Realistic fisheries management reforms could mitigate the impacts of climate change in most countries

Although climate change is altering the productivity and distribution of marine fisheries, climate-adaptive fisheries management could mitigate many of the negative impacts on human society. We forecast global fisheries biomass, catch, and profits to 2100 under three climate scenarios (RCPs 4.5, 6.0, 8.5) and five levels of management reform to (1) determine the impact of climate change on national fisheries and (2) quantify the national-scale benefits of implementing climate-adaptive fisheries reforms. Management reforms accounting for shifting productivity and shifting distributions would yield higher catch and profits in the future relative to today for 60-65% of countries under the two least severe climate scenarios but for only 35% of countries under the most severe scenario. Furthermore, these management reforms would yield higher cumulative catch and profits than business-as-usual management for nearly all countries under the two least severe climate scenarios but would yield lower cumulative catch for 40% of countries under the most severe scenario. Fortunately, perfect fisheries management is not necessary to achieve these benefits: transboundary cooperation with 5-year intervals between adaptive interventions would result in comparable outcomes. However, the ability for realistic management reforms to offset the negative impacts of climate change is bounded by changes in underlying biological productivity. Although realistic reforms could generate higher catch and profits for 23-50% of countries experiencing reductions in productivity, the remaining countries would need to develop, expand, and reform aquaculture and other food production sectors to offset losses in capture fisheries. Still, climate-adaptive management is more profitable than business-as-usual management in all countries and we provide guidance on implementing – and achieving the benefits of – climate-adaptive fisheries reform along a gradient of scientific, management, and enforcement capacities.

Introduction potential for fisheries reform to mitigate the impacts of climate change on human livelihoods is 86 to examine the performance of more realistic productivity adaptations at the country-level. 87 88 Here, we use the Gaines et al. [ productivity and distributions along a gradient from no adaptation (a.k.a., business-as-usual 93 management) to full adaptation, including scenarios with realistic intervals between 94 management interventions. Overall, we (1) forecast the impacts of climate change on national 95 fisheries and (2) quantify the national-scale benefits of implementing climate-adaptive fisheries 96 reforms. We conclude with a brief overview of promising methods for achieving the benefits of 97 climate-adaptive fisheries reform along a gradient of scientific, management, and enforcement 98 capacities.

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Methods 101 102 Overview 103 104 We used the Gaines et al. [19] climate-linked fisheries bioeconomic model to examine 105 country-level changes in fisheries status, catches, and profits under three emissions scenarios 106 (RCPs 4.5, 6.0, and 8.5; Table S1) and five management scenarios ( year based on the resulting changes in range size from the SDM assuming a 1:1 proportional 148 change (see [19] for a justification of this assumption).

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The harvest rate is based on the following five management scenarios: business-as-usual 151 (i.e., no adaptation), productivity shift adaptation only, range shift adaptation only, full 152 adaptation, and "realistic" adaptation (see Tables 1 and 2  assumes that both challenges are addressed: the dynamic economically-optimal harvest policy 162 is implemented and maintained even as stocks shift across boundaries. Realistic adaptation 163 refines the full adaptation scenario by implementing productivity shift adaptations at plausible 164 management intervals: it determines the economically-optimal harvest rates on 5, 10, or 20-165 year intervals and maintains these rates until the next management intervention. 166 167 Country-level fisheries outcomes 168 169 We undeveloped fishery is an expected consequence of economically optimal management and 199 should only be perceived negatively when the decrease reduces biomass below the target.

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Impacts of climate change on maximum sustainable yield 204 205 Maximum sustainable yield (MSY) of the evaluated stocks is forecast to decrease by 206 2.0%, 5.0%, and 18.5% from 2012-2021 to 2091-2100 under RCPs 4.5, 6.0, and 8.5, respectively 207 (Figure 1). Note that these values differ slightly from those reported in Gaines et al.
[19] 208 because we excluded mixed-species stocks and measured changes in MSY using decadal means. 209 Across emissions scenarios, MSY is projected to decrease for equatorial countries and increase 210 for poleward countries (Figure 1). Particularly dramatic reductions in MSY are predicted for the 211 equatorial West African countries. Even under the least severe emissions scenario, nineteen 212 countries, fifteen of which are in West Africa, are projected to experience reductions in MSY of 213 50-100%. The number of countries projected to experience dramatic losses in MSY, and the 214 intensity of these losses, expands under the more severe emissions scenarios. In the most 215 severe scenario, 51 countries are expected to experience reductions in MSY of 50-100% ( Figure  216 1). All eighteen West African countries south of Senegal and north of Angola (including these 217 two countries) are forecast to experience reductions in MSY greater than 85%. The equatorial 218 Indo-Pacific and South America are also projected to experience considerable losses in MSY 219 under the three emissions scenarios, with especially pronounced losses under RCP 8.5 ( Figure  220 1). Twenty-two countries are projected to experience increases in MSY under all three 221 emissions scenarios with seven of these countries showing a 15% average increase in MSY 222 across scenarios. The five most consistent and pronounced climate change "winners" are: 223 Finland, Antarctica, Norway (4 EEZs: Norway plus Bouvet Island, Jan Mayen, and Svalbard), 224 Portugal (3 EEZs: Portugal plus Azores and Madeira), and Fiji.

