OCB5635 Week 10

Marine megafauna

- A number of megafauna species, including humpback whales & northern elephant seals, have recovered to historical baselines following protection; however, rates of recovery depend on the life history of the species: some large whales may require more than 100 years to recover, whereas smaller pinnipeds may only need several decades. - Sea turtles have recovery timescales of up to 100 years, although some populations have partially recovered much faster (for ex, green turtles in Hawaii increased sixfold between 1973 & 2016). - Seabird populations typically require a few decades to recover

Roadblocks

- A number of roadblocks may delay / prevent recovery of some of the critical components of marine life. - These include natural variability & intensification of environmental extremes caused by anthropogenic climate change, unexpected natural / social events, & a failure to meet commitments to reduce existing pressures & mitigate climate change. - In addition, the growing human population, which will probably exceed 9 billion individuals by 2050, will create additional demands for seafood, coastal space & other ocean resources. - Accordingly, if all necessary recovery wedges are stacked, a 2050 target of substantial to complete recovery (that is, 50-100% increase relative to the present) for most rebuilding components appears realistic & achievable. - Partial to substantial (10 to >50%) recovery can be targeted for deep-sea habitats, where slow recovery rates lead to a modest rebuilding scope by 2050, & for coral reefs, where existing & projected climate change severely limits the rebuilding prospects. - A major roadblock to recovery for intertidal habitats, such as mangroves & salt-marshes, is their conversion to urban areas, aquaculture ponds / infrastructure. - However, even in large cities, such as New York & Shenzen, some restoration of degraded habitats has been achieved. - Incentives to develop alternative sources of livelihood, relocate landholders, mediate land-tenure conflicts & improve land-use planning can release more habitat for coastal restoration. - Tools are emerging to prioritize sites for restoration based on past experience & a broad suite of biophysical & socio-economic predictors of success. - Reduced sediment supply due to dam construction in watersheds is also an important challenge for the recovery of salt marshes & mangroves, & these challenges are exacerbated by sea-level rise & climate change. - However, these habitats may be less vulnerable than previously thought, with a recent assessment concluding that global gains of 60% of coastal wetland area are possible under sea-level rise. - By contrast, enhanced sediment load from land clearing is often responsible for losses of nearshore coral reefs & hinders their capacity to recover from coral bleaching.

Conclusion

- Based on the data reviewed here, we conclude that substantial rebuilding across many components of marine life by 2050 is an achievable Grand Challenge for science & society. - Meeting this challenge requires immediate action to reduce relevant pressures, including climate change, safeguarding places of remaining abundance, & recovering depleted populations, habitats & ecosystems elsewhere. - This will require sustained perseverance & substantial commitment of financial resources, but we suggest that the ecological, economic & social gains will be far-reaching. - Success requires the establishment of a committed & resilient global partnership of governments & societies aligned with this goal, supported by coordinated policies, adequate financial & market mechanisms, & evolving scientific & technological advances that nurture a fast learning curve of rebuilding interventions. - Meeting the challenge of substantially rebuilding marine life would be a historic milestone in humanity's quest to achieve a globally sustainable future

Reversing the decline of marine life

- By the time the general public admired life below water through the television series 'The Undersea World of Jacques Cousteau' (1968-1976), the abundance of large marine animals was already greatly reduced. - Since the first frameworks to conserve & sustain marine life were introduced in the 1980s, the abundance of marine animals & habitats that provide essential ecosystems services has shrunk even further. - Currently, at least 1/3 of fish stocks are overfished, 1/3 to 1/2 of vulnerable marine habitats have been lost , a substantial fraction of the coastal ocean suffers from pollution, eutrophication, oxygen depletion & is stressed by ocean warming, & many marine species are threatened with extinction. - Nevertheless, biodiversity losses in the ocean are less pronounced than on land & many marine species are capable of recovery once pressures are reduced / removed. - Substantial areas of wilderness remain in remote regions & large populations of marine animals are still found, for example, in mesopelagic (200-1,000 m depth) ocean waters. - Regional examples of impressive resilience include the rebound of fish stocks during World War I & World War II following a marked reduction in fishing pressure, the recovery since 1958 of coral reefs in the Marshall Islands from 76 megatons of nuclear tests & the improved health of the Black Sea & Adriatic Sea following a sudden reduction in the application of fertilizers after the collapse of the Soviet Union. - Although these rapid recoveries were unrelated to conservation actions, they helped to inform subsequent interventions that have been deployed in response to widespread ocean degradation. - These interventions include a suite of initiatives to save threatened species, protect & restore vulnerable habitats, constrain fishing, reduce pollution & mitigate climate change.

