Forecasting global fisheries biomass, catch and profits to 2100 under three climate scenarios and five levels of management reforms
Marine fisheries provide a vital source of food for over half the world’s population and support the livelihoods of more than 56 million people globally. However, the ability for marine fisheries to provide these services is threatened by climate change, compromising the contribution of the oceans to sustainable development goals. Ocean warming has already reduced the productivity of many fisheries around the globe, with some regions having experienced up to 35 percent declines in maximum sustainable yield.
An ensemble of marine ecosystem models forecasts by Lotze and co-workers continued showing decreases in marine animal biomass of 4.8 to 17.2 percent by 2100 under low- to high-end emissions scenarios, respectively. In general, productivity is predicted to decrease in tropical and temperate regions and increase towards the poles, as marine organisms shift distributions to maintain their thermal niches. These regional shifts in productivity, range, and fishing opportunity will result in regional discrepancies in food and profits from fisheries. Under current policies, these effects will be unevenly distributed with tropical developing countries and small island developing states exhibiting the greatest vulnerability to the impacts of climate change on fisheries.
Gaines and co-workers provided a critical step towards understanding the opportunities for fisheries reforms to mitigate the impacts of climate change at a global-level. They showed, at a global scale, that business-as-usual fisheries management would exacerbate the negative impacts of climate change, but that climate-adaptive fisheries reforms would maintain global fisheries health, harvest, and profits into the future under all but the most severe emissions scenario evaluated.
However, the effectiveness and feasibility of these reforms is likely to vary regionally, with higher capacity, poleward countries gaining productivity and lower capacity, tropical countries losing productivity. A critical next step then in understanding the potential for fisheries reform to mitigate the impacts of climate change on human livelihoods is to examine the performance of more realistic productivity adaptations at the country level.
This article – adapted and summarized from the original publication (Free, C.M. et al. 2020.) – reports on a study based on the Gaines, et al., model that evaluated the impacts of climate change and management reform on global fisheries and forecast the impacts of climate change on national fisheries; quantified the national-scale benefits of implementing climate-adaptive fisheries reforms; and presented an overview of promising methods for achieving the benefits of climate-adaptive fisheries reform along a gradient of scientific, management, and enforcement capacities.
We used the climate-linked fisheries bioeconomic model by Gaines and co-workers mentioned before to examine country-level changes in fisheries status, catches and profits under three emissions scenarios (RCPs 4.5, 6.0 and 8.5) and five management scenarios from 2012 to 2100.
Projections were made through 2100 using the following general procedure: (1) distributions were updated based on a modified version of the species distribution model by García Molinos and co-workers; (2) carrying capacities were assumed to change in proportion to changes in range size, i.e., a 10 percent increase in range size results in a 10 percent increase in carrying capacity; and (3) biomass, catch and profits were then updated based on a modified version of the Costello, et al., the bioeconomic model by Costello and co-workers and the selected management scenario.
For detailed information on the study design; species distribution model; bioeconomic model and management scenarios; country-level fisheries outcomes; and the guiding principles for climate-adaptive fisheries management, refer to the original publication.
Results and discussion
Overall, our results indicate that climate change will dramatically alter the distribution and productivity of marine fisheries, but plausible climate-adaptive management reforms could minimize or eliminate negative impacts in most countries. This reinforces and expands upon the work of Gaines and co-workers in two important ways.
First, whereas they document the benefits of management reform at a global-level, we focus on the distributional consequences of these global effects by evaluating the benefits that individual countries stand to gain from climate-adaptive fisheries reforms. Second, we recognize that perfectly adapting to changing productivity will be a challenge in even the most sophisticated fisheries systems and evaluate a more realistic scenario that implements well-intentioned, yet imperfect, adaptation to productivity shifts. These expansions are important because they place more realistic bounds on the ability for management to mitigate the impacts of climate change and present practitioners with a tool for investigating the impacts of climate change and opportunities for reform in their respective country’s fisheries.
