Snow Crab Collapse Linked to Energetic Limitations During a Marine Heatwave

Snow crabs sampled during the 2024 eastern Bering Sea bottom trawl survey. Credit: NOAA Fisheries / Erin Fedewa

A new study addresses one of the largest mass mortality events in recent marine history-the abrupt collapse of the snow crab populationin the eastern Bering Sea. The research team was led by NOAA Alaska Fisheries Science Center fishery biologist Erin Fedewa. They set out to identify potential factors driving the increased mortality, population collapse, and recovery potential of Bering Sea snow crab.

Erin and her team measured snow crab energy reserves during and after the 2018-2019 marine heatwave and population collapse. This approach is the first of its kind for Bering Sea crab stocks. Findings show that warmer conditions and high population density led to energetic limitations-likely causing the snow crab population collapse.

NOAA scientist Erin Fedewa collects snow crab hepatopancreas samples during the 2023 eastern Bering Sea bottom trawl survey to monitor energetic reserves of juvenile snow crab. Credit: NOAA Fisheries / Emily Markowitz

A New, Rapid Energetic Condition Metric for Fisheries Management

Poor energetic condition is a state in which crabs have low energy reserves. This can increase starvation and mortality risk, emphasizing the importance of monitoring the energetic condition of individuals in marine populations. Techniques used to monitor energy reserves in fish often rely on data collected rapidly during annual scientific surveys, such as the weight-at-size of individual fish.

However, snow crabs have exoskeletons; they molt as juveniles, which uses a lot of

their energy stores. They require more sensitive techniques to detect energy reserves beyond simple length-weight measurements. These techniques are often time-intensive and costly. Scientists have not routinely monitored energetic condition in Bering Sea crab populations to date.

To address this critical need, the team looked for other options. In 2019, they began collecting hepatopancreas samples from juvenile snow crabs collected during the annual Bering Sea trawl surveys. The hepatopancreas is a unique organ in some marine animals-including crabs, lobsters, shrimp, and squid-that combines functions of the liver and pancreas. It's the primary energy storage organ in crustaceans and it can be isolated and weighed.

"This effort allowed us to successfully validate an indirect and rapid method for monitoring snow crab energetic condition. We demonstrated a strong positive relationship between the easy-to-measure hepatopancreas percent dry weight and total fatty acid concentration, which is more complex to measure," said NOAA scientist and co-author Louise Copeman.

Scientists collect snow crab hepatopancreas samples to monitor energetic reserves of juvenile snow crab during the 2023 eastern Bering Sea bottom trawl survey. Credit: NOAA Fisheries / Emily Markowitz

This practical energetic condition metric offers an efficient, early-detection system to track snow crab energetic limitations and mortality risk. These measures are crucial for management decisions and forecasting future collapses. "Snow crab energetic condition estimates generated from this study are communicated to stakeholders and fishery managers in the Snow Crab Ecosystem and Socioeconomic Profile," added Fedewa, "which provides a near-real time measure of stock health to inform snow crab management decisions."

Warming and High Population Density Created a "Perfect Storm"

Most notably, the study found that during the snow crab collapse, decreases in abundance were associated with dramatic declines in the energetic condition of juveniles. Fedewa continues, "It's always a concern when you see impacts to juveniles. They represent the future of the population and the fishery. In this case, their energetic deficiencies were attributed to high population density-which likely created too much competition for food-and the warm conditions brought on by the marine heatwave."

Snow crab have historically been associated with the cold pool in the Bering Sea (cold bottom waters <2°C). Authors of the study were surprised to find that much colder temperatures below 0°C appear necessary for promoting elevated energy reserves and high survival in snow crab. "Our results underscore the consequences associated with elevated temperatures and high population density and point towards the critical importance of cold-water habitat for snow crab recovery in the eastern Bering Sea," said Mike Litzow, the director of the Shellfish Assessment Program in Kodiak and a co-author on the study.

Energetic Rebound Suggests Snow Crab Population Recovery

In a promising sign for the future of the stock, the study shows that energetic condition in the eastern Bering Sea snow crab population rebounded quickly following the marine heatwave. This increase in energy reserves coincided with strong recruitment and increasing population abundance from 2021 to 2024. "Support for initial population recovery is very encouraging," said Fedewa, "but strong recruitment in the near-future is critically dependent on conditions that promote high energetic reserves and survival of juvenile snow crab. The population is still vulnerable to another marine heatwave."

Scientists collect additional data on individual snow crab sampled for hepatopancreas, including weight, size, and maturity status. Credit: NOAA Fisheries / Emily Markowitz

The new research is an important first step in improving our understanding of snow crab collapse and recovery dynamics. The authors of the study emphasize the importance of continued energetic-based sampling in the Bering Sea. Copeman says she's particularly excited about the next steps, noting that "food deprivation experiments conducted at the science center's Newport Lab were used to verify observations from the surveys to determine critical energetic thresholds." Fedewa adds "There's a lot of really exciting snow crab research right now. The 2018-2019 snow crab population collapse was a complete surprise, but I'd like to hope that we're better prepared if it happens again."

This research was funded by the North Pacific Research Board.