Overcoming the climate change roadblock

- Climate change is the critical backdrop against which all future rebuilding efforts will play out. - Current trajectories of greenhouse gas emissions lead to warming by 2100 of 2.6 to 4.5 °C above preindustrial levels, far exceeding the long-term goal of the Paris Agreement (holding the increase in global average temperature to well below 2 °C above preindustrial levels). - Much stronger efforts to reduce emissions are needed to reduce the gap between target emissions & projected emissions under the present voluntary NDCs126 a challenging but not impossible task. - Efforts to rebuild marine life need to consider unavoidable impacts brought about by ocean warming, acidification & sea-level rise already committed by past emissions, even if the climate mitigation wedge, represented by the Paris Agreement, is fully implemented. - These changes include projected shifts in habitats & communities at subtropical-tropical (coral to algal turf & seaweed), subtropical- temperate (kelp to coral & urchin barrens, salt-marsh to mangrove) temperate-Arctic (bare to kelp, ice fauna to pelagic) & intertidal (coastal squeeze) boundaries, propelled by species displacements & mass mortalities from future heat waves11. - Mapping the areas where the likelihood of these transitions is high can help to prioritize where & how restoration interventions should be deployed. - For instance, conserving & restoring vegetated coastal habitats will help to defend shorelines against increased risks from sea-level rise while helping to mitigate climate change. - Well-managed MPAs may help to build resilience to climate change. - However, many of them are already affected by ocean warming & further climate change may potentially compromise their performance in the future. - Rebuilding coral reefs carries the highest risk of failure, as cumulative pressures (for ex., overfishing & pollution) that drove their historical decline are now increasingly compounded by warming-induced bleaching. - The IPCC (Intergovernmental Panel on Climate Change) projects that global warming to 1.5 °C above preindustrial levels will result in very high risks & losses of coral reefs unless adaptation occurs faster than currently anticipated. - A recent study shows that while coral bleaching has increased in frequency & intensity in the last decade, the onset of coral bleaching is now occurring at significantly warmer temperatures (around 0.5 °C) than previously, suggesting that the remaining coral populations now have a higher thermal threshold for bleaching, due to a decline in thermally vulnerable species & genotypes and/or acclimatization. - However, the capacity to restore coral reefs lags behind that of all other marine habitats, because coral-reef restoration efforts typically have a very small footprint, & are expensive & slow. - Coral restoration often fails because the original causes of mortality remain unchecked, & despite decades of effort, only 10s of hectares have been regrown so far. - Our growing knowledge of ecological processes in coral reefs provides opportunities to catalyse recovery by reducing multiple pressures while repairing key processes, including herbivory & larval recruitment. - Mitigating the drivers of coral loss, particularly climate change, & developing innovative approaches to restoration within this decade are imperative to revert coral losses at scale. - Efforts are underway to find corals that are resistant to the temperatures & acidity levels expected by the end of the twenty-first century, to understand the mechanisms of their resistance & to use 'assisted evolution' to engineer these characteristics into other corals. - However, these efforts are in their infancy & their benefits currently unproven. - Overall, the societal benefits that would accrue from substantially rebuilding marine life by 2050 will depend on the mitigation of greenhouse gas emissions & on the development of efficient CO2 capture & removal technologies to meet or, preferably, exceed the targets of the Paris Agreement

Potential for rebuilding

- Efforts to rebuild marine life cannot aim to return the ocean to any particular past reference point. - Our records of marine life are too fragmented to compose a robust baseline, & the ocean has changed considerably and—in some cases—irreversibly, including the extinction of at least 20 marine species. - We argue instead that the focus should be on increasing the abundance of key habitats & keystone species, & restoring the three-dimensional complexity of benthic ecosystems. - The yardstick of success should be the restoration of marine ecological structure, functions, resilience & ecosystem services, increasing the capacity of marine biota to supply the growing needs of an additional 2 to 3 billion people by 2050. - To meet this goal, rebuilding of depleted populations & ecosystems must replace the goal of conserving & sustaining the status quo, & swift action should be taken to avoid potential tipping points beyond which collapse may be irreversible. - Here we examine the rates of recovery of marine species & habitats to date, & propose a tentative timeframe in which substantial recovery of marine life may be possible, should major pressures, including climate change, be mitigated. - We broadly define recovery as the rebound in populations of marine species & habitats following losses, which can be partial (that is, 10-50% increase), substantial (50-90% increase) / complete (>90% increase)

