minimizing their damage on the eelgrass.

In all, they removed over one million crabs over three summers. As a result, the eelgrass rebounded by 10 percent each year for three  years. The little eelgrass that had survived the invasion (only 1.3 percent of its former abundance) was able to seed and propagate an almost instantaneous recovery.
If green crabs as lobster bait takes off as practical, affordable and effective, it would be a win-win for Maine. We could help reduce the negative impact of a destructive invader on the near-shore ecosystem and put less pressure on forage fish populations, like herring, currently used as bait.

Heather Deese is the Island Institute’s vice-president for strategic development; Susie Arnold is a marine scientist with the organization.