Modeling and mapping research project tracks and predicts climate change’s influence on cold and warm water-adapted species in Long Island Sound

By Juanita Asapokhai

Tides of Change

Like an aquatic hotel, at any point in the year, Long Island Sound plays host to a diverse array of species, including over 170 different species of fish and invertebrates. Changes in the species, location, and distribution of fish residing within the Sound shift with the seasons and temperatures. In fact, many fish in the Sound have evolved to survive in specific temperature ranges, and migrate to other habitats when the seasons cycle. Cold-adapted fish, like winter flounder, skates, and hake, which dwell and feed at the bottom of water bodies, swim away from the Sound and toward colder waters when the summer and fall months hit. On the other hand, warm-adapted fish, among them black sea bass and summer flounder, continue to thrive as the temperatures rise. As global warming drives temperatures in the Sound upwards, the ability of warm-adapted fish to continue living in Long Island Sound when the tides turn towards warmer weather gives them a critical advantage over their cold-adapted counterparts. Learning about how the Sound has changed over time and the impact these changes have on the distribution and survival of cold-adapted fish is a crucial step in developing management strategies to support and protect these creatures in their changing habitat.

As part of the research to inform these strategies several students, including Robyn Linner, a PhD candidate at Stony Brook University, is working with Dr. Yong Chen, Professor of Marine Sciences, and a bi-state team of marine scientists and resource managers in developing habitat suitability indices (HSIs), which will analyze a variety of environmental factors to determine ideal habitat conditions for various  cold- and warm-adapted species  in the Sound. The team has already completed an initial literature review for the project, assembling research about the environmental needs and temperature thresholds for several cold and warm water-adapted species of Long Island Sound over time. The team is using long-term datasets collected by the Connecticut Department of Energy and Environmental Protection and the New York State Department of Environmental Conservation to identify environmental-related changes to cold- and warm-adapted fish distribution that have occurred over time and map the area and structure of the fishes’ Sound habitat. The goal is that this data will provide valuable information that reinforces or potentially redirects current Sound ecology monitoring programs and species conservation efforts.

animinated map with different time periods from 1985-2017 map that shows fall median bottom water temperatures (in degrees Farhenheit) increasing over time in the Long Island Sound. These maps were created using data from the Finite Volume Community Ocean Model (FVCOM).
Fall median bottom water temperatures (in degrees Fahrenheit) are increasing over time in the Long Island Sound. These animated maps were created using data from the Finite Volume Community Ocean Model (FVCOM).

 Assessing the Sound: The Habitat Suitability Index

The relationship between a species and its habitat is essential to its survival. A fish’s habitat is characterized by many variables, from salinity to sediment type, to temperature and levels of dissolved oxygen, to food-chain dynamics between predators and prey. Changes to these conditions make or break a species’ ability to remain and thrive in a habitat.

A habitat suitability index (HSI) will factor in all of these variables over a specific period of time to calculate a number that represents the appropriateness of the habitat, on a scale of 0 (least suitable) to 1 (optimal). The HSIs can then be used to identify key areas within the Sound that are best suited to the environmental needs of different cold- or warm-water-adapted species.

“We run the analysis and try to understand what the ideal ranges are for each variable such as salinity,  sediment, etc.,” Linner explained. “And then we create  maps that have information about the [conditions] in that location.”

The HSIs can be used to identify key areas within the Sound that are best suited to the environmental needs of different cold- or warm-water-adapted species. “Since we’re looking at habitat through time, we can see how their distribution has changed, but also how we might expect them to change in the future under different climate scenarios,” Linner said. “So, it’s a really good visual way to identify some of these habitats and predicted changes.”

Robyn Linner poses with an Atlantic cod, a cold water-adapted species, while participating in the fisheries survey that she ran for the first 4 years of her PhD program in the Gulf of Maine. Photo courtesy of Linner.
Robyn Linner poses with an Atlantic cod, a cold water-adapted species, while participating in the fisheries survey that she ran for the first 4 years of her PhD program in the Gulf of Maine. Photo courtesy of Linner.

Alongside Chen, Linner, who has worked in Chen’s fisheries ecology lab for five  years, and the research team are studying eight cold-adapted fish, and seven warm-adapted fish species found in the Sound. The cold-adapted species include windowpane flounder, little skate, fourspot flounder, winter flounder, spotted hake, silver hake, and red hake. The warm-adapted fish species selected included scup, butterfish, summer flounder, black sea bass, striped sea robin, and northern sea robin. The team is also studying the biology and environmental needs of two crustacean species, the cold-adapted American lobster and warm-adapted blue crab. The fish and crustacean species examined in the study were selected by the team because of their strong temperature preferences, and the significant implications that climate change and changing temperatures have on the abundance and health of the species.