Habitat restoration

- Evidence that mangrove restoration can be achieved at scale first came from the Mekong Delta mangrove forest, possibly the largest (1,500 km2 ) habitat restoration undertaken to date. - Global loss of mangrove forests has since slowed to 0.11% per year, with stable mangrove populations along the Pacific coast of Colombia, Costa Rica & Panama, & increasing populations in the Red Sea, Arabian Gulf & China. - Large-scale restoration of salt-marshes & oyster reefs has occurred in Europe & the United States. - Restoration attempts of seagrass, seaweed & coral reef ecosystems are also increasing globally, although they are often small in scale. - Notably, a global inventory of total restored area is missing.

Water-quality improvement.

- Policies to lower inputs of nutrients & sewage to reduce coastal eutrophication & hypoxia were initiated four decades ago in the United States & European Union (EU), leading to major improvements today. - Many hazardous pollutants have been regulated / phased out through the Stockholm Convention and, specifically in the ocean, by the MARPOL Convention, often reinforced by national & regional policies. - Recent attention has focused on reducing & preventing plastic pollution from entering the ocean, which remains a growing problem; inputs of plastic are currently estimated at between 4.8 to 12.7 million metric tons per year

Call to action

- Rebuilding marine life requires a global partnership of diverse interests, including governments, businesses, resource users & civil society, aligned around an evidence-based action plan supported by a sound policy framework, a science & educational plan, quantitative targets, metrics for success & a business plan. - It also requires leadership to assemble the scientific & socio-economic knowledge & the technologies required to rebuild marine life & the capacity to deploy them. - A concerted global effort to restore & protect marine life & ecosystems could create millions of new—and in many cases—wellpaying jobs. - Thus, commitments of governments, which are required to meet the UN SDGs by 2030, need to be supported & reinforced by commitments from society, non-government organizations (NGOs) & other agents, such as philanthropic groups, corporations & industry. - The sectors that operate in the ocean spaces, which bear considerable responsibility for the losses thus far experienced and, in many cases, are likely to be the main beneficiaries of efforts to rebuild marine life, must change their ethos to commit to a net positive conservation impact as part of their social license to operate in the ocean space. - The use of the ocean by humans should be designed for net positive conservation impact, creating additional benefits that increase prosperity & catalyse political will to deploy further efforts in a positive feedback spiral of ocean bounty. - The long-term commitment to rebuilding marine life requires a powerful narrative, supported by scientific evidence that conveys its feasibility in the face of climate change & a growing human population, its alignment with societal values, & its widespread societal benefits. - Growing numbers of success stories could shift the balance from a wave of pessimism that dominated past scientific narratives of the future ocean to evidence-based 'ocean optimism' (for ex., #oceanoptimism in social media), conveying solutions & opportunities for actions that help to drive positive change. - This optimism must be balanced with transparent & robust communication of the risks posed by relevant pressures that are yet to be mitigated. - Rebuilding marine life will benefit from nations declaring, analogous to the Paris Agreement on climate change, NDCs towards rebuilding marine life. - NDCs aimed at rebuilding marine life will be essential for accountability, auditing milestones & forecasting success in reaching goals. - NDCs can include both commitments for action within national Economic Exclusive Zones, as well as a catalogue of actionable opportunities available to investors, corporations & philanthropists. - The global policy framework required to rebuild marine life is largely in place through existing UN mechanisms (targets to be adopted in 2020 under the Global Biodiversity Framework of the CBD, SDGs & Paris Agreement of the UNFCC), if their most ambitious goals are implemented, along with additional international conventions such as the Bonn Convention on the Conservation of Migratory Species of Wild Animals, the Moratorium on Commercial Whaling of the International Whaling Commission (1982), Ramsar Convention on Wetlands of International Importance & CITES, among others. - High-level coordination among all UN instruments & international policies addressing the oceans, including the high seas, is needed. - The UN initiated, in 2018, an Intergovernmental Conference to reach a new legally binding treaty to protect marine life in the high seas by 2020. - This proposed treaty could enhance cooperation, governance & funds for conservation & restoration of high-seas & deep-sea ecosystems damaged / at risk from