Two recent research reports focused on halibut spatial dynamics, habitat occupation, and spawning dynamics suggest that new management considerations of commercial stocks may be warranted.
The first document, published in the Canadian Journal of Fisheries & Aquatic Sciences this past spring, focuses on identifying halibut spawning dynamics, including locating spawning grounds, and identifying the conditions occupied and the timing of occupation on these grounds, notes Austin Flanigan, a fisheries master student at the University of Alaska Fairbanks, and principal author of both papers.
Researchers attached pop-up satellite telemetry tags to large female halibut in the Northern Bering Sea, with time series data and tag reporting locations being used to infer spawning behavior and to identify occupied spawning habitat conditions, location, and timing
The research team found that these halibut occupied spawning habitats later and farther north than previously described. Their spawning habitat was occupied from January to May and reached as far north as the Russian continental shelf. They also observed that 42 percent of mature halibut never occupied presumed spawning habitat, suggesting the presence of skip spawning behavior. Such findings, they said, suggest that Pacific halibut exhibit unique spawning dynamics in the Northern Bering Sea, which may result in reduced reproductive potential within the northern population component.
Flanigan said that understanding reproductive output would require fecundity (number of eggs produced) data, as if skip spawning Pacific halibut have the same fecundity as those that spawn annually, then they would produce fewer offspring.
"However, it is also possible that a skip spawning individual may produce a larger number of eggs when spawning relative to annually spawning individuals," he said. "Due to this, we instead discussed the implications of skip spawning behavior reducing the effective spawning stock biomass within a given year, rather than making assertions regarding the relative fecundity of Pacific halibut within the northern Bering Sea."
Skipped spawning in halibut is a reproductive phenomenon where mature individuals, despite having developed gonads, forego spawning in a given year. This behavior, confirmed in both Pacific and Atlantic halibut, is likely caused by poor nutritional condition but can also be influenced by environmental factors, such as cold temperatures at the edge of their range, which can reduce reproductive potential.
The second study, published in Environmental Biology of Fish in late August, focuses on halibut spatial dynamics and habitat occupation. In this study, researchers "assessed where and when fish moved within the Bering Sea, as well as what conditions (depths and temperatures) they occupied during these movements," Flanigan said.
Spatial dynamics in fisheries refers to the study of how fish populations and fishing effort are distributed and change over time and space, driven by environmental factors, biological processes like migration and recruitment, and human activities such as fishing. Understanding these dynamics helps fisheries managers to identify vulnerable habitats, predict impacts of climate change, and design more effective conservation strategies that align with the biological structure of fish stocks.
The report, led by Flanigan, describes their research as the first evaluation of halibut habitat occupation and movements at the northern extreme of their distribution. These halibut exhibited seasonal migratory behaviors between shallow-inshore and deep-offshore waters, presumably as they moved between foraging and spawning habitats, their report said. However, these halibut were seen overwintering in colder and more northerly waters than previously identified for this species.
Researchers also observed that all tagged halibut seasonally emigrated from the Northern Bering Sea continental shelf, likely due to sub-freezing winter temperatures that make the region inhospitable.
This suggests that the timing of migratory movements and extent of halibut occupation within the Bering Sea are likely variable across years and may be limited by the spatial and temporal extent of the cold pool, they said.
Their report identified management implications, as migratory contingent behavior and cross-boundary movements during the commercial fishing season suggest that halibut display more complex spatial dynamics than currently assumed. Such findings may warrant additional consideration, as not accounting for contingent structure in stock assessment and harvest policy may lead to a loss in contingent diversity, potentially degrading the robustness of the population against environmental variability and harvest pressure, they said.
Still many questions remain unanswered because their findings are limited to the movement dynamics of a select number of large female halibut from the northernmost extent of the Bering Sea sub-population.
The dynamics of the Russian halibut fleet must also be assessed before its impacts on the northern Bering Sea halibut contingent can be understood, they said.
