Evaluating spatial management strategies to optimize sustainability of transboundary sablefish
by NOAA Fisheries 18 Jun 20:58 UTC
Sablefish near the seafloor © NOAA Fisheries
Sablefish don't recognize political boundaries. These highly migratory fish move freely across the state and national lines within which they are managed.
New research suggests that accounting for the biological spatial boundaries that do matter to sablefish could lead to more sustainable fisheries in some areas.
Scientists and fishermen from Alaska, British Columbia, and the U.S. West Coast collaborated to develop a management strategy evaluation for northeast Pacific sablefish. The goal was to investigate how different spatial management approaches might affect economic and conservation outcomes for this valuable transboundary species. The team used innovative modeling simulations based on 60 years of fishery survey data to project possible harvest outcomes of seven different spatial management strategies through 2040. The findings will help managers balance economic and conservation tradeoffs to identify the most effective strategy to optimize sustainability of the sablefish population.
"This research answers key questions about how northeast Pacific sablefish biological spatial boundaries might affect management outcomes. We identified a promising, simple way to evaluate the impacts of spatial population structure on management," said study lead Maia Kapur, NOAA Fisheries Alaska Fisheries Science Center/University of Washington. "In the bigger picture, this is one of the first transboundary management strategy evaluations based on real data and including stakeholder input. It sets the gold standard for how transboundary Management Strategy Evaluations could be done around the world."
Sablefish: a highly valuable, highly mobile transboundary species
Sablefish has traditionally been one of the highest valued finfish in Alaska and Pacific Northwest commercial fisheries. It is currently managed separately off the coasts of Alaska, British Columbia, and the U.S. West Coast. Each region uses a different assessment and management framework.
However, recent work shows that sablefish mix across their northeastern Pacific range. A single genetic population spans the three management jurisdictions. Tagging studies confirm that these highly mobile fish readily traverse the 2,000 miles from southern California to Southeast Alaska within 1 or 2 years. Since sablefish can live for more than 90 years, they have plenty of time to make the trip.
Furthermore, stock assessments conducted separately for each region have shown synchronous trends in sablefish populations. These include a large-scale decline from 2010 to 2018, followed by increased production and large year classes in recent years.
"Researchers and fishermen in each region saw long-term declines in sablefish numbers, and saw that the declines seemed to match in magnitude and timing across the entire northeast Pacific. We were concerned that sablefish spatial dynamics might be posing risks for our management system," Kapur said. "We undertook this research to answer the question: Could spatial management approaches improve sustainability of transboundary northeast Pacific sablefish?"
Management strategy evaluation helps identify effective spatial management approaches
Understanding the spatial structure of a fish population is crucial for effective management of sustainable fisheries. Research has shown that management units should be scaled to match biological population units. Management strategies that account for fish population spatial structure have been found to perform better in meeting conservation and economic goals. If a population segment is isolated, but not managed separately, it may have a higher risk of depletion.
Transboundary stocks like Pacific sablefish that cross international or other boundaries present an additional challenge to spatial management approaches because they require political cooperation.
Transitioning to spatially-structured management is an immense, potentially costly undertaking for any fishery. Management strategy evaluation helps managers assess these tradeoffs before committing to a new management strategy. This computer modeling approach simulates future outcomes of different management actions. It can help managers identify which strategies are likely to perform best before making a formal change.
Developing a management strategy evaluation for Northeast Pacific sablefish
In 2018, scientists from the U.S. and Canada began development of a transboundary management strategy evaluation for sablefish. The goal was to investigate whether spatially-structured management might result in better conservation and economic outcomes for northeastern Pacific sablefish.
"Through computer simulations, we created a simulated sablefish population with a particular biological spatial structure," said Kapur. "Then we looked at how management performed if it did or didn't consider that spatial structure."
This management strategy evaluation was one of the first in the world based on fishery data—from 1960 to 2019.
"The power of the simulations was driven by the breadth and depth of data available from decades of fisheries surveys," said Kapur.
The model evaluated seven management scenarios representing a spectrum of population spatial complexity. These ranged from complex biologically based structure, to the status quo determined by political boundaries, to no structure—a single population across regions.
Two types of metrics were used to quantify the performance of each management strategy: biological and economic objectives. These were identified during stakeholder workshops.
"Working together with fishermen allowed us to look at tradeoffs in performance objectives that fishermen care about," Kapur said.
Performance objectives identified in these workshops included:
- Minimize risk of stock being overfished
- Avoid depleted populations
- Maintain minimum catch level
- Minimize annual catch variability
- Maximize catch in the near- and long-term
- Maximize long term profitability
The collection and analysis of multiple, multidisciplinary datasets was possible due to the cooperation of a dedicated team.
"We had a committed international, interdisciplinary team who knew their data inside and out. I am especially grateful to the fishermen whose participation made these results realistic, not just a desktop analysis," Kapur said. "We built a collaboration and partnerships that will continue to work together."
Impacts of spatial structure on management outcomes vary by region
This study found that spatial models of intermediate structural complexity can satisfy stakeholder objectives and avoid negative outcomes for the sablefish fishery. That includes models resembling the current management approach.
"The good news is, we found no serious red flags under the current management paradigm. Even if we continue under a 'business as usual' scenario, we did not see more than a 10 percent risk of the population falling below target biomass levels in any region," Kapur said.
However, differences among spatial management strategies differed among regions. Differences in economic and biological outcomes among management strategies were relatively small in Alaska and British Columbia. The impact of ignoring spatial structure was most striking in the U.S. West Coast region.
"We found that spatial mismatches of management and biological units pose a crucial risk of localized depletion in the southern U.S. West Coast," Kapur said. "Our results also suggest that the whole system is likely protected by the size of the Alaska stock. Sablefish spawned in Alaska make their way down south and replenish the local populations in British Columbia and the U.S. West Coast."
Other findings from the study included:
- A strategy where the three management regions are treated as separate, unconnected stocks—the closest to the current approach—was not preferred for any region, but did not result in catastrophic outcomes for any performance metric
- Management strategies that include movement between regions performed better for the U.S. West Coast and Alaska, with tradeoffs in catch variability
- Alaska may obtain better catch outcomes under management strategies that consider mixing between areas, but the difference in near-term catch across strategies is less than 10 percent
"These results are phase one of this work," Kapur said. "We already have projects under way that use the tools we've built and extend these results to incorporate socioeconomic information and linkages to climate."
The methods developed and the results of the research have both regional and global relevance.
"Findings from this study will benefit everyone involved with the sablefish fishery in the northeast Pacific—assuring that the full range of the stock is considered in management," Kapur said. "And the tools we've developed can provide a model for anyone who works with transboundary stocks around the world."