commercial interests. - This mandate would require funding of around US$30 million annually, which could be financed through long-term bonds in international capital markets / taxes on resource extraction. - Internationally agreed contributions will also be required, because populations of many species are shared across Exclusive Economic Zones of multiple nations. - This approach could follow the model of the Regional Fisheries Management Organizations, bringing together nations to manage shared fish stocks that straddle national waters & the high seas. - For ex., in September 2010 the Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR) established the world's first MPA network on the high seas covering 286,200 km2. - Rebuilding marine life will also require active oversight, participation & cooperation by local, regional & national stakeholders. - A readiness & the capacity to implement recovery wedges differs across nations, & cooperation to rebuild marine life should remain flexible to adapt to variable cultural settings; locally designed approaches may be most effective1. - Past failures in some nations can inform new governance arrangements to avoid repeating the same mistakes elsewhere. - Rebuilding marine life should draw on successful marine policy formulation, management actions & technologies to nurture a learning curve that will propel future outcomes while reducing cost. - For instance, many developed nations have already implemented nutrient reduction plans; however, fertilizer use is rising globally, supported mainly by demands from developing nations that also continue to develop their shorelines. - Adopting the measures now in place in developed nations to increase nitrogen-use efficiency in South & East Asia could lower global synthetic fertilizer use by 2050, even under the increased crop production required to feed a growing population. - Calls for international assistance to support recovery, whether it is for coastal wetlands to reduce risks of damages from natural disasters / marine life generally, should include assistance to improve governance & build institutional capacities. - However, the capacity of both developed & developing nations to deploy effective recovery actions is already substantial. - Mangrove restoration projects are considerably larger & cheaper but similarly successful (about 50% survival reported) in developing nations compared with developed countries, & small-island states are showing growing leadership in response to plastic pollution & the marine impacts of climate change. - However, many developing countries need particularly high levels of investment to conserve & restore habitats that protect populations at risk in low-lying coastal areas, which could be financed through international climate change adaptation funds. - Currently, the UN's Green Climate Fund has mobilized US$10.3 billion annually to assist developing countries to adapt to climate change, with a goal of US$100 billion per year in 2020. - Allocating a sizable fraction of these funds to developing countries for the conservation 7 restoration of 'blue infrastructure' (for ex., salt-marshes, oyster & coral reefs, mangroves & seagrass beds) could increase the resilience of coastal communities to climate change & to extreme events while improving their livelihoods

Fish stocks

- Recovery can also refer to achieving resilient populations that support the full extent of ecosystem functions & services that characterize them. - For instance, fish stock recovery is often defined in terms of biomass increases to the level that enables the maximum sustainable yield (BMSY), which fisheries harvest theory predicts to be between 37% & 50% of the virgin biomass (B0), depending on the particular model used. - This range is consistent with an empirical estimate of B0 for 147 exploited fish stocks, which found that contemporary BMSY values were 40% of B0, on average, with a range of 26% to 46% across taxa. - Reported recovery times to BMSY for overexploited finfish & invertebrate stocks range between 3 & 30 years, which is consistent with palaeoecological reconstructions of prehistoric collapse & recovery ofanchovy, sardine & hake stocks, data from fisheries closures & fish stock assessments. - However, BMSY should be considered to represent a minimum recovery target, as it does not account for ecosystem interactions, & might provide only limited resilience in the face of environmental uncertainty & change. - Minimum recovery times of populations are set by the maximum intrinsic rate of population increase (rmax), which is typically higher than observed rates, resulting in longer recovery times. - Recovery rates also depend on the fishing pressure imposed on the stock; for ex., rebuilding depleted populations to BMSY may take less than a decade, if fishing mortality is rapidly reduced below FMSY. - Longer recovery times are expected if fishing pressure is reduced more slowly. - Recovery for longer-lived, slow-growing species such as most elasmobranchs (sharks, rays & skates), depleted coral reef fish & deep-sea species may take much longer

