2023 Casco Bay Ecosystem Monitoring Report

Reports | Feb 16, 2024

Over the last several decades, we've seen dramatic ecosystem changes in the Gulf of Maine. We've observed population spikes and declines, habitat loss and recovery, and the appearance of new species that typically occur in regions to our south.

Scientists at the Gulf of Maine Research Institute (GMRI) are conducting long-term, nearshore monitoring to learn about the pace and direction of these changes in our region. This inaugural edition of our Casco Bay Ecosystem Monitoring Report summarizes key findings.

This is a graphic with paper cut style colored squares scattered across the image.

About This Report

GMRI scientists began collecting field data in 2014 as part of a long-term, nearshore ecosystem monitoring effort in the Gulf of Maine. For 10+ years, this monitoring has provided us with insights about complex, nearshore ecosystem changes. It also provides a baseline for context and comparison in the future. Continuously collecting samples in (and adjacent to) the bay over time enables us to detect, understand, project, and communicate change.

This report highlights key trends from our observations and offers context on the impact of ocean warming on critical components of the Gulf of Maine ecosystem with a particular emphasis on Casco Bay. Read on to see results from our Casco Bay Aquatic Systems Survey (CBASS), anecdotal observations, and more.

Report Highlights

  • The Casco Bay ecosystem is changing as a result of ocean warming.
  • In 10 years of monitoring, we have caught 50 unique species in our beach seine.
  • Our eight most common catches are green crab, Atlantic silverside, winter flounder, mummichog, alewife, Atlantic herring, Atlantic tomcod, and American sand lance.
  • In colder years, Atlantic herring is the most commonly caught fish, followed by Atlantic silverside. This relationship flips in warmer years.
  • There is clear evidence that warming ocean temperatures in the Gulf of Maine are adversely affecting cold-adapted native species, particularly Atlantic tomcod and Atlantic herring.
  • We are tracking the recent appearance and increase in abundance of permit, crevalle jack, white mullet, and summer flounder in Casco Bay. These species are more commonly found in warmer regions to the South, but are now appearing in Casco Bay.
  • Fishermen observations play a key role in our understanding of Casco Bay ecosystem changes (see the On the Water section of this report).

About Casco Bay

Casco Bay (Figure 1) is a semi-enclosed bay located within the broader Gulf of Maine. This relatively shallow body of water covers approximately 230 square miles, and 578 miles of coastline.

This is a map of casco bay.
Figure 1. This is a map of Casco Bay. Dark blue dots represent sampling locations where data has been collected as a part of our long-term monitoring effort, CBASS. Locations in Harpswell were collected by Quahog Bay Conservancy (QBC) as a part of our collaborative CBASS expansion.

Physical factors generate high productivity in Casco Bay and the Gulf of Maine. First, two key oceanographic currents, the cold Labrador Current and the warm Gulf Stream, meet just offshore. They mix nutrients, algae, and plankton that are then available to fish, seabirds, and marine mammals to eat. Second, these systems have a complex bottom topography created from glacial scour at the last Ice Age (20,000 years ago). The topography includes deep trenches, shallow banks, and prominent ridges. These features provide critical habitat for the region’s diversity of invertebrates, fish, and mammals.

Since 2012, Casco Bay and the Gulf of Maine have anomalously warmed. As a result, the marine ecosystem is experiencing changes, including:

  • Shifts in plankton community structure.
  • Declines in plankton prey quality.
  • Profound changes in groundfish distribution and abundance.
  • Increased observations of warm-water species such as black sea bass and squid.
  • Changes in the occurrence of benthic species such as lobsters, crabs, and mussels.

Drivers of Change

The driving forces of marine ecosystem change are among the most profound documented in the Gulf of Maine’s geologic record. Unprecedented alterations in wind strength and direction, coupled with ocean heating, have resulted in cascading changes in the abundance, types, and interactions of marine species living along the Northeast continental shelf and adjacent coastlines.

In this Gulf of Maine, Explained video, GMRI Climate Center Director Dr. Dave Reidmiller explains why the Gulf of Maine is warming faster than almost any ocean surface on the planet, and why that matters.

