Get students out of the classroom and up close to science they can touch

(Retired) European Green Crab Mitigation

What makes a species "invasive"?

Invasive species are species that are non-native in an ecosystem. Typically, they grow and reproduce quickly and spread aggressively with the potential to cause harm. Often their presence disrupts the natural order of the ecological web. These species increase predation pressures as well as competition for food and shelter resources, and if their presence is strong enough, they can cause native species to become extinct in those areas.

Even within the same country, a species may be considered invasive in another location. For example, lake trout are native to the Great Lakes. However, in Yellowstone Lake in Wyoming, they are considered invasive because they compete with native cutthroat trout for habitat.

Approximately 42% of threatened or endangered species are at risk because of invasive species, making them the leading threat to native wildlife. They may also threaten human health and create a heavy financial burden on affected communities. These combined impacts can cost billions of dollars each year.

Invasive species may become introduced in a new ecosystem because of human intervention, but increasingly, climate change is the cause. Climate change has accelerated the impact of invasive species in sensitive ecological areas worldwide. Various species are beginning to inhabit new ecosystems to escape the pressures of climate change in their native environments, while creating cascading pressures on the native species in their new ecosystem.

What are European Green Crabs?

The European Green Crab is an invasive species that lives in the waters around the Great Marsh and Cape Ann. This crab negatively impacts the intertidal communities as well as the shellfish industry.

The European Green Crab is one of three invasive species on the North Shore that Seaside is working to mitigate. Green crabs as well as Asian crabs can threaten native species and add to predation pressures, which creates long term effects such as reduction in biodiversity. In turn, the rocky intertidal communities can suffer severely.

Seaside works with volunteers to periodically check traps that are placed along the waterways to track the European Green Crab populations and their effects on the native species. When crabs are caught, Seaside removes them from economically important clam beds. Local students also get to help out as part of their science education. The goal is to remove as many crabs as possible, but especially egg-laying females, to aid in the recovery of the sensitive ecosystem.

You can join us on a trapping field trip or participate in a laboratory exercise at your school to learn more about this invasive species!

Our Green Crab project is in partnership with the city of Gloucester and the Merrimack Valley Planning Commision (MVPC).

European Green Crab Mitigation Program for Students

We offer a comprehensive 1.5 hour hands on workshop available for college and high school laboratories that will be brought to your site for presentation.

Learn about the invasive green crab species and what actions can be taken to reduce their numbers and impact on the ecological web. We will be measuring and collecting data on invasive species, while determining a male from a female. Learning about this common species gives us insight into wildlife management practices, and protections of native crab species largely affected by the green crab.

Invasive Species on the North Shore

Learn about some examples of invasive species you can find on the North Shore below that Seaside is actively working to eliminate, and how you can help with this effort.


Phragmites Australisis a non-native grass present along the marshes of the North Shore. These plants often grow very densely, blocking out the sunlight native species need.

As these plants continue to be a problem, Seaside Sustainability is monitoring their populations on the North Shore to experiment with organic alternatives to the current eradication methods. This initiative is used as a means of restoring the marsh, organically, safely, and responsibly.

Because phragmites can reduce biodiversity and threaten the native flora and fauna in the invaded habitat, it is important to know a few methods to reduce the phragmites population.

Ways to Reduce:

  • Landowners can eliminate or reduce fertilizer, herbicide and pesticide use

  • Do not purposely plant it; instead consider planting only native species in gardens

  • Avoid transporting Phragmites either via equipment or as compost


Pepperweed is a perennial that can be found on islands and beaches around the North Shore. Like phragmites, pepperweed reduces biodiversity and threatens the native flora and fauna in the ecosystem it inhabits. There are also negative impacts on local waterfowl and fish populations.

Every Spring and Summer, Seaside Sustainability partners with a local Audubon office to conduct field surveys to map and mitigate the invasive Pepperweed populations. Through these efforts, we strongly believe that the marsh can be restored.

Ways to reduce:

  • Treating with herbicides is most effective during flower bud/flowering stage

  • If within small patches, repeated pulling of small patches is effective

  • After removal, plant desirable and competitive species

Green and Asian Crabs

Green and Asian Crab species can threaten native intertidal species around Cape Ann and add to predation pressures. Their presence can create long-term effects such as reduction in biodiversity, thereby causing intertidal communities and the shellfish industry to suffer.

It is important to measure the effects the rise in this invasive crab's population has on other marine animals. Seaside Sustainability has set traps off the waters of the North Shore that are monitored periodically; where we record the number of crabs that are present in the water.

Local students also get to help out as part of their science education. This program also offers a chance for a sustainable economy by providing green crabs to businesses, so they can be used for a sustainable local meal and be ‘repurposed’ as high nutrient compost.

Ways to reduce:

  • Mitigate through removal processes like eating them or using them as bait

  • Prevent the spread through ballast water and other sea transportation methods

  • Spread awareness of the damages that is caused by these invasive crabs

Final Mudflat Acidification Infographic.