New research tracks Pacific halibut in northern Bering Sea
Recent research showed that Pacific halibut in the northern Bering Sea spend their winters in spawning grounds along the continental shelf but likely only spawn every other year.
Austin Flanigan, a doctoral student at the University of Alaska Fairbanks, gave a Strait Science presentation last week to discuss his project to understand the movements and reproductive habits of Pacific halibut in the region.
“In this region we really have a limited understanding of spatial dynamics and spawning behavior,” Flanigan said.
Knowing where halibut travel could inform fisheries management decisions about quota, allocation and harvest timing and pressure, he explained. And though spawning metrics have often been treated as constant across the range of a fish, recent studies have indicated that these can vary across different latitudes.
“What we’ve seen is that at northern extremes, we often see a reduction in overall reproductive potential,” Flanigan said.
For example, fish in the northern end of a species’ range might show reduced spawning frequency, reduced fecundity and a shortened spawning season.
“This being critical information for management, we wanted to go ahead and address this and really inform both management and local stakeholders on these aspects of Pacific halibut within this region,” Flanigan said.
The work by Flanigan and his colleagues was funded by the Norton Sound Economic Development Corporation, or NSEDC. With the help of local fishermen, they captured and tagged halibut in waters near two northern Bering Sea locations from 2019 to 2022.
The International Pacific Halibut Commission treats the species like a singular coast-wide stock, but they divide up the region into distinct regulatory areas. Region 4 covers the Bering Sea. The first location for the halibut-tagging was around Nome, falling within regulatory area 4E. The second was near Savoonga, on St. Lawrence Island, which falls within regulatory area 4D.
“When we were tagging, we were seeking large fish,” Flanigan said. “We targeted fish specifically over one 100 centimeters in length. This gave did two things. It gave us a high probability that that was both a mature fish and that it was a female, due to the sexual dimorphism that’s seen within the species. Over the course of our four years, we managed to tag 84 individuals.”
Those tags recorded depth, temperature and light intensity data while attached to the fish, and then at a pre-scheduled date, the tags would pop up and float to the surface and transmit that data.
The researchers found that some subpopulations of halibut in the northern Bering Sea made extensive migrations out to the continental shelf edge each year to spend their winters in warmer waters. The furthest recorded journey was about 1,000 kilometers, or 620 miles. Many of those individuals were also moving into Russian waters. Though the halibut spread out over a large swathe of the shelf edge in the winter, many exhibited homing behavior to their foraging grounds, returning to back to areas where they were originally tagged. They also didn’t leave region and travel to areas like the Gulf of Alaska.
Pacific halibut in the northern Bering Sea additionally appear to be biennial spawners, according to the new data.
“That’s the first we’ve seen that kind of behavior with Pacific halibut,” Flanigan said. “Skip spawning has been theorized before, but not at this magnitude.”
The researchers were able to guess whether a fish was spawning by checking on its depth.
“Two-hundred meters has been used as a threshold for which fish cannot spawn above, so for any fish that didn’t descend below 200 meters, we assumed that it was a skip-spawner based on the fact that it wasn’t occupying suitable spawning habitat,” Flanigan said.
Flanigan explained that of those 84 tagged fish, 61 tags that gave him data, and 28 had a really good amount of time series data. Of those 28, he saw that 46 percent had gone down to spawning depths while 54 percent skipped spawning.
The researchers had another motivation in conducting this study: Learning about whether halibut might alter their habitat as climate change increases ocean temperatures and alters ecosystems. The results of the study suggest that halibut could withstand temperatures they would find further north, in the Chukchi Sea, but for whatever reason, they are not migrating into that area.
“Pacific halibut are capable of occupying the thermal conditions available within the Chukchi Sea, but they don’t appear to be doing so at this time,” Flanigan said. “That’s probably a result of limited forging opportunities or simply migratory distance to reach those grounds. It’s possible that within the southern Chukchi Sea, there’s just not enough there to merit them to move that distance.”