Our model predicts shifts in productivity that are consistent in both pattern and magnitude with a recent ensemble model that averaged the predictions of six other peer-reviewed marine ecosystem models. We estimated 2.0 and 18.5 percent decreases in maximum sustainable yield from 2012 to 2100 under RCPs 4.5 and 8.5, respectively. By comparison, Lotze and co-workers in their ensemble model estimated 8.6 percent (±6.0 percent standard deviation, SD) and 17.2 percent (±10.7 percent SD) decreases in marine animal biomass in the absence of fishing from 1990 to 2100 under the same two emissions scenarios. This ensemble model, its constituent models and our model all predict increases in productivity in poleward regions and decreases in productivity in tropical to temperate regions. The slight differences in the productivity shifts predicted by our model and the ensemble model are unsurprising given the differences in the structure, mechanistic drivers and taxonomic scope of our model and the ensemble’s constituent models.
Importantly, however, our approach differs from these studies, because, in addition to forecasting the impact of climate change on the biological potential of fisheries, we consider the impact of alternative human responses to these changes, which could either exacerbate or alleviate the impacts of changing biological potential. Indeed, our results indicate that all countries would benefit from reforming current management to account for shifting distributions and productivity and that many countries could even see higher catch and profits than today with such reforms. However, the ability for management reform to mitigate the impacts of climate change is dependent on swift efforts to reduce greenhouse gas emissions.
Even perfect climate-adaptive management (“full adaptation”) is unable to maintain current catch and profits under high-end greenhouse gas emissions (RCP 8.5). Furthermore, although perfect adaptation could maintain global catch and profits under partial emission reductions (RCP 6.0), tropical and temperate regions would still incur dramatic losses in fisheries benefits. This underscores the fact that emission reductions consistent with the Paris Agreement could have significant impacts on the ability for fisheries to feed and employ people into the future.
The development and implementation of stock assessment methods and management strategies necessary to achieve benefits in the face of climate change is nascent but rapidly developing. For example, researchers reviewed 1,250 stock assessments from around the world and found that only 2 percent incorporated ecosystem information into either the final stock assessment model or the recommended harvest control rule. In the United States, other scientists found greater, though still limited, incorporation of ecosystem information into stock assessments: 24 percent of 206 evaluated assessment reports included ecosystem information in either the exploratory or final stock assessment models.
The effective incorporation of environmental information into management strategies is similarly challenging but is also increasing in frequency and effectiveness. Researchers reviewed management strategy evaluation (MSE) studies that test procedures for setting environmentally linked harvest control rules and found that, in general, these procedures were only effective when the environmental drivers were well understood. This emphasizes the need for increasing monitoring and process-oriented lab and field studies in conjunction with the development and testing of more sophisticated analytical techniques.
Furthermore, achieving the benefits of climate-adaptive fisheries reform will require accounting for shifting productivity and distributions along a gradient of scientific, management and enforcement capacities. Many countries lack the monitoring programs required to detect and describe shifts in distribution and productivity, the scientific capacity for conducting either climate-agnostic or climate-adaptive stock assessments, and the management capacity for setting and enforcing fisheries regulations.
This is frequently the case for the tropical developing countries that are forecast to experience the greatest losses in fisheries catch and profits under climate change and exhibit the greatest vulnerability to these reductions in food and income. The tools for enacting climate-adaptive fisheries reforms and achieving biological and socioeconomic resilience to climate change will have to span this gradient of capacity. Fortunately, a growing body of literature provides guidance on accounting for shifting distributions and productivity in fisheries assessment and management and for fostering socioeconomic resilience to climate change in diverse fisheries systems.
Guiding principles for climate-adaptive fisheries management
These methods are promising for achieving the benefits of climate-adaptive fisheries reform along a gradient of scientific, management and enforcement capacities.
