Chapter 16. Invasive Species

Invasive Species Remain a Persistent Threat to Casco Bay

A Changing Climate May Control Invader Abundance

At a Glance

Volunteer efforts create the capacity to do long-term and large-scale monitoring, collecting thousands of observations each year, which helps researchers detect new invasions, track changes, and identify emerging threats.

Volunteer monitoring is also a valuable way to help the community learn more about invasive species, equipping them to take actions that help prevent new introductions.

Since the 2021 Report

Volunteer involvement in invasive species monitoring has grown over the last two decades, in both the quantity of volunteers and observation sites. While monitoring sites and volunteer observations declined slightly in 2022 due to the removal of several monitoring sites, new sites have since been added.

Invasive species are organisms that are not native to an area but arrive primarily through human activity. Marine invasives are mainly introduced by the transportation of species from one place to another. Global shipping, recreational boating, aquaculture, and other forms of marine transportation have greatly increased the movement of species around the world, allowing invasives to hitchhike on ship hulls, in ballast water, or on marine equipment.

Key characteristics of invasives include aggressively outcompeting native species and growing in population rapidly. This can harm native ecosystems by altering the structure of the food web which can cause negative impacts to the environment, the economy, and public health.

Community Volunteer Scientists Crucial to Invasive Monitoring Efforts

The Wells National Estuarine Research Reserve (Wells NERR) leads one of Casco Bay’s largest invasive species data collection efforts. The Marine Invader Monitoring and Information Collaborative, also known as MIMIC, engages community scientists along the coast and trains them to be able to identify and record 23 species of marine invasives.

The community scientists visit their sites once a month from May to October to sample for the presence and abundance of invasive species. Volunteers are trained to identify 23 species including tunicates, bryozoans, algae, and crustaceans. The data collected since 2008 suggests that invasive species are now established throughout much of Casco Bay. While individual species fluctuate from year to year, the overall number of observations have increased over the past decade.

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Photo: Jeremy Miller from Wells National Estuarine Research Reserve (aka "Wells NERR")

Case Study: Colonial Tunicate

Didemnum vexillum (pictured on the right) has had a growing presence in Casco Bay since it appeared in 2014. Photo: Jeremy Miller from Wells National Estuarine Research Reserve (aka “Wells NERR”)

One of the clearest examples of how an invasive species can establish and spread in Casco Bay is the colonial tunicate, Didemnum vexillum. This species first started to appear in Casco Bay in 2014 forming masses over rocks, shells, and other organisms. While initial sightings were rare, more observations were gradually documented in the following years and since 2020, there has been a steady upward trend. Not only has the colonial tunicate been observed more frequently over the years, but its abundance grows rapidly during the summer, and declines in the fall when the water gets colder.

FIgure 1. Observations of invasive Didemnum vexillum in Casco Bay from 2020 through 2024.

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Photo: Jeremy Miller from Wells National Estuarine Research Reserve (aka "Wells NERR")

Survey May Indicate Decline in Overall Biodiversity

The Gulf of Maine Rapid Assessment Survey (RAS) is a regional survey of invasive species that is conducted every 5 years and surveys the coast from Maine to New York during mid-summer. This program started in 2000 and is led by the Massachusetts Office of Coastal Zone Management and Massachusetts Institute of Technology Sea Grant College Program.

The Brewer South Freeport Marine survey location has been the longest consistently monitored site in Casco Bay. The site has experienced a decline in native species observations and only small fluctuations in invasive species observations over the last few surveys. As a result of these changes, invasive species made up a larger portion of the ecological community than native species in 2023. This may indicate that invasive species are maintaining their presence while overall biodiversity declines.

Photo: Jeremy Miller from Wells National Estuarine Research Reserve (aka "Wells NERR")

Increased Sightings of Blue Crabs and Other Emerging Species

While many species that arrive in Casco Bay are introduced through human activities, some non-native species make their way into different environments on their own. Typically, these species, known as “emerging species”, move as a result of changing environmental conditions which shifts the range of their ideal habitat.

Blue crabs (Callinectes sapidus) are a species endemic to the western Atlantic Ocean, from south of Cape Cod to the Gulf of Mexico. While the species was once rarely observed in Maine, it is now being seen more frequently along the southern coast. It is believed that the Gulf of Maine’s warming waters have created a more habitable environment for this species, explaining this range shift.

As waters warm, other non-native species can be expected to follow this trend. Other species of crab such as the fiddler crab (Uca pugnax), lady crab (Ovalipes ocellatus), and smooth mud crab (Hexapanopeus angustifrons) have also been making their way up the coast in recent years.

The blue crab, Callinectes sapidus, has been seen more frequently in Casco Bay due to a recent range expansion. Photo: Jeremy Miller from Wells National Estuarine Research Reserve (aka “Wells NERR”)

Environmental Conditions Impact Short-Term Growth of Invasive Species

While long term trends show that invasives are becoming more abundant in Casco Bay, not every year is the same. The winter of 2024–2025 was colder than recent years and included periods of exceptionally low temperatures. These conditions appeared to affect many invasive species during the subsequent monitoring season. 

During the summer of 2025, MIMIC volunteers observed fewer invasive species and lower abundances compared to recent years. Several species that had become common at monitoring sites were observed less frequently, suggesting that colder winter temperatures may have reduced survival rates. Similarly, fewer blue crabs sightings were also reported by other partners that summer.

While a single year does not reverse the long-term trend of increasing invasions, it highlights how environmental conditions can temporarily slow the spread and growth of invasive populations. Understanding these relationships can improve our understanding of invasive species dynamics and predict how they may respond to future environmental changes.

Comparison of kelp conditions in August 2023, showing substantial fouling, and August 2025, where kelp remained relatively free of invasives following an unusually cold winter.

2023 Kelp Fouling in August | Photo: Jeremy Miller

2025 Clean Kelp in August | Photo: Jeremy Miller

LOOKING AHEAD TO 2030

There are regulations in place to try and limit the spread of damaging invasive species and efforts to control them or reduce their impacts, but invasive species are expected to continue to arrive and spread in Casco Bay in the coming years. The continued efforts of the MIMIC volunteers are important to tracking these invasive species or recording the appearance of new ones, and to understanding how systems respond to their presence. Wetlands, riparian zones, lakes, and other ecosystems in the Casco Bay watershed are also impacted by invasive species, and efforts are evolving to monitor these invasions as well.