Stress tests for the sea: can fisheries withstand ocean acidification?

Photo credit: A. Obaza (NOAA)

Photo credit: A. Obaza (NOAA)

In the aftermath of the recent global financial crisis, financial regulators began subjecting banks to “stress tests” to determine whether banks and their assets were vulnerable to scenarios such as rising interest rates or falling stock prices. These tests aim to understand the risk inherent in current policies, and how assets will perform if there is a shift from the present day economic environment. The expectation is that such tests will identify areas where adjustment of regulation will avoid an undesirable level of risk. An analogous thought process may benefit fisheries managers struggling to incorporate ocean acidification into their already-complicated list of tasks.

Ocean acidification has the potential to alter marine ecosystems by fundamentally changing their chemistry. Many marine species are sensitive to changes in pH and the concentration of carbon dioxide, carbonate ions, and bicarbonate ions brought about by ocean acidification, However, they differ in the degree of their sensitivity, what physiological functions are altered under acidified conditions, and their expected ability to acclimate or adapt to ocean acidification. Because of the high variability in sensitivity and response to ocean acidification among species, the phenomenon has a strong potential to reorganize marine food webs and ecosystems.

While the implications of climate change on fisheries have received much attention over the past few decades, ocean acidification is a relatively new challenge. Because fisheries management varies by stock, there will not be a one-size-fits-all solution for addressing ocean acidification. Methods for ocean acidification “stress tests” will depend on the species and management context. Moreover, such stress tests will need to accommodate the reality that ocean acidification is not a phenomenon that occurs in isolation, but is part of a suite of inextricably linked processes connecting temperature, oxygen, carbon processes and more.

Here we consider three case studies drawn from the U.S. West Coast. As one of the three global hotspots for the progression of ocean acidification [1], the region is seen as a “natural laboratory.” If model projections are accurate, by 2050, over half of the nearshore water mass in the central region off the U.S. West Coast will be corrosive to aragonitic calcium carbonate structures year-round [1], meaning that many shell-forming organisms may suffer effects of acidification. Thus, the impacts of ocean acidification on the marine ecosystem and, subsequently, its fisheries might be felt earlier in this region than many others. For example, ocean acidification already lowered production of an oyster hatchery in Oregon [2] and has been shown to disrupt shell integrity of a zooplankton species, the pteropod Limacina helicina [3]. We present three case studies below for fisheries in a rich, moderate, and poor environment for data and modeling sophistication. These examples illustrate how existing fishery management strategies might work to anticipate or manage in the face of ocean acidification.

Case studies

Data-rich example: A single-species stock assessment model for Pacific whiting

The Pacific whiting (Merluccius productus, also known as hake) fishery is the largest fishery on the U.S. West Coast by tons landed, with more than 229,000 metric tons landed by U.S. vessels in 2013 and 55,000 metric tons landed by Canadian vessels [4].

As a result, the Pacific whiting fishery benefits from relatively high-quality fishery information, population data, and modeling resources. The stock is managed jointly by the U.S. federal government and Canada. Each year, a stock assessment is conducted based on extensive data, including those from on-board fisheries observers and a dedicated biennial coast-wide research survey. The stock assessment uses models with statistical estimation techniques to calculate population size, and harvest control rules reduce catch if abundance falls below pre-set threshold levels.

To date, there is no evidence of direct biological impact of ocean acidification on Pacific whiting. However, Pacific whiting feed heavily on krill. Although the sensitivity of North Pacific krill species is unknown, laboratory research on Antarctic krill suggests that hatch rate decreases under acidified conditions [5]. While Pacific whiting, particularly larger adults, may be able to shift to other prey sources, reduction of krill populations could lead to reduced productivity and abundance of this fish.

Despite the potential risks to Pacific whiting from ocean acidification, we expect that this data-rich situation provides safeguards for the stock against some impacts. Statistical stock assessment methodology [6] is designed to detect changes in productivity and abundance over time, and managers reduce catch if abundance declines. Additionally, the stock assessment allows for changes in fish growth through time [4], which might be expected if krill abundance declines.

Thus, despite the risk Pacific whiting face due to potential acidification effects on their food base, we expect current fishery management – at least in the short term – can adjust fishing rates given the high-quality data and models for this species. We expect computer simulations (e.g., Punt et al. 2014) will continue to be used to stress test the current thresholds and set the annual harvest. Fast management feedback (i.e., extensive monitoring, stock assessment, and setting of catch limits) may be the best way to cope with climate change and ocean acidification [8].

The frequent collection of data and application of models also can elucidate trends in the recent past driven by acidification or other global change. For data-rich species, with quick (annual) management response built in, there may be little value to formal attempts to embed detailed mechanisms or relationships about climate change and acidification into stock assessments [9]. Simulation tests suggest that performance rarely improves substantially when environmental factors – along with realistic uncertainty in mechanisms and data – are explicitly included in management [7].

A data-moderate example: Ecosystem modeling approach on the U.S. West Coast

The previous data-rich example does not represent the normal state of affairs for most fisheries. Unlike Pacific whiting, most federally managed groundfish on the U.S. West Coast undergo fishery stock assessments every two years at most. Others are assessed very infrequently. However, data for most species are available on an annual basis from on-board observers, research trawl surveys, and fishery sampling. The ability of fishery managers to detect and adjust to impacts of acidification will depend on the extent and frequency with which each groundfish species is sampled and assessed and the magnitude of the effects of ocean acidification.