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Ability for management reform to mitigate global climate impacts 227 228 Business-as-usual (BAU) management results in both lower catches and profits in the 229 future relative to today under all three emissions scenarios (Figure 2). In contrast, full 230 adaptation yields both higher catches and profits in the future in all but the most severe 231 emissions scenario (RCP 8.5); in this scenario, full adaptation yields higher profits but lower 232 catches in the future relative to today. Addressing productivity shifts and range shifts in 233 isolation is insufficient for jointly maintaining catch and profits into the future under any of the 234 emissions scenario (Figure 2). However, realistic adaptation, which recalibrates productivity 235 management at 5, 10, and 20-year intervals and maintains this management regime as stocks 236 shift across boundaries, frequently achieves better outcomes in the future relative to today 237 (Figure 2). Notably, realistic adaption that implements adaptive management at 5-year intervals 238 performs comparably to full adaptation and generates both higher catch and profits in the 239 future relative to today under the two least severe emissions scenarios (Figure 2).

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Ability for management reform to mitigate country-level climate impacts 242 243 While business-as-usual management results in lower catches and profits relative to 244 today for the majority of countries (82-85% of countries), full adaptation yields higher catches 245 and profits for a majority of countries in all but the most severe emission scenario (Figures 3 246 and S1). In this scenario, only 35% of countries experience both increased profits and catches, 247 while 59% of countries experience both reduced catches and profits (Figure 3). Realistic 248 adaptation implemented at 5-year intervals achieves outcomes quite similar to full adaptation: 249 it results in higher catch and profits for the majority (56-63%) of countries under RCPs 4.5 and 250 6.0 but lower catch and profits for the majority (59%) of countries under RCP 8.5 (Figure 3). The 251 ability for adaptation to maintain or increase fisheries outcomes under climate change is 252 sensitive to the direction and magnitude of changes in underlying productivity (Figures 3-5). For 253 example, the West African countries projected to experience the greatest losses in MSY are also 254 projected to have the most limited ability to mitigate these impacts (Figures 1 and 4). Although 255 realistic adaptation (5-yr) could increase both catch and profits for 51% of the countries 256 projected to lose underlying productivity (i.e., lower MSY) in the least severe emissions 257 scenario, it could increase outcomes despite losses in productivity for only 23% of countries in 258 the most severe emissions scenario (Figure 4). In comparison, realistic adaptation (5-yr) could 259 increase both catch and profit for a much larger proportion of countries projected to gain 260 underlying productivity: 78% of these countries (n=69) could increase both catch and profits in 261 the least severe emissions scenario and this percentage actually increases to 95% in the most 262 severe emissions scenario as these poleward countries (n=22) inherit even more productivity 263 (Figure 4). Neither realistic (5-yr) nor full adaptation are sufficient to maintain fisheries 264 outcomes into the future for all countries, but they are nearly always preferable to business-as-265 usual management. In all but the most severe emissions scenario, both full adaptation and 266 realistic adaptation yield both higher cumulative catches and profits than business-as-usual 267 management for nearly all countries (98-99% of countries; Figure 6). In the most severe 268 scenario, full adaptation and realistic adaptation yield higher cumulative profits than business-269 as-usual management, but achieve lower cumulative catches for 40-41% of countries (Figure 6) al.

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[5] ensemble model, its constituent models, and our model all predict increases in productivity 294 in poleward regions and decreases in productivity in tropical to temperate regions. The slight 295 differences in the productivity shifts predicted by our model and the ensemble model are 296 unsurprising given the differences in the structure, mechanistic drivers, and taxonomic scope of 297 our model and the ensemble's constituent models.