Principle #1: Implement best practices in fisheries management
Historically, well-managed fisheries have been among the most resilient to climate change and our results predict that well-intended, albeit imperfect, management will continue to confer climate resilience. Overall, fisheries best practices confer ecological resilience by maintaining healthy stock sizes, age structures and genetic diversity and socioeconomic resilience by providing a portfolio of options to fishers and a buffer against climate-driven losses in any one target stock.
Principle #2: Be dynamic, flexible and forward-looking
Adapting to climate change will require dynamic, flexible and forward-looking management. This can be achieved by aligning management policies with the spatio-temporal scales of climate change, ecosystem change and socioeconomic responses. In lower capacity systems, forward-looking fisheries management could include precautionary management to buffer against uncertainty as well as management strategies that preserve population resilience, age structure and genetic diversity. For example, size limits, seasonal closures and protected areas can be used to protect the big, old, fecund females that disproportionately contribute to reproductive output and to maintain the genetic diversity required to promote evolutionary adaptations to climate change.
Principle #3: Foster international cooperation
Shifting distributions are already generating management challenges and the rates of these shifts and associated conflicts are expected to increase with climate change. New or strengthened international institutions and agreements will be necessary to ensure that management remains sustainable as stocks shift between jurisdictions.
Principle #4: Build socioeconomic resilience
The impact of climate change on fishing communities can be reduced through measures that increase socioeconomic resilience and adaptive capacity to environmental variability and changing fisheries. Across low to high-capacity systems, these measures include (1) policies that facilitate flexibility, such as diversification of access to fisheries and alternative livelihoods; (2) policies that provide better assets, such as the enhancement of fisheries technology and capacity; (3) policies that provide better organization in the system, including multilevel governance, community-based management and other governance structures; and (4) policies that promote agency and learning.
Aquaculture could help compensate for losses in capture fisheries
Even the best climate-adaptive management will be unable to maintain current catch and profits in most tropical developing countries. Although these countries should still pursue climate-adaptive reforms to maximize catch and profits from capture fisheries, they will also need to develop, expand and reform other sectors to compensate for capture fishery losses and meet growing production demands. Marine aquaculture, or mariculture – the cultivation of marine animals and plants – presents a particularly promising substitute for capture fisheries.
The biological potential for mariculture is enormous and exceeds both current production and projected demand even after accounting for economic feasibility and the availability of feed for fed-finfish mariculture. This potential is expected to decrease under climate change but breeding a larger proportion of stocks for fast growth could more than offset these negative impacts. Expanding mariculture under climate change will require (1) streamlining permitting for sustainable mariculture in highly regulated countries where mariculture growth has often been slow; (2) promoting better practices in weakly regulated countries where mariculture growth has often been rapid and less sustainable; and (3) promoting access to financial resources such as credit and insurance in countries where mariculture production has yet to develop.
Although mariculture has the potential to feed millions of people, it also poses a number of environmental problems including pollution, habitat conversion, disease and parasite transmission and escapement and hybridization. The expansion of large-scale mariculture for increased food and employment opportunities will thus require a better understanding of these environmental trade-offs and the best practices for managing them.
Although climate change is expected to reduce the productivity of marine fisheries globally, climate-adaptive fisheries management reforms could mitigate many of the negative impacts on the food and income provisioning potential of the ocean. Our results suggest that climate-adaptive fisheries could result in higher catch and profits than business-as-usual management in all countries. For most countries, climate-adaptive management reforms could result in higher catch and profits in the future than today.
However, the ability for management reforms to offset negative impacts is diminished under increasingly severe greenhouse gas emission scenarios. Thus, swift actions to reduce emissions will be necessary to limit the impacts of climate change on fisheries, especially in developing tropical countries. For many of these countries, even the best climate-adaptive fisheries reforms will be insufficient to maintain current levels of catch and profits into the future. Adaptation in these countries will require innovations in sustainable mariculture and other food sectors to ensure that countries are able to meet the food and nutrition requirements of their growing populations.
The ocean will become an increasingly important source of nutrition. Achieving these benefits will depend on swift and innovative management actions.
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