Satellite Tags Reveal Red King Crab Migrations Are Linked to Environmental Conditions

The Bristol Bay red king crab fishery was once one of the largest and most valuable crab fisheries in the world, but it collapsed in the early 1980s. Managers established closure areas to help the population rebound, but the boundaries of these closure areas are fixed and cannot adapt to changing environmental conditions and shifting crab distributions. Previous research has documented northward movements of the Bristol Bay red king crab population. Changes in their seasonal migration patterns may limit the effectiveness of static closure areas like the Red King Crab Savings Area .

In response to this pressing management need, a new satellite tagging study tracked and modeled movements of mature male red king crab in Bristol Bay from late spring through the fall during 2021-2024. The research team's "movement models" predicted that red king crab generally moved to the west or southwest by the fall-into deeper and cooler areas in central Bristol Bay. In years with warmer bottom water temperatures, crab moved further west. The model's predictions were validated by commercial catches from the fall Bristol Bay red king crab fishery and researchers are currently applying this approach to other seasons.

Future applications in the winter and spring may provide important insights about movements during mating and molting. These are the times of the year when the crab are more vulnerable. This information will help fishery managers evaluate current closure area boundaries and inform future boundary changes that may be needed to better align with present and projected seasonal migration patterns.

Filling in the Data Gaps

NOAA Fisheries conducts annual bottom trawl surveys in the eastern Bering Sea. These fisheries-independent surveys provide foundational data for stock assessments for many commercially important species. We conduct surveys from approximately late May to early August and sample from east to west. This means Bristol Bay is sampled in late May through June, which provides good insight into red king crab distributions in late spring.

The commercial fishery for Bristol Bay red king crab typically operates from October 15 to January 15, with the majority of the catch taken in the first month. Data from fisheries observers, logbooks, and fish landings provide fisheries-dependent data. However, there's a data gap in Bristol Bay from late June through mid-October.

This study sought to understand crab movement during this time period and what habitat characteristics are associated with seasonal movements. As part of the 2021-2024 eastern Bering Sea bottom trawl surveys, scientists affixed pop-up satellite archival tags to mature male red king crab. Tagging took place in late May through mid June, and scientists released the crabs immediately after tagging.

These tags cannot transmit real-time information like tags deployed on whales and seals, which spend lots of time at the surface. Instead, these specialized tags are programmed to release, or pop up, on a specific date. For this study, that was between October 11 and 15. Once at the surface, they transmitted their data via the satellite network to the collaborative team from:

• NOAA's Alaska Fisheries Science Center
• Alaska Department of Fish and Game
• University of Alaska Fairbanks College of Fisheries and Ocean Sciences
• Bering Sea Fisheries Research Foundation

Where Do King Crab Go in Late Summer and Fall?

You may or may not realize it, but we frequently-if not always-carry a form of satellite tags around in our pockets and purses. Our mobile phones can pinpoint our location by constantly registering with satellites through the Global Positioning System . So, how does this work for tagged crabs scurrying around on the seafloor with no real-time satellite communication?

Short answer: It doesn't. The satellite tags only provide the start and end location, along with daily temperatures. From that information, modelers can develop spatial movement models that incorporate habitat preferences-the environmental conditions like temperature and depth that crab seek out. This novel approach offers new insights to better explain why crab move throughout Bristol Bay at specific times of the year. And that's how they concluded that king crab are, in general, moving to the southwest and further offshore from June to October.

The shading in the graphs above indicate preferred habitat for red king crab in October. The darker green corresponds to more ideal habitat, while darker red corresponds to less ideal habitat. The darker green areas exhibited cooler bottom temperatures along with varying combinations of depth and oceanographic conditions. These included shallow habitats with slower tidal currents and deeper habitats with faster currents.

In years with warmer bottom temperatures, models predicted crab would move farther west. "And after the red king crab fishing season, when we compared our model projections with actual catch data from the fishery, they generally aligned," said Sean Hardison, lead author, former NOAA affiliate, and Research Associate at the University of Alaska Fairbanks.

Just as the models predicted that the centers of projected crab distributions in the fall would move west in years with warmer bottom temperatures, so did the commercial fishing fleet. Because fishermen follow the actual presence of crab, this fisheries-dependent data can be used to evaluate the models' accuracy. The alignment between model predictions and fishery catches is promising. It has given Hardison and the team confidence that this tagging and modeling approach can be applied to other seasons, and to female red king crab.

Significance of the New Movement Model

"Satellite tagging has been ongoing in Bristol Bay, but the tagging data haven't really seen a formal evaluation such as the one we've presented here," continued Hardison. "Our unique approach combines movement models developed from the satellite tagging data with bottom trawl survey data, allowing us to project where crab would migrate in mid-October when the commercial fishery opens."

"By showing that fall red king crab distributions shift based on bottom temperatures in Bristol Bay, our findings suggest that dynamic, rather than static, spatial closure boundaries may be more effective for red king crab," added Erin Fedewa, co-author and fishery biologist at NOAA Fisheries. "Our modeling framework provides a tool to inform these spatial management measures, which is a crucial step forward in managing fisheries under changing ocean conditions."

Recently, tagging efforts have been expanded to include both males and females in winter and spring, and the research team is currently developing similar movement models to unravel migration patterns during those times of the year.

Bolstering the Red King Crab Population

Fisheries scientists typically have a limited window to understand animal distributions, especially in cold, deep, and remote environments like red king crab habitat in Bristol Bay. This study offers new insight into how we can combine multiple types of data to paint a better picture of red king crab distributions and migrations across multiple seasons.

Hardison concludes, "While our results don't prescribe new closure boundaries, they show how crab movement, survey, and environmental data can be integrated to identify critical red king crab habitat and evaluate the potential effectiveness of proposed closure area adjustments. And this may help us find more effective ways to reduce bycatch, protect essential fish habitat, and bolster the Bristol Bay red king crab population following more than a decade of poor recruitment."

Franz Mueter, co-author and professor of fisheries at the University of Alaska Fairbanks, offered more. "A more comprehensive understanding of crab movements could allow other fisheries to incorporate targeted dynamic closures to minimize bycatch."

The Bristol Bay red king crab fishery has suffered from two multi-year closures-from 1994 to 1995 and again for the 2021/22 to 2022/23 seasons. With warmer years and marine heatwaves becoming more common in recent decades, environmental conditions are anticipated to have significant impacts on red king crab distribution in future years. The team's goal is to understand these changes through continued research, and provide fishery managers with the tools needed for responsive, adaptive management.