We document this ocean warming in our warming updates — quarterly and annual reports on sea surface temperatires (SST) in the Gulf of Maine. The most recent warming update indicates average SSTs are warming at a rate of 0.87 ºF per decade — a rate that is three times faster than the average for the global ocean (0.29 ºF per decade). This makes the Gulf of Maine one of the most rapidly warming bodies of water on the planet.

In addition to climate-induced ocean warming, there are numerous other environmental factors that influence the dynamics and functioning of ocean systems including:

  • Increasing human activities in both the coastal domain and offshore.
  • Novel species and shifting marine community interactions.
  • Ocean acidification.
  • Harmful algal blooms.

The bottom-line is that our oceans are changing both physically and chemically. Our models, corroborated by field observations, indicate rising temperatures, increases in precipitation, decreases in salinity, shifting current patterns, and stronger stratification.

As a result of these changes, scientists project declines in cold-adapted species, increases in warm-water species, and associated variations in predator-prey dynamics among these communities.

However, it is important to note that the large-scale models that examine global responses to climate change may not accurately describe the the dynamics of local ecosystems. Regional properties such as bathymetry, local fronts and eddies, shore zone features, turbulence, and mixing influence the larger system. This can occasionally lead to very different ecosystem outcomes.

This is why local monitoring is imperative. We must collect local data to better describe expected outcomes.

This is a landscape shot with a blue sky and emerald green water, and two fishermen have cast a long white seine net into the water.
A woman is hauling up a seining net on a small white skiff.

Current Ecosystem Baseline

To date, we have caught more than 50 different fish and crustaceans species in our beach seine. The eight most frequently captured species (caught in at least 10% of all sampling attempts) are:

  • green crab (Carcinus maenas)
  • Atlantic silverside (Menidia menidia)
  • winter flounder (Pseudopleuronectes americanus)
  • mummichog (Fundulus heteroclitus)
  • alewife (Alosa pseudoharengus)
  • Atlantic herring (Clupea harengus)
  • Atlantic tomcod (Microgadus tomcod)
  • American sand lance (Ammodytes americanus)

Some of these species are year-round residents of nearshore waters, like American sand lance, mummichog, and Atlantic silverside. Green crabs, an invasive species introduced to North America, have been year-round residents of Casco Bay for more than a hundred years. Other species, like winter flounder and Atlantic herring, depend on Casco Bay and similar estuarine environments as nursery areas for juveniles. As adults, they will make regular seasonal migrations using the Gulf of Maine’s inshore areas. A third group of species, like Atlantic tomcod and alewife, are diadromous, meaning they spawn in the fresh waters of coastal rivers and migrate to the ocean as adults. The populations of all migrant species are affected not only by environmental conditions in Casco Bay but also in the other riverine and oceanic habitats they use.

The status of the wider Gulf of Maine ecosystem therefore depends on the health of all its components, and heavily on resource-rich nearshore habitats like Casco Bay.

Most of the species we commonly observe are in the southern half of their native range, meaning their tolerance for warming waters may be limited. These species may suffer population declines or be forced out of their historical habitat as Casco Bay warms.

Species such as green crabs, which are native to Northern Africa and Europe, and Atlantic silversides and mummichogs, which are frequently found in nearshore waters as far south as Florida, are likely to better tolerate warming waters. They may even flourish.

With ten years of data on the Casco Bay ecosystem, we can analyze how the abundance, distribution, and growth of commonly collected species shifts with changing environmental conditions. Using SST data from the Portland Harbor NOAA tide gauge as an approximation of Casco Bay’s nearshore temperatures, we divided data into the five coldest (2014 – 2015, 2017 – 2019) and five warmest (2016, 2020 – 2023) years. This revealed pronounced differences in the relative abundances of fish species between colder and warmer periods (Figure 1).

Figure 2. Nearshore Casco Bay ecosystem community structure in the five coldest (2014 – 2015, 2017 – 2019) and five warmest (2016, 2020 – 2023) years of CBASS seine survey data collection, as illustrated by scaled indices of relative abundance for the top eight most frequently caught fish and crustacean species.
Figure 2. Nearshore Casco Bay ecosystem community structure in the five coldest (2014 – 2015, 2017 – 2019) and five warmest (2016, 2020 – 2023) years of CBASS seine survey data collection, as illustrated by scaled indices of relative abundance for the top eight most frequently caught fish and crustacean species.

In colder years, Atlantic herring is the most commonly caught fish, followed by Atlantic silverside. This relationship flips in warmer years.

