Last summer, a story in the Atlantic Fisherman outlined the ongoing efforts to control the spread of the invasive and destructive European green crab in this region.
According to the Department of Fisheries and Oceans (DFO), green crabs were first found in Canadian waters in 1951 in southwest New Brunswick and have since expanded to many other locations in Atlantic Canada. They entered Nova Scotia waters in 1953/1954 and reached just south of Halifax in 1966. By 1982/1983, green crabs were present along the eastern shore of Nova Scotia.
The damage green crabs have on eelgrass beds and other shellfish species is well documented.
In an effort to control or even eradicate the invasive species, there have been trapping programs going on for some time in Eastern and Southwestern Nova Scotia.
Commercial fishermen are now involved in controlling the green crab and are actually generating revenue for themselves by selling the tiny crustaceans as bait. It began as a pilot program in Southwest Nova Scotia in 2010, became permanent in 2014 and was expanded to Eastern Nova Scotia in 2017. There are currently 20 licences in Eastern Nova Scotia and 70 in Southwestern Nova Scotia.
DFO explained that the commercial fishery was established as a management measure to help reduce green crab impacts.
Due to the wide range of the green crab in Nova Scotian waters, for now, conservation efforts are aimed at controlling the harm this small crustacean causes — not total eradication of the species. And according to the latest scientific research, this surprisingly might be the best approach.
A recent study led by the University of California found that some invasive species targeted for total eradication bounce back with a vengeance, especially in aquatic systems.
The study, published recently in the journal Proceedings of the National Academy of Sciences, chronicles the effort — and failure — to eradicate invasive European green crabs from a California estuary. The crabs increased 30-fold after about 90 per cent had been removed.
In 2009, researchers began intensive efforts to eradicate the European green crab from Stinson Beach’s Seadrift Lagoon. And initial efforts appeared to be working. By 2013, the population had decreased from 125,000 to fewer than 10,000 individuals.
But one year later, in 2014, the population exploded to about 300,000 green crabs in the lagoon — a 30-fold increase over 2013 levels and nearly triple the pre-eradication population size, according to the study.
The study found that the population explosion was due, in part, to the fact that adult green crabs typically cannibalize younger individuals. When most adults were removed, juveniles grew unchecked and overcompensated for the loss of adults.
The study notes this short-term overcompensation drove a process called the “hydra effect,” named after a mythical serpent that grew two new heads for each one that was removed. The scientists likened it to the Sorcerer’s Apprentice in the Disney film Fantasia, in which several spellbound brooms emerge from just one chopped by apprentice Mickey.
The study also had a precautionary tale for natural resource managers: “Don’t try to get them all, or it could come back to bite you.”
“Instead of a one-size-fits-all approach, this study highlights the need to evaluate possible unintended consequences in selecting management strategies and tailoring these to the particular context and expected outcome,” explained Greg Ruiz, a co-author of the study and marine biologist with the Smithsonian Environmental Research Center.
Regarding the conservation approaches to the invasive green crab in our water, the researchers advise what they call the “Goldilocks level” approach, where the population is low enough to protect native species and ecosystem functions without risking a population explosion of the invasive species.
