Chapter 15. Marine Community

Casco Bay’s Marine Community Experiencing Changes Across Several Indicators

Shifts in Many Species’ Abundance, Biomass, Growth, and Migration and Reproduction Timing May Be Linked to Warming Waters

At a Glance

Increases in the abundance and biomass of some anadromous fish might reflect the success of continuing efforts to improve fish passage in Casco Bay rivers.

Southern species, including “Gulf Stream Orphans,” are becoming increasingly common.

Warming waters are affecting growth rates of some fish in addition to shifting seasonal occurrences and community composition.

Since the 2021 Report

One to two sentence summary of any major changes since the last report

Casco Bay’s marine community comprises dozens of species that interact in varied and complex ways. The organisms in this community range from algae, small crustaceans, and juvenile shellfish that live as plankton in the water column, to recreationally and commercially important fish and shellfish species, to megafauna such as seals, whales, and sharks that frequent the Bay.

A diverse and resilient marine community helps maintain the ecological and economic health of Casco Bay, and changes or declines in key species can indicate trouble. By tracking species’ populations, growth rates, and seasonal changes, we can gain insight into the health of the Bay and understand how changing conditions might impact fisheries.

Species often live in different parts of the Bay at different points in their life. Nearshore environments offer high productivity and protection that makes them an important nursery habitat for juveniles of many species, including lobster, groundfish, and herring species. Once adults, these species often migrate offshore, joining the community of organisms that live in the deeper waters of the Bay. Monitoring both of these communities – nearshore and in deeper water – helps us understand the overall health of Casco Bay’s marine community.

How Nearshore Data Was Collected

The Casco Bay Aquatic Systems Survey (CBASS) nearshore monitoring program at the Gulf of Maine Research Institute (GMRI) has been monitoring the nearshore marine community of the Casco Bay since 2014.

The program monitors 12 locations in the western Bay, with an additional six locations in the eastern Bay added in 2023. Multiple times each summer, CBASS uses beach seines to collect fish within swimming distance of shore at each site, then records the species and size of captured fish.

Common Name Scientific Name
Lobster Homarus americanus
Winter Flounder Pseudopleuronectes americanus
Alewife Alosa pseudoharengus
Atlantic Herring Clupea harengus
Silver Hake Merluccius bilinearis
Sand Dab Scophthalmus aquosus
Longhorn Sculpin Myoxocephalus octodecemspinosus
Blueback Herring Alosa aestivalis
Red Hake Urophycis chuss
Rock Crab Cancer irroratus
Crangon Shrimp Crangon septemspinosa
Long-Finned Squid Doryteuthis pealeii
Rainbow Smelt Osmerus mordax
White Hake Urophycis tenuis
Butterfish Peprilus triacanthus
Cunner Tautogolabrus adspersus
Dichelo Shrimp Dichelopandalus leptocerus
Shad Alosa sapidissima
Jonah Crab Cancer borealis
Toad Crab Hyas araneus
Montagui Shrimp Pandalus montagui
Mackerel Scomber scombrus
American Plaice Hippoglossoides platessoides
Atlantic Cod Gadus morhua

Table 1. The 25 marine species most commonly caught in DMR’s trawl surveys of Casco Bay, ordered from most commonly caught to less commonly caught.

DCIM100GOPROGOPR0240.JPG

Nearshore Data Trends Show Increased Presence of Southern Species

In the mid-2010’s, the Gulf of Maine underwent a period of rapid warming due to northward shifts in the Gulf Stream, which carries warm water to the Northeastern US.

In this same time period, CBASS researchers began to find increasing numbers of “southern species” in their surveys. These southern species included species like white mullet (Mugil curema) and summer flounder (Paralichthys dentatus) that typically live south of Cape Cod but may seasonally show up in Casco Bay.

Southern species also included species rarely observed in Casco Bay and more frequently associated with subtropical waters, such as permit (Trachinotus falcatus), crevalle jack (Caranx hippos), and Atlantic moonfish (Selene setapinnis). GMRI believes these subtropical fish to be “Gulf Stream Orphans” that were trapped and transported by the Gulf Stream, but will not be able to survive annually or establish breeding populations in the cooler waters of Casco Bay.

 

Figure 1. The abundance of southern species (white mullet, summer flounder, permit, crevalle jack, and Atlantic moonfish) found in the CBASS beach seine surveys has increased in recent years. The first southern species was recorded in 2015, and southern species abundance peaked in 2023 and 2024, followed by a slight decline in 2025. (Warmer colors represent higher relative abundance. Colors on the y axis indicate yearly relative temperature trends; red represents warmer years in the survey period and blue represents cooler years.)

Atlantic Herring and Atlantic Silversides

In CBASS beach seines, early season catch (June – July) is typically dominated by Atlantic herring, while late season catch (August – October) is dominated by Atlantic silverside. This community shift is likely driven by water temperature.

