With a current estimate of ~1,000 million tons, mesopelagic fishes likely dominate the world total fishes biomass. However, recent acoustic observations show that mesopelagic fishes biomass could be significantly larger than the current estimate. Here we combine modelling and a sensitivity analysis of the acoustic observations from the Malaspina 2010 Circumnavigation Expedition to show that the previous estimate needs to be revised to at least one order of magnitude higher. We show that there is a close relationship between the open ocean fishes biomass and primary production, and that the energy transfer efficiency from phytoplankton to mesopelagic fishes in the open ocean is higher than what is typically assumed. Our results indicate that the role of mesopelagic fishes in oceanic ecosystems and global ocean biogeochemical cycles needs to be revised as they may be respiring ~10% of the primary production in deep waters.

The UN Sustainable Development Goal 14 aims to "conserve and sustainably use the oceans, seas and marine resources for sustainable development". Achieving this goal will require rebuilding the marine life-support systems that deliver the many benefits society receives from a healthy ocean. In this Review we document the recovery of marine populations, habitats and ecosystems following past conservation interventions. Recovery rates across studies suggest that substantial recovery of the abundance, structure, and function of marine life could be achieved by 2050, should major pressures, including climate change, be mitigated. Rebuilding marine life represents a doable Grand Challenge for humanity, an ethical obligation, and a smart economic objective to achieve a sustainable future. The ability of the ocean to support human wellbeing is at a crossroads. The ocean currently contributes 2.5% of global GDP and provides employment to 1.5% of the global workforce 1 , with an estimated output of US$1.5 trillion in 2010, expected to double by 2030 1. And there is increased attention on the ocean as a source of food and water 2 , clean energy 1 , and as a means to mitigate climate change 3,4. At the same time, many marine species, habitats and ecosystems have suffered catastrophic declines 5-8 and climate change is further undermining ocean productivity and biodiversity 9-14 (Fig. 1). The conflict between growing human dependence on ocean resources and declining marine life under human pressures (Fig. 1) is focusing unprecedented attention on the connection between ocean conservation and human well-being 15. The UN Sustainable Development Goal 14 (SDG14 or "life below water") aims to "conserve and sustainably use the oceans, seas and marine resources for sustainable development" (https://sustainabledevelopment.un.org/sdg14). Achieving this goal will require rebuilding marine life, defined in the context of SDG14 as the life-support systems (populations, habitats, and ecosystems) that deliver the many benefits society receives from a healthy ocean 16,17. Here we show that, in addition to being a necessary goal, substantially rebuilding marine life within a human generation is largely achievable, if the required actions, prominently mitigating climate change, are deployed at scale. Slowing the decline of marine life and achieving net gains By the time the general public admired life below water through the "Undersea World of Jacques Cousteau" (1968-1976), the abundance of large marine animals was already greatly reduced 5-7,18. And the abundance of marine animals and habitats that support ecosystems services has shrunk to a fraction of what was in place when the first frameworks to conserve and sustain marine life were introduced in the 1980s (Fig. 1), to a fraction of pre-exploitation levels 5,6,19,20. Currently, at least one-third of fish stocks are overfished 21 , one-third to half of vulnerable marine habitats have been lost 8 , a substantial fraction of the coastal ocean suffers from pollution, eutrophication, oxygen d...

In this article, we review evidence of how climate change has already resulted in clearly discernable changes in marine Arctic ecosystems. After defining the term 'footprint' and evaluating the availability of reliable baseline information we review the published literature to synthesize the footprints of climate change impacts in marine Arctic ecosystems reported as of mid-2009. We found a total of 51 reports of documented changes in Arctic marine biota in response to climate change. Among the responses evaluated were range shifts and changes in abundance, growth/condition, behaviour/phenology and community/regime shifts. Most reports concerned marine mammals, particularly polar bears, and fish. The number of well-documented changes in planktonic and benthic systems was surprisingly low. Evident losses of endemic species in the Arctic Ocean, and in ice algae production and associated community remained difficult to evaluate due to the lack of quantitative reports of its abundance and distribution. Very few footprints of climate change were reported in the literature from regions such as the wide Siberian shelf and the central Arctic Ocean due to the limited research effort made in these ecosystems. Despite the alarming nature of warming and its strong potential effects in the Arctic Ocean the research effort evaluating the impacts of climate change in this region is rather limited.

Community respiration ( R ) rates are scaled as the two-thirds power of the gross primary production ( P ) rates of aquatic ecosystems, indicating that the role of aquatic biota as carbon dioxide sources or sinks depends on its productivity. Unproductive aquatic ecosystems support a disproportionately higher respiration rate than that of productive aquatic ecosystems, tend to be heterotrophic ( R > P ), and act as carbon dioxide sources. The average P required for aquatic ecosystems to become autotrophic ( P > R ) is over an order of magnitude greater for marshes than for the open sea. Although four-fifths of the upper ocean is expected to be net heterotrophic, this carbon demand can be balanced by the excess production over the remaining one-fifth of the ocean.