Atlantic tomcod also appears to be sensitive to warmer temperatures and are more abundant in colder years. A number of temperature-linked factors could drive these changes in relative abundance including food and habitat availability, changes in current patterns, or changes in fish reproductive ability.

Note that changes in abundance might reflect catchability rather than actual changes in Casco Bay's absolute abundance. For example, as waters warm, Atlantic herring may move to slightly deeper waters out of range of our seine which can sample ~100 feet from shore and six feet of water. Additionally, we sample from June to September and may miss abundance trends outside of this window.

Emergent Trends

Warming ocean temperatures in the Gulf of Maine adversely affect cold-adapted native species, particularly Atlantic tomcod and Atlantic herring. In warm years, tomcod and herring have decreased abundance in seine samples and leave the survey area earlier as water warms seasonally (Figures 3 and 4).

Figure 3. Atlantic herring relative abundance during CBASS seine survey operations, 2014 – 2023. The color of the year on the vertical axis indicates whether that year was in the top five warmest (red) or the top five coldest (blue) years of the time series. Each colored cell indicates the relative abundance of Atlantic herring for one week of the sampling season. Gray indicates that seine sampling was conducted, but no herring were observed. Cooler colors (purples and blues) indicate that relatively few herring were caught in that week, while warmer colors (yellows) indicate that relatively many herring were caught in that week. This plot shows reduced catches of herring since 2020.
Figure 3. Atlantic herring relative abundance during CBASS seine survey operations, 2014 – 2023. The color of the year on the vertical axis indicates whether that year was in the top five warmest (red) or the top five coldest (blue) years of the time series. Each colored cell indicates the relative abundance of Atlantic herring for one week of the sampling season. Gray indicates that seine sampling was conducted, but no herring were observed. Cooler colors (purples and blues) indicate that relatively few herring were caught in that week, while warmer colors (yellows) indicate that relatively many herring were caught in that week. This plot shows reduced catches of herring since 2020.
Figure 4. Atlantic tomcod relative abundance during CBASS seine survey operations, 2014 – 2023. The colors represent the same things as in the above figure on herring. This plot shows reduced catches of tomcod since 2020.
Figure 4. Atlantic tomcod relative abundance during CBASS seine survey operations, 2014 – 2023. The colors represent the same things as in the above figure on herring. This plot shows reduced catches of tomcod since 2020.

Tomcod and herring prefer significantly colder waters than the average SST temperature. This means they may be actively avoiding warmer waters. Although not shown here, separate analyses of herring growth rates revealed a trend of faster growth and larger fish in colder years.

Warmer temperatures may negatively impact herring in various ways, including changes in growth and habitat use. Both of these changes may lead to poorer recruitment and lower population productivity. Monitoring the population within Casco Bay gives us insight into future fluctuations in herring abundance in the broader region.

We are tracking the recent appearance and increase in abundance of permit, crevalle jack, white mullet, and summer flounder in Casco Bay (Figure 5). These fish are not native to the Gulf of Maine, and their distributions have historically been bounded to the north by Cape Cod.

As the influence of the cold Labrador Current decreases and temperatures rise within the Gulf of Maine, warm-adapted species carried to our region by the Gulf Stream Current have a better chance of surviving milder winters, spawning in the spring, and becoming permanent residents.

Non-native species can outcompete native species for food and resources in a warming environment, so it will be important to monitor their populations.

Figure 5. “Southern” species (white mullet, permit, crevalle jack, and summer flounder) relative abundance during CBASS seine survey operations, 2014 — 2023. The color scheme is the same as the above temporal density plots.
Figure 5. “Southern” species (white mullet, permit, crevalle jack, and summer flounder) relative abundance during CBASS seine survey operations, 2014 — 2023. The color scheme is the same as the above temporal density plots.

On the Water

The summer of 2023 was notable to commercial and recreational fishermen in Casco Bay for a number of reasons. It was the seventh wettest summer recorded in Portland and set the record for warmest average temperature caused by warm overnight lows (Portland Press Herald, 2023). The spring and summer experienced multiple significant rain events and many fishermen blamed poor pelagic fishing on increased freshwater runoff. Brown offshore surface water, originally thought to be caused by that freshwater runoff, was eventually identified as a significant bloom of the dinoflagellate Tripos muelleri (University of New Hampshire, 2023).