A roadmap to recovery

- Steps taken to rebuild marine life to date have involved a process of trial & error that delayed positive outcomes (for ex., reduction of excessive nutrient inputs in the EU & United States), but that generated know-how to cost-effectively propel subsequent efforts at scale. - Improved ocean stewardship, as required by UN SDG 14, is a goal shared across many nations, cultures, faiths & political systems, occupying a more-prominent place in the agendas of governments, corporations, philanthropists & individuals than ever before. - This provides a window of opportunity to mitigate existing pressures over the next decade while supporting global initiatives to achieve substantial recovery of marine life by 2050. - we can choose between a legacy of a resilient & vibrant ocean / an irreversibly disrupted ocean, for the generations to follow. - Some of the interventions required to rebuild marine life have already been initiated, but decadal time lags suggest that the full benefits are yet to be realized. - Because most policies to reduce local pressures & prompt recovery of marine life were introduced after the 1970s, it is only now that comprehensive benefits are becoming evident at a larger scale. - Similarly, as most current MPAs are less than 10 years old, their full benefits, which increase with the age of the reserve, are yet to be realized, particularly for MPAs that are properly managed & enforced.

Necessary investments & expected returns

- Substantial rebuilding of marine life by 2050 requires sustained effort & financial support, with an estimated cost of at least US$10-20 billion per year to extend protection actions to reach 50% of the ocean space & substantial additional funds for restoration. - This is comparable to establishing a global MPA network that conserves 20-30% of the ocean (US$5-19 billion annually). - Yet the economic return from this commitment will be considerable, around US$10 per US$1 invested & in excess of one million new jobs. - Ecotourism in protected areas provides 4-12 times greater economic returns than fishing without reserves (for ex., AUS$5.5 billion annually & 53,800 full time jobs in the Great Barrier Reef). - Rebuilt fisheries alone could increase the annual profits of the global seafood industry by US$53 billion. - Conserving coastal wetlands could save the insurance industry US$52 billion annually by reducing storm flooding, while providing additional benefits of carbon sequestration, income & subsistence from harvesting, & from fisheries supported by coastal wetlands. - A global rebuilding effort of exploited fish stocks could increase fishing yields by around 15% & profits by about 80% while reducing bycatch mortality, thereby also helping to promote recovery in non target species. - Rebuilding fish stocks can be supported by market based instruments, such as rationalizing global fishing subsidies, taxes & catch shares, to end perverse incentives & by the growth of truly sustainable aquaculture to reduce pressure on wild stocks. - Whereas most regulatory measures focus on commercial fisheries, subsistence & recreational fishing are also globally relevant & need to be aligned with rebuilding efforts to achieve sustainability.

Management of fisheries.

- Successful rebuilding of depleted fish populations has been achieved at local & regional scales through well-proven management actions, including catch & effort restrictions, closed areas, regulation of fishing capacity & gear, catch shares & co-management arrangements. - These interventions require detailed consideration of socio-economic circumstances, with solutions being tailored to the local context. - Persistent challenges include harmful subsidies, poverty & lack of alternative employment, illegal, unregulated & unreported fishing, & the disruptive ecological impacts of many fisheries.

Habitat protection & restoration

- The need to better protect sensitive habitats, including non-target species, has inspired the use of Marine Protected Areas (MPAs) as a comprehensive management too. - In 2000, only 3.2 million km2 (0.9%) of the ocean was protected, but MPAs now cover 26.9 million km2 (7.4% of ocean area, / 5.3% if only considering fully implemented MPAs. - MPA coverage continues to grow at about 8% per year - The twenty-first century has also seen a global surge of active habitat protection & restoration initiatives, even in challenging environments adjoining coastal megacities. - These efforts have delivered benefits, such as improved water quality following oyster reef restoration. - Additionally, Blue Carbon strategies, submitted within the nationally determined contributions (NDCs) of more than 50 nations—at the heart of the Paris Agreement—are being used to mitigate climate change & improve coastal protection by restoring seagrass, salt marsh & mangrove habitats.