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Importantly, however, our approach differs from these studies, because, in addition to 300 forecasting the impact of climate change on the biological potential of fisheries, we consider 301 the impact of alternative human responses to these changes, which could either exacerbate or 302 alleviate the impacts of changing biological potential [13]. Indeed, our results indicate that all 303 countries would benefit from reforming current management to account for shifting 304 distributions and productivity and that many countries could even see higher catch and profits 305 than today with such reforms. However, the ability for management reform to mitigate the 306 impacts of climate change is dependent on swift efforts to reduce greenhouse gas emissions. 307 Even perfect climate-adaptive management ("full adaptation") is unable to maintain current 308 catch and profits under high-end greenhouse gas emissions (RCP 8.5 principles as well as specific strategies for achieving the benefits of climate-adaptive 348 management reforms. We offer recommendations for higher and lower capacity fisheries 349 systems as well as recommendations for countries where even the best management reforms 350 will be unable to offset the negative impacts of climate change.

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Guiding principles for climate-adaptive fisheries management 353 354 Principle #1: Implement best practices in fisheries management 355 Historically, well-managed fisheries have been among the most resilient to climate 356 change [4], and our results predict that well-intended, albeit imperfect, management will 357 continue to confer climate resilience. Together, these results indicate that the wider 358 implementation of best practices in fisheries management will mitigate many of the negative 359 impacts of climate change. In higher capacity systems, best practices include scientifically-360 informed catch limits, accountability measures, regional flexibility in policy practices, and 361 protection New or strengthened international institutions and agreements will be necessary to ensure that 403 management remains sustainable as stocks shift between jurisdictions. First, this will require 404 sharing data between Regional Fisheries Management Organizations (RFMOs) or countries to 405 identify, describe, and forecast shifting stocks. Second, it will require a commitment to use 406 these shared data to inform collaborative management. For example, these data could be used 407 to regularly and objectively update national allocations of catch or effort based on changes in 408 distribution rather than historical allocations (e.g., [59,60] Even the best climate-adaptive management will be unable to maintain current catch 448 and profits in most tropical developing countries. Although these countries should still pursue 449 climate-adaptive reforms to maximize catch and profits from capture fisheries, they will also 450 need to develop, expand, and reform other sectors to compensate for capture fishery losses 451 and meet growing production demands [73]. Marine aquaculture (hereafter called mariculture), 452 the cultivation of marine animals and plants, presents a particularly promising substitute for 453 capture fisheries. The biological potential for mariculture is enormous [74] and exceeds both 454 current production and projected demand even after accounting for economic feasibility and 455 the availability of feed for fed-finfish mariculture [75]. This potential is expected to decrease 456 under climate change [76] but breeding a larger proportion of stocks for fast growth could 457 more than offset these negative impacts [77]. Although mariculture has the potential to feed 458 millions of people, it also poses a number of environmental problems including pollution, 459 habitat conversion, disease and parasite transmission, and escapement and hybridization [78].

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The expansion of large-scale mariculture for increased food and employment opportunities will 461 thus require a better understanding of these environmental tradeoffs and the best practices for 462 managing them [79].

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Conclusions 465 466 Although climate change is expected to reduce the productivity of marine fisheries 467 globally [5], climate-adaptive fisheries management reforms could mitigate many of the 468 negative impacts on the food and income provisioning potential of the ocean [19]. Our results 469 suggest that climate-adaptive fisheries could result in higher catch and profits than business-as-470 usual management in all countries. For most countries, climate-adaptive management reforms 471 could result in higher catch and profits in the future than today. However, the ability for 472 management reforms to offset negative impacts is diminished under increasingly severe 473 greenhouse gas emission scenarios. Thus, swift actions to reduce emissions will be necessary to 474 limit the impacts of climate change on fisheries, especially in developing tropical countries. For 475 many of these countries, even the best climate-adaptive fisheries reforms will be insufficient to 476 maintain current levels of catch and profits into the future. Adaptation in these countries will 477 require innovations in sustainable mariculture and other food sectors to ensure that countries 478 are able to meet the food and nutrition requirements of their growing populations [73]. As 479 land-based sources of food also falter [80], the ocean will become an increasingly important 480 source of nutrition. Achieving these benefits will depend on swift and innovative management 481 actions. 482    intervals. Grey shading indicates countries without marine territories. 736