The river herring spawning season on the Presumpscot River occurred later than normal in 2023, despite many other alewife runs in the state starting earlier. Water temperature and river flow trigger the timing of these runs. The water temperature needs to be above ~50 °F and the flow has to be low enough that the fish can physically ascend Presumpscot Falls at the head of tide, typically below 1,000 cubic feet per second (cfs).

Alewives began their migration earlier, but the main surge occurred later than usual. We received reports on May 8th of alewives at the Highland Lake dam, three weeks earlier than normal. This was likely encouraged by the anomalously warm Gulf of Maine (GMRI, 2023). However, significant rain on May 20 caused the dam's discharge to nearly double in a 24-hour period (USGS, 2024). This delayed the fish and the main surge of Presumpscot and Highland Lake fish did not pass the falls until late June, almost 10 days later than usual.

Most southern Maine fishermen agreed that, anecdotally, most pogie (Brevoortia tyrannus) schools stayed further offshore this year, with Portland based seiners traveling out of the bay after June to find fish. Striped bass (Morone saxatilis) followed the pogies offshore and large bass became scarce inshore. At the same time, tuna fishermen reported striped bass as bycatch, which are typically rare.

Maine Marine Patrol and NOAA Office of Law Enforcement confirmed observing, and prosecuting, multiple instances of illegal targeting of striped bass in Federal waters by fishermen who would not have had the opportunity to poach in a typical year. Striped bass fishermen and guides in Casco Bay reported inconsistent fishing this season, with low fish numbers low but bigger fish. Many people caught their personal best fish, while at the same time remarking on how hard it was to find fish this year.

In talks with groundfish fishermen, we heard about an increase in American plaice (Hippoglossoides platessoides) closer to shore than in recent memory. This allowed fishing closer to home and led to increased profitability. Additionally, fishermen saw an increase of blue crab sightings, a species normally associated with the middle Atlantic and Chesapeake Bay. Lobstermen are catching them in their traps, alongside evidence of lobster mortality. The increase in blue crab sightings aligns with Gulf of Maine Blue Crab Network research led by Wells National Estuarine Research Reserve (Wells NERR) — research on which GMRI collaborates. Wells NERR have trapped more than 200 blue crabs this year in an effort to monitor the increase in blue crab range.

This is a photo of a blue crab.
A lobsterman gave this blue crab to GMRI after catching it in a lobster trap off Cliff Island in Casco Bay in 2023. It was 195 mm (7.68 inches), which is larger than New York's largest recorded blue crab (7.19 inches).
This is a photo of a young fisherman holding a seahorse.
Seahorse caught by lobsterman Grady Kemp, 25 yards off Two Lights. Sept 2, 2023.
This is a photo of a crevalle jack fish.
Crevalle Jack, (Caranx hippos), caught by GMRI researchers.

During our CBASS seining we captured schools of white mullet (Mugil curema) in both eastern and western Casco Bay. This is the first time we saw this species in enough abundance to form schools. This increase in abundance suggests a species range shift as the Gulf of Maine becomes more suitable to more warm water species.

We will continue to closely monitor white mullet through our CBASS sampling. Sightings of other warm water species include the collection of a seahorse captured off of Two Lights Lighthouse in September 2023, a juvenile summer flounder (fluke), and a couple juvenile crevalle jack. At present, we aren't sure whether these sightings represent catches of transient species, or whether they are indicative of range shifts and the colonization of Casco Bay as a new habitat.

This is a picture of a white mullet.
White mullet, (Mugil curema), caught by GMRI researchers.
This is a summer flounder.
Summer flounder, (Paralichthys dentatus), caught by GMRI researchers.

Conclusion

The Casco Bay marine ecosystem is changing, but we need to continue collecting data to better understand what that means for our region's ecosystem health, blue economy, and our coastal communities. Observational data collected as part of a time series allow us to characterize the accelerated pace of marine ecosystem change, put climate-mediated changes into historical context, distinguish between natural and human-induced changes, identify underlying causal mechanisms, and project changes moving into the future.

We will continue to collect samples from Casco Bay to keep track of the health of our region's marine ecosystem, and to keep you informed. Keep an eye out for our annual report on the status of Casco Bay's ecosystem.

Read More