Intro

- The ocean currently contributes 2.5% of global gross domestic product (GDP) & provides employment to 1.5% of the global workforce1 , with an estimated output of US$1.5 trillion in 2010, which is expected to double by 20301 . - increased attention on the ocean as a source of food & water, clean energy & as a means to mitigate climate change. - However, many marine species, habitats & ecosystems have suffered catastrophic declines, & climate change is further undermining ocean productivity & biodiversity. - conflict between the growing dependence of humans on ocean resources & the decline in marine life under human pressures is focusing the attention on the connection between ocean conservation & human wellbeing. - The United Nations Sustainable Development Goal 14 (UN SDG 14 / 'life below water') aims to "conserve & sustainably use the oceans, seas & marine resources for sustainable development" - Achieving this goal will require rebuilding marine life, defined in the context of SDG 14 as the life-support systems (populations, habitats & ecosystems) that deliver the many benefits that society receives from a healthy ocean - substantially rebuilding marine life within a human generation is largely achievable, if the required actions—mitigation of climate change—are deployed at scale.

Coastal habitats

- The recovery of coastal habitats after the removal of stressors / following active restoration of the habitat typically occurs on a similar timescale as fish stock recovery, less than a decade for oyster reefs & other invertebrate populations, & kelp-dominated habitats, between one to two decades for saltmarsh & mangrove habitats, & 1 to several decades for seagrass meadows. - Deep-sea corals & sponges grow more slowly & recovery times from trawling disturbance / oil spills may range from 30 years to more than a century. - Recovery timescales of coral reefs that are affected by local stressors range from a few years to more than a decade. - However, recovery from severe coral bleaching has taken well over a decade & will slow in the future as ocean warming shortens the interval between bleaching events, with an associated steep reduction in coral-reef recruitment. - In summary, available data suggest that many marine species & habitats require one to three decades to approach undisturbed / reference abundance ranges & fish stock biomass that supports maximum sustainable fish catches after removal of the causes of decline, with longer recovery times required for some slow-growing groups

Impactful interventions

- The regulation of hunting. - The protection of species through the Convention on International Trade of Endangered Species & the global Moratorium on Commercial Whaling (1982) are prominent examples of international actions to protect marine life. - These actions have been supplemented by national initiatives to reduce hunting pressure on endangered species & protect their breeding habitat

Recovery times

- The time that is required to rebuild components of marine life depends on the extent of previous declines, which are often substantial. - The reduction in species abundance & biomass relative to predisturbance baselines averages about 44 & 56%, respectively, across affected marine ecosystems. - Similarly, the Living Blue Planet Report estimated a 49% decline in the abundance of marine animal populations between 1970 & 2012, although many species & habitats have declined further since. - Moreover, although the maximum rates of recovery of marine populations typically range from 2 to 10% per year, rates slow down as carrying capacity is approached. - Assuming a reported average annual recovery rate of 2.95% (95% confidence interval, 2.42-3.41%) across marine ecosystems & a characteristic rebuilding deficit of about 50% of predisturbance baselines, we provisionally estimate that the average time to reach 90% of undisturbed baselines (that is, achieve substantial recovery) would be about 21 years (95% confidence interval, 18-25 years). - However, the expectation of an average recovery time of about two decades is compromised by the fact that many species & habitats continue to decline & some pressures, such as climate change & plastic pollution, are still increasing. - Thus, substantial (50-90%), rather than complete (>90%), recovery may be a more realistic target for rebuilding marine life in the short term. - Based on the case studies examined, we provisionally propose three decades from today (2050) as a target timeline for substantial (that is, 50-90%) recovery of many components of marine life, recognizing that many slow-growing, severely depleted species & threatened habitats may take longer to recover, & that natural variability may delay recovery further. - Importantly, achieving substantial recovery by 2050 requires that major pressures are mitigated soon, including climate change under the Paris Agreement. - Climate change affects the demography, phenology & biogeography of many marine species & compromises the productivity of marine ecosystems. - Current impacts of realized climate change on many coral reefs raise concerns about the future prospects of these ecosystems. - If we succeed in mitigating climate change & other pressures, we may witness a trend change from a previous steep decline to stabilization and, in many cases, substantial global recovery of marine life in the twenty-first century.