Data-rich species can be managed effectively based on the short-term forecasts available from stock assessments and current data sources. However, this single-species stock assessment cycle cannot help managers and policymakers forecast long-term changes to the ecosystem from directional forces, like ocean acidification. Long-term projections of the effects of stressors such as ocean acidification require strategic tools, such as ecosystem models. These strategic tools also may be a stopgap measure for stocks that are not frequently assessed, allowing ”stress tests” of the effects of acidification with a focus on the entire fish community. This ecosystem-scale approach gives insight into potential changes that emerge when we consider species interacting within a network, revealing expected and unexpected indirect effects of environmental forcing.

Ecosystem models include predator-prey interactions and population dynamics, often in a map-based framework. Typically, predators in these simulations can adjust diets if a prey item (e.g., krill) declines, but cannot necessarily fully compensate for the lost diet item.

On the U.S. West Coast, we have applied ecosystem models to test the impacts of ocean acidification [10-12]. For instance, imposing a simple representation of ocean acidification on an Atlantis model [10] led to strong declines in abundance and long-term harvests of a flatfish and shark species that prey upon shelled invertebrates. Projections from ecosystem models will improve as more laboratory studies are conducted regarding acidification’s impacts on species, and as new, long-term, high-resolution projections of ocean acidification become available for the U.S. West Coast.

The role of these models in the U.S. and internationally is not for annual management, but for considering global change, predator-prey interactions, and decadal-scale, cumulative impacts of fishing. Thus, they should be understood as scenario-based, long-term projections intended for strategic use, not for annual decision-making. However, they illustrate broad risks, trade-offs, and inter-dependencies in the marine ecosystem. As such, they function as another type of stress test, providing managers with advance insight into potential causes for concern.

Data-poor fisheries: Risk assessments for prioritizing action and resources among many fisheries

The examples above illustrate fisheries management tools that contain options for recognizing, including, and responding to ocean acidification impacts. However, a large number of fisheries along the West Coast exist outside of these heavily monitored, data-rich frameworks, including emerging fisheries (e.g., sea cucumber or hagfish) and some high-value fisheries, such as market squid (Doryteuthis opalescens, $69 million per year since 2010) and Dungeness crab (Metacarcinus magister, >$100 million per year). This alternate management style requires different tools to respond to the unanticipated effects of ocean acidification.

The pink shrimp (Pandalus jordani) fishery along the U.S. West Coast is a good example of a data-poor fishery. This coast-wide fishery is managed through cooperation between California, Oregon, and Washington, and bases fisheries yield on a maximum sustainable yield estimate that was last updated in 1981. Fisheries management includes gear designed to allow escape of pre-reproductive individuals and seasonal closure during reproductive periods. Additionally, extensive efforts have resulted in gear changes to minimize bycatch. The Oregon pink shrimp fishery is certified as sustainable by the Marine Stewardship Council.

Pink shrimp stocks appear healthy under the current management structure, in part due to economic factors. The marketable catch remains well below the maximum sustainable yield estimated in 1981, while catch-per-unit-effort remains high. At the same time, there are known risks, such as a high number of latent permits (fishers “sitting out” of the fishery) which could result in substantial additional pressure to the stock should the economics of the fishery change.

The current management structure offers few opportunities to anticipate or proactively respond to ocean acidification impacts. Though there is a paucity of research on the impacts of acidification on pink shrimp, there is evidence that acidification may delay physiological development of some shrimp species. This would be a concern for a fishery such as this where part of the management structure relies on set seasons to protect certain life stages.

Potential impacts of acidification are unlikely to trigger large-scale stock assessments or ecosystem modeling for every fishery along the West Coast. Those approaches are likely not feasible in the short term given the time, data, and expense that these methodologies require, compared to the low value of some of these fisheries. Risk assessments may be an appropriate tool for this group of fisheries [13]. Risk assessments can identify both species and management frameworks most vulnerable to ocean acidification.

Various risk assessment frameworks are being deployed on West Coast fisheries, including the National Oceanic and Atmospheric Administration (NOAA) Climate Vulnerability Assessment, and many include ocean acidification as a potential stressor. A useful aspect of some risk assessment frameworks is their ability to provide relative measures of vulnerability, even in very data-poor situations. By flagging the most vulnerable species, they can help direct management resources to where there is the highest potential benefit. Risk assessments also can help identify target species with biology or management that puts them at less risk from ocean acidification. Thus, even for those fisheries with the fewest data and modeling resources in place, tools exist to provide crude “stress tests.”

Conclusions

There will be no single best way to incorporate ocean acidification into fisheries management. At first glance, it may seem that there is little that can be done without detailed knowledge of how ocean acidification will impact targeted species and the ecosystem of which they are a part. The examples above show, however, that existing fisheries management tools and approaches offer multiple avenues for considering ocean acidification, should they be fully employed in management frameworks. Looking for opportunities to tweak current approaches can serve the same role as financial stress tests: identifying the problems before they emerge, and giving managers time and insight with which to respond. As such, flexibility in the management system emerges as a key to sound fisheries management under ocean acidification. Early warnings from data and models, no matter the type, will provide for sound management into the future and allow fisheries management systems the opportunity to adapt to ocean acidification as impacts are observed and management approaches evolve.

D. Shallin Busch, Michael J. O’Donnell, Isaac C. Kaplan & Errin Ramanujam, Earthzine, 3 June 2015. Article.


  • Reset

Subscribe

OA-ICC Highlights


%d bloggers like this: