A study suggests that warming ocean waters are changing the number of fish

The following is courtesy of NOAA Fisheries Alaska:

Several haddock swimming in open blue waterAlaska pollock. Credit: NOAA Fisheries.

As the ocean warms, marine fish are moving beyond their traditional habitats and across international borders. Understanding these movement patterns is essential to predict change and manage climate resilient fisheries.

A new collaborative study from NOAA Fisheries analyzes the movement patterns of multiple species of fish across the entire Bering Sea shelf over decades. Scientists from the Alaska Fisheries Science Center collaborated with Russian scientists to combine data from the eastern, western, and northern Bering Sea shelf. A groundbreaking analysis distilled dominant patterns of fish movement over time from these data. The research advances our understanding of how the ecosystem is responding to climate change.

“International collaboration is likely to become increasingly important to the sustainable management of Bering Sea fisheries,” said study leader Lukas DeFilippo of NOAA Fisheries Alaska’s Fisheries Science Center. “As fish move into new habitats, we need to take a larger-scale approach to monitoring to support sustainable fisheries management.”

Map of the study area in the Bering Sea
Map of the Bering Sea showing the spatial extent of all survey data analyzed in the study. The shallow area (less than 200 meters deep) shown in light blue is the shelf.

Fish moving across international borders

The Bering Sea is home to some of the largest fisheries in the world. As such, it has been relatively well studied. However, most of the research has focused on the US (Southeast, Northeast) or Russian (West) shelf areas separately. The objective has been to understand the fish populations in the waters of each country.

But in recent decades, unprecedented warming and loss of sea ice have caused drastic changes in the distribution of fish. Scientists have observed large-scale shifts to the north and increased movements between the east and west of the Bering Sea.

“We need to understand movement patterns at the scale of the entire platform so we can tailor our monitoring and management,” DeFilippo said. “The Western data that the Russian scientists shared with us is key to characterizing those patterns.”

Distilling the dominant patterns

The team set out to identify shelf-wide movement patterns of 10 fish species over more than three decades. To achieve this goal, they worked with scientists from the University of Washington, using an innovative method to analyze data from international surveys.

“Empirical analysis of orthogonal functions (EOF) can extract the dominant signals from measurements in space and time,” DeFilippo explained.

EOF is commonly used in physical oceanography. But only recently has it been applied to biological data.

The survey data covered the years 1982-2018. Data for the eastern and northern Bering Seas were collected from Alaska Fisheries Science Center bottom trawl surveys. The western data was collected by the Pacific branch of the Russian Federal Research Institute of Fisheries and Oceanography.

Scientists collecting data during survey
Scientists collect data during an Alaska Fisheries Science Center survey. Credit: NOAA Fisheries/Mabel Baldwin-Schaeffer.

The set of groundfish studied included:

Several Pacific cod swim among smaller rockfish in open water above plants and substrate.
Pacific cod with northern rockfish in Alaskan waters. Credit: NOAA Fisheries.

Cold pool variations and a long-term northward slip

The results of the study corroborated a well-established pattern observed in previous research in the eastern Bering Sea. But they also revealed another unexpected sign.

The expected pattern (Factor 2) showed movements correlated with the extent of the cold pool. The cold pool is a body of water near the bottom with a temperature of 2°C or less. It has previously been shown to be a key influence on species distribution and ecosystem structure in the Bering Sea. This signal varied each year or during stanzas of years.

However, the dominant pattern (Factor 1) showed movements that were independent of cold pool variability.

“We found a consistent northward trend until around 2011,” DeFilippo said.

Graph showing gradual movement of groundfish populations northward over decades
Top panel: The dominant signal was a gradual, long-term northward movement over decades. Bottom panel: Movement in relation to the extent of the cold pool (blue lines) varied over the stanzas of the years.

This pattern emerged when all the species were combined; it does not necessarily mean that all of them are moving north. The trend was strongest for Pacific cod, Alaska pollock and Alaska plaice. Northern rock sole, Pacific halibut, flathead sole and Greenland turbot were affected to a lesser extent.

The cause of the northward shift remains unknown. “We can speculate that it is the climatic effects that are not captured by the changes in the cold pool. It could be physiology, prey availability, competition, fishing pressure, or predation,” DeFilippo said.

Understanding future change to build climate resilience

DeFilippo points to the importance of expanded monitoring to provide the information we need to adapt to climate change in the future.

As populations of fish such as pollock and cod converge into the north from the east and west, continued monitoring through the Northern Bering Sea Study will be crucial. Some fish may also be moving deeper in response to warming. “Without that knowledge, we are missing part of the picture,” DeFilippo said.

Oscar Dyson Survey Vessel
The NOAA research vessel Oscar Dyson in Alaskan waters. Credit: NOAA Fisheries/Kim Rand.

And international cooperation will be essential to monitor and manage fish stocks in the Bering Sea and beyond.

“All of our data shows that species are on the move, and they may not always be moving in a way that can be predicted using only data from US waters,” DeFilippo said. “Marine animals cross international borders and are likely to do so even more in the future. Tools for combining international data will be key to successfully monitoring and predicting change in transboundary ecosystems around the world.”