Many cold-adapted fishes migrate towards colder water offshore or to the north, including the Gulf of Maine, when temperatures rise in the summer and stay warm in the fall. But for the fish that don’t and remain in the Sound and are forced to tolerate the suboptimal warmer temperatures, they may face reduced movement activity, feeding, and decline in weight, which may have negative effects on the reproduction and egg count of cold-adapted fish species.

On the other hand, warm-adapted species have the opportunity to expand their range in Long Island Sound as they continue to fare in the warmer temperatures. Species like the butterfish, black sea bass, and blue crab are experiencing increases in abundance that were not previously observed in the Sound due to their ability to spread out into formerly uninhabited waters in the Sound. Their expansion has several consequences. The Sound acts as a supportive nursery for several species to spawn their young and for juvenile species to develop before maturation, safe from access by predators. Voracious predator species like black sea bass and others consume young fish and may threaten the nursery function of the Sound, Linner said.

Cold-adapted fish displaced from the Sound may run into difficulty finding a habitat that has the unique conditions present in Sound. Further, as warmer temperatures enable warm-adapted fish to remain in the Sound for longer periods of time and extend their range, cold-adapted species, like the winter flounder, find themselves in greater competition with warm-adapted species . “Moving into these new areas that allow them [warm-adapted fish] to thrive can have really unpredictable consequences,” said Linner. “We aren’t used to seeing this combination of species in the past.” The impact that changing fish distribution has had on fishery economies and the livelihood of commercial fishers is also significant, demonstrated dramatically in the case of the American lobster, whose major decline toppled over a booming lobstering industry.

Mapping the Sound

Linner and the research team are close to completing the analysis and development of  maps that show changes in fish distribution over time, using spatial indicators like center of gravity and evenness to track the species’ movements. “Center of gravity” indicates where in the water a species tends to congregate, and can help identify changes in the east-west and north-south distribution of a population within the Sound. “Evenness” refers to how clustered or aggregated a species is, which reflects whether a species is clumping together or spreading out. Whether or not a species is aggregating  can affect the catchability of a species, and can make estimating their abundance difficult.

“These (spatio-temporal maps) help to get just a quick visual snapshot of some of these changes through time, before we even run the habitat suitability models, to get a sense of which species have experienced the most change,” Linner said.

Much of the information about abundance over time used in this study was collected through the Connecticut Department of Energy and Environmental Protection’s (CT DEEP) Trawl Survey. While aboard a research vessel, researchers survey the entire Long Island Sound in the fall and spring, recording data about fish length, weight, and abundance for various species. Linner and the research team are working with data collected through the CT Deep Trawl Survey between 1984, when the program started, and 2021.

The HSI  maps will support another objective of the research project: creating hindcasting and forecasting models to form a complete picture of the changes that cold- and warm-adapted fish in the Long Island Sound have experienced over time, and what changes can be expected to happen in the future in a variety of different climate scenarios (e.g. changes in salinity conditions, temperature conditions, etc.). The forecasted climate scenarios combine information about climate changes that have already occurred with modeled estimates about the rates of change in the future. By looking at different climate scenarios, Linner said, “You can evaluate different levels of severity. So, if we end up making some changes (to our carbon footprint), we can look at how that might impact these species, instead of only the worst-case scenario.”

Protecting the Sound’s Biodiversity in a Changing Climate

The results of the research team’s work will provide valuable guidance for ecosystem management and monitoring programs in the Sound. It can help identify critical species that may be most negatively or positively impacted by predicted changes in climate, and call attention to key areas where cold-adapted species can go to find thermal refuge. Linner hopes that the research can be used to inform necessary spatial planning policy, or direct monitoring towards sentinel species that will signal if more urgent climate-related changes are taking place within the Sound.

Interventions like these “might help mitigate how quickly some of these changes occur, which may allow some of these species to grow their populations to a point that allows them to be a little more resilient,” Linner said.

“It’s really about trying to highlight specific species, specific areas, and specific issues that we think that we might be able to make some changes to, to help these species deal with the changing temperatures.” 

Juanita Asapokhai was a Communications Intern for the Long Island Sound Study in summer 2023. She attends Tufts University and will be graduating with degrees in Community Health and Sociology in the spring of 2024. 

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