Recovery wedges

- There is no single solution for achieving substantial recovery of marine life by 2050. - Rather, recovery requires the strategic stacking of a number of complementary actions, here termed recovery wedges, each of which will help to increase the recovery rate to reach / exceed the target of 2.4% increase per year across different ecosystem components. - These wedges include protecting vulnerable habitats & species, adopting cautionary harvesting strategies, restoring habitats, reducing pollution & mitigating climate change. - The strength of the contribution of each of these wedges to the recovery target can be expected to vary across species & ecosystems. - For instance, mitigating climate change is the critical wedge to set coral reefs on a recovery trajectory, whereas improved habitat protection & fisheries management are the critical wedges for the recovery of marine vertebrates & deep-sea habitats. - Ongoing efforts to remove pressures on marine life from anthropogenic climate change, hunting, fishing, habitat destruction, pollution & eutrophication must be expanded & made more effective. - A new framework to predict risks of new synthetic chemicals is required to avoid circumstances in which industry introduces new chemicals faster than their risks can be assessed. - Challenges remain for persistent legacy pollutants (for example, CO2, organochlorines & plastics) that are already added to the atmosphere & oceans, the removal of which requires novel removal technologies & protection of long-term sinks, such as marine sediments, to avoid their remobilization. - MPAs represent a necessary & powerful recovery wedge across multiple components of the ocean ecosystem, spanning from coastal habitats to fish & megafauna populations. - The current growth of MPAs is currently on track to meet ambitious targets, 10% of ocean area protected by 2020, 30% by 2037 & 50% by 2044. - Many fish stocks could recover to BMSY by 2030, assuming global management reforms couple the use of closed & protected areas to measures that reduce overall fishing pressure & collateral ecosystem damage that are adapted to the local context. - However, projected climate impacts on ocean productivity & an increase in extreme events can delay recovery and, depending on emission pathways, may prevent recovery of some components altogether. - The current focus on quantitative targets of the percentage of the ocean area that is protected has prompted concerns over the quality & effectiveness of MPAs. - Although 71% of assessed MPAs have been successful in enhancing fish populations, the level of protection is often weak (94% allow fishing), & many areas are undermined by insufficient human & financial capacity101. - Improving the effectiveness of MPAs requires enhanced resourcing, governance, level of protection & siting to better match the geography of threats & to ensure desired outcomes. - The current surge in restoration efforts can, if sustained, be an instrumental recovery wedge to meet rebuilding targets for marine habitats by 2050. - For instance, assuming a mean project size of 4,197 ha, restoring mangroves to their original extent of 225,000 km2 by 2050 would require the initiation of 70 projects per year. - This is not unrealistic, as realization of the benefits, such as reducing storm damage in low-lying areas, encourages further growth in restoration efforts. - Past coastal restoration projects have reported average success rates ranging from 38% (seagrass) to 64% (salt-marshes & corals); however, reasons for failure are well understood, which should improve future outcomes. - Much can be learned from increased reporting of failed attempts, because the published literature may be biased towards successful restoration projects. - Emerging technologies are now being developed to restore coral species in the presence of climate change, although long-term testing is required before their effectiveness & lack of negative consequences are demonstrated. - Kelp restoration at a national scale in Japan provides a successful model, rooted in cultural practices, for linking restoration to sustainable fishing. - More broadly, these practices recognize that sustainable harvest of marine resources ought to be balanced by broader restoration actions embedded in a socio-ecological context, including reducing greenhouse gas emissions, restoring habitats, removing marine litter / managing hydrological flows to avoid hypoxia. - These restoration experiences also show that involvement of local communities is essential, because of their economic dependence, commitment to place & ownership. - Removing pollution is a critical recovery wedge for seagrass meadows, coral reefs & kelp forests. - Three decades of efforts to abate coastal eutrophication have provided valuable knowledge on how actionable science can guide restoration successes. - Additional interventions (for ex., restoring hydrological flows / rebuilding oyster reefs) can catalyse the additional removal of nutrients while improving biodiversity. - Seaweed aquaculture can help to alleviate eutrophication & reduce hypoxia. - Nutrient reduction has the additional benefit of locally reducing coastal acidification & hypoxia directly & indirectly through the recovery of seagrass meadows. - Reducing sulfur dioxide precipitation, hypoxia, eutrophication, emissions & runoff from acidic fertilizers also helps to reduce acidification of coastal waters. - Large-scale experiments in anoxic basins of the Baltic Sea, for ex., have shown that treatment of sediments with phosphorus-binding agents helps to break biogeochemical feedback loops that keep ecosystems in an alternative anoxic stable state. - Oil spills from oil tankers should decline further with the incoming International Maritime Organisation (IMO) requirement (13F of Annex 1 of MARPOL) for double hulls in new large oil tankers, although deepwater drilling, illustrated by the catastrophic Deepwater Horizon spill in 2010, & increasing risks of oil spills from future oil drilling & oil tanker routes in the Arctic present new challenges. - Noise pollution from shipping & other industrial activities, such as drilling, pile driving & seismic surveys, should be reduced. - Similarly, worldwide efforts to reduce / ban single-use plastic (initiated in developing nations), taxes on plastic bags, deposits & refunds on bottles, & other market-based instruments are being deployed to reduce marine litter, while providing incentives to build a circular economy for existing plastics while developing safer materials.