Atlantic herring are a cold water associated species and Casco Bay is in the southern half of their range. Atlantic silversides can tolerate warmer waters and Casco Bay is in the northern portion of their range.

While this seasonal change in community composition has remained consistent, GMRI researchers have observed changes in the growth rates of Atlantic silversides, with silversides growing significantly faster in years that were warmer than average. Over this same period, the growth rate of Atlantic herring remained unchanged. Changes in the available sizes of fish, or the timing when certain sizes are present, could potentially lead to shifts in the role these species play in the food web.

Figure 2. Atlantic herring relative abundance typically peaks early in the summer, between late June and mid-July.

Figure 3. Atlantic silverside relative abundance typically peaks late in the summer, between late August and mid September.

Figure 4. Atlantic silversides have higher growth rates in years that are warmer than average, while Atlantic herring growth rates are unchanged between warmer and cooler years. (Dots represent individual fish lengths, and lines represent growth rates modeled on weekly length distributions.)

School of circling Alewives herring fish
School of circling Alewives herring fish

How Deep Water Data Were Collected

Deeper water marine species community data come from the Maine-New Hampshire Inshore Trawl Survey, which has been carried out in fall and spring of each year since Fall 2000. The survey is carried out by a partnership of commercial fishermen and state resource scientists. The trawl survey samples areas from 5 to >100m depth and collects near-bottom dwelling species in a non-targeted manner. Data from the inshore trawl surveys are used to inform fisheries management decisions across both states.

Deepwater Data Trends Show an Increase in Alewife Numbers, With Neutral or Declining Numbers in Other Species of Interest

Over the 25 years of the survey, 97 different species have been caught in Casco Bay. Most of these species are found rarely however, and only 25 show up in more than 20% of surveys. Those 25 common species include 10 species of interest to commercial fisheries in Maine and 4 anadromous fish that are of interest in river restoration and fish passage projects (see Aquatic Connectivity chapter). Of these common species of interest, only alewife show signs of positive population trends. All others show declining, neutral, or unclear trends.

The trawl survey samples locations along the entire coast of Maine and New Hampshire, but we restricted our analyses to trawls carried out within Casco Bay. This approach allows us to report on population trends that are directly observable in Casco Bay, but does not put the results in a broader regional context. A recent University of Maine study demonstrates that some species in the Gulf of Maine are shifting across latitudes or depths, so population declines in Casco Bay may indicate either that the total number of individuals of a species is decreasing, or that range of the species is shifting out of Casco Bay.

Species Results of trend analyses
Anadromous and Comm. Fishery Species
Alewife Increasing catch frequency
Anadromous Species
Blueback Herring Inconsistent results, possible increase in catch frequency
Rainbow Smelt Abundance and biomass decreasing
American Shad No trend
Commercial Fishery Species
Crustaceans
American Lobster Decreasing abundance and biomass
Jonah Crab Decreasing catch frequency and abundance
Rock Crab No trend
Pelagic fish
Atlantic Herring No trend
Mackerel Decreasing abundance
Groundfish
American Plaice Decreasing catch frequency and abundance
Atlantic Cod Decreasing catch frequency and biomass
White Hake Decreasing catch frequency, abundance, and biomass
Winter Flounder No trend

Table 2. Most commercial fishery species that are commonly captured in DMR’s trawl surveys are showing signs of decline in Casco Bay. Of the anadromous fish that are commonly captured, Rainbow Smelt are showing signs of decline, but Alewife and Blueback Herring are showing signs of increasing or stable populations. This may indicate populations are recovering in response to ongoing efforts to improve anadromous fish passage in rivers, or could be a response to other environmental or ecological factors.

Plankton are organisms drifting in oceans and seas.
Plankton are organisms drifting in oceans and seas. Zooplankton.

Data Gaps in Some Areas Pose Challenges to Understanding Population Trends

The low seasonal frequency of trawl surveys (two months per year) and the frequency of zeros in the dataset when certain species aren’t found complicate the detection of population trends. Even models of some of the most commonly caught species returned inconsistent results. This may be partially because only data from trawl surveys in Casco Bay were analyzed and working with the entire dataset may provide more clarity. Higher confidence in Casco Bay specific results will require more frequent trawl surveys in Casco Bay.

 

LOOKING AHEAD TO 2030

GMRI’s CBASS program has grown into a highly valuable source of data for nearshore fish communities in Casco Bay. The program is continuing to grow and expand to collect new types of data thanks to the efforts of GMRI and their strong partnerships with other organizations.

There are data gaps in the lower levels of the Casco Bay marine community, specifically around zooplankton, which form the base of the marine food and provide an energetic link between primary producers and higher level consumers. If the zooplankton community is experiencing composition changes, those changes will likely affect higher trophic levels. There have been recent efforts to start a zooplankton monitoring program (see Emerging Data chapter) so future reports may be able to consider the role food web dynamics play in driving species abundances.