The science of sustainable seafood, explained

What kind of MPAs are most effective to reduce bycatch?

Bycatch, or unwanted catch often discarded at sea, is unproductive in the worst way. Fishermen pull up nets and sort through animals to keep or those to throw back, sometimes alive but frequently dead or injured. Biodiversity is harmed and fishermen waste time and resources.

Though bycatch has improved—just 10% of global catch is discarded, a rate down over 40% from the 1990s—reducing bycatch remains a priority in fishery management and seafood sustainability.

There are many ways to reduce accidental catch, but which are the best policies to implement? Closing parts of the ocean to fishing in marine protected areas (MPAs) is one way growing in popularity: A recent campaign to close 30% of the ocean by 2030 (30×30) has made its way to the oval office.

The science behind the 30×30 campaign has been slow to catch up to the policy push, but a paper out last month in PNAS, Pons et al. 2022, has begun to bridge the gap. It evaluates bycatch reduction in several types of potential closures under the 30×30 framework, and found that a typical 30×30 proposal of static area closures in bycatch hotspots would reduce bycatch by only 16%.

However, other types of regulations, namely dynamic no-take areas, would be much more effective, reducing bycatch up to 57%.

Scientists examined confidential data from 15 real-world fisheries and modeled how spatial and temporal closures (i.e., closing fishing areas or times) would impact target catch and bycatch. Crucially, researchers ran the model under two different realistic fishing scenarios:

  1. That fishing pressure remains constant, but moves to the 70% open to fishing.
  2. Catch remains constant, so total fishing pressure may increase if areas of high target catch were closed.

These scenarios mimic reality as MPAs generally lead to a displacement of fishing, not a removal—the areas still open to fishing are fished harder to make up for the closed areas and bycatch increases in those areas.

Aren’t dynamic closed areas just another name for fishery management?

Policies that regulate fishing are all part of fishery management. The management toolbox includes everything from fishing gear restrictions to MPAs, but universally, effective fishery management restricts fishing in certain areas or at certain times of the year to maintain fish stocks and ecosystems.

Dynamic, closed no-take areas—flexible and adaptable in time and space—are a newer method of fishery management that can quickly identify high bycatch areas and close them. High bycatch areas can be identified by real-time data collected each day or predicted from historical data based on oceanographic conditions. Dynamic management works because the location of high bycatch areas changes from year to year or season to season just as ocean conditions and fish distribution change.

Dynamic vs Static protection

Pons et al. 2022 used detailed spatial data to calculate the tradeoff between catch and bycatch under dynamic and static closed areas. They also compared differences between a single large closed area and a mosaic of small protected areas. Mosaic protected areas are smaller networks of closed areas meant to protect important sites, like a spawning aggregation or bycatch hotspot.

Researchers were able to use confidential data on exact fishing locations from 15 fisheries around the world. Using that data, researchers modeled four different types of protected areas where 30% of the fishing area was closed. Two different variables were combined and compared for four possible protected area types: static vs dynamic and single area (centroid) vs mosaic.

Figure 2 from Pons et a. 2022. Showin the difference between static and dynamic protected areas.
Figure 2 from Pons et al. 2022 shows static vs. dynamic protected areas as well as single area (centroid) vs. mosaic.

Dynamic protected areas can move from year to year while static remain in the same place over time. Single area protected areas are continuous while mosaic protections are scattered strategically.

So which of the protected area types performed best at reducing bycatch while maintaining target catch?

Results from Pons et al. 2022 showing catch vs bycatch under dynamic vs static closures and centroid vs mosaic protected areas.
Figure 4 from Pons et al. 2022 showing bycatch reductions under dynamic vs static and centroid vs mosaic protected areas.

Averaging across all fishing scenarios and assuming a closure of 30% of the fishing area, changing from a classic static single closure design to a mosaic design increased bycatch reductions from 16% to 42%, and when adding the dynamic component (changing year to year) that reduction increased up to 57%.

“We found we can significantly reduce bycatch without decreasing the catch of target species by closing small fishing areas that can move year to year,” said lead author Maite Pons. “This dynamic approach is increasingly valuable as climate change drives species and fisheries into new habitats, altering these interactions.”

Bycatch is fishery-dependent, but dynamic protection is not

Bycatch is not uniform. Different fisheries with different fishing gears have different efficiencies, bycatch rates, and bycatch species. For example, shrimp fisheries have the highest bycatch rates (27% of all bycatch comes from shrimp), while purse seines that scoop up a school of sardines have hardly any.

Though this study had a wide representation of fisheries from around the world, there were fisheries where bycatch was significantly reduced (e.g. closing longline fisheries in sea turtle hotspots) and not at all (e.g. French purse seine tuna fisheries in the Indian Ocean where bycatch and target catch are highly correlated in space and time). 

Map from pons et al. 2022 showing the 15 fisheries used in the study
Map from Pons et al. 2022 showing the 15 fisheries used in the paper.

Regardless, dynamic fishery management is the way forward when considering spatial fisheries management to reduce accidental catch. Flexible regulations that can change through time are the best way to manage and mitigate bycatch while also preserving food benefits from fisheries. However, even well-enforced static area closures can be appropriate in the right conditions. Further, closures should accompany other kinds of mitigation measures like fishing gear and operational updates and improved handling and release practices to increase post-release survival.

“We hope this study will add to the growing movement away from permanently closed areas to encourage more dynamic ocean management,” said senior author Ray Hilborn, founder of this site. “Also, by showing the relative ineffectiveness of static areas, we hope it will make conservation advocates aware that permanent closed areas are much less effective in reducing accidental catch than changes in fishing methods.”

However, time and area closures may not be the most effective fishery management tool to reduce bycatch. Changes in fishing gear and practices, e.g. turtle excluder devices on trawl nets, are generally seen as the most effective way to reduce bycatch. Those changes, in addition to fishing closures, are the way forward in management to reduce bycatch. The team of researchers behind Pons et al. 2022 has an analysis in preparation looking at which gear and operational modifications to reduce bycatch are most effective.

Max Mossler

Max Mossler

Max is the managing editor at Sustainable Fisheries UW.

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