Reduction in pollution

- Time-series analyses show that legacy persistent organic pollutants have declined even in marine environments that tend to accumulate them (for example, the Arctic). - The transition towards unleaded gasoline since the 1980s has reduced lead concentrations to concentrations comparable to baseline levels across the global ocean by 2010-2011. - Similarly, the total ban in 2008 of the antifouling chemical tributyltin (TBT) has led to rapid declines of imposex (females that develop male sexual organs)—a TBT-specific symptom—in an indicator gastropod. - Improved safety regulations have also led to a 14-fold reduction in large oil spills from oil tankers from 24.7 events per year in the 1970s to 1.7 events per year in 2010-2019. - Whereas evidence of improved coastal water quality following nutrient reductions was equivocal a decade ago, multiple success stories have now been confirmed, with positive ecosystem effects such as the net recovery of seagrass meadows in the United States, Europe, the Baltic Sea & Japan

Recovery of fish stocks

- Using a comprehensive stock-assessment database, we find that fish stocks with available scientific assessments are increasingly managed for sustainability. - The proportion of stocks with fishing mortality estimates (F) below the level that would produce a maximum sustainable yield (F < FMSY) has increased from 60% in 2000 to 68% in 2012. - Many fish stocks that are subject to such management interventions display positive trends, & globally aggregated stock assessments suggest a slowing down of the depletion of fish stocks, although this trend cannot be verified for the majority of stocks, which lack scientific assessments. - The most recent report of the Food & Agriculture Organization on global fisheries also suggests that two thirds of large-scale commercial fish stocks are exploited at sustainable rates—although, again, this figure does also not account for smaller stocks / non-target bycatch species, which are often not assessed & in poor condition. - Available data suggests that scientifically assessed stocks generally have a better likelihood of recovery owing to improved management & regulatory status compared with unassessed stocks, which still represent the majority of exploited fish stocks, especially in developing countries.

Recovery to date

Reductions to the risk of extinction. - The proportion of marine species assessed by the IUCN (International Union for Conservation of Nature) Red List as threatened with global extinction has decreased from 18.0% in 2000 to 11.4% in 2019 (s.d. = 1.7%, n = 1,743), with trends being relatively uniform across ocean basins & guidelines. - In part, this reflects the growing number of species that have been assessed. - However, many assessed species have improved their threat status over the past decade. - For marine mammals, 47% of 124 well-assessed populations showed a significant increase over the past decades, with 40% unchanged & only 13% decreasing. - Some large marine species have exhibited particularly notable rebounds, even from the brink of extinction. - Humpback whales migrating from Antarctica to eastern Australia have been increasing at 10% to 13% per year, from a few hundred animals in 1968 to more than 40,000 currently. - Northern elephant seals recovered from about 20 breeding individualsin 1880 to more than 200,000 today, & grey seal populations have increased by 1,410% in eastern Canada & 823% in the Baltic Sea since 1977. - Southern sea otters have grown from about 50 individuals in 1911 to several thousand at present. - While still endangered, most sea turtle populations for which trends are available are increasing in size, with increases in green turtle nesting populations ranging from 4 to 14% per year.

Abstract

Sustainable Development Goal 14 of the United Nations aims to - "conserve & sustainably use the oceans, seas & marine resources for sustainable development". Achieving this goal will require - rebuilding the marine life-support systems that deliver the many benefits that society receives from a healthy ocean. - recovery of marine populations, habitats & ecosystems following past conservation interventions. - Recovery rates across studies suggest that substantial recovery of the abundance, structure & function of marine life [could be achieved] by 2050, if major pressures—including climate change—are mitigated. - Rebuilding marine life represents a doable Grand Challenge for humanity, an ethical obligation & a smart economic objective to achieve a sustainable future