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D:\841166211.doc Impacts of climate change on marine ecosystems and fisheries K. M. Brander DTU Aqua, Denmark [email protected] Abstract Marine ecosystems have always been affected by changes in climate at timescales from decades to millions of years. Since the industrial revolution in the 19th century the increase in greenhouse gases has caused an accelerating rise in global temperature whose effects on marine biota can now be detected. The rising level of CO2 and consequent acidification of the oceans is having an impact on metabolism and calcification in many organisms, with damage to vulnerable ecosystems, such as coral reefs, already occurring. The pH of the oceans is already lower now than it has been for the past 600,000 years. We depend on the oceans and coastal seas for many ecosystem services (supporting, provisioning, regulating and cultural) and there is real cause for concern that these services will be damaged and degraded by climate change. Can we identify areas and ecosystems that are particularly vulnerable? How soon could climate have an impact on marine ecosystem services? How great is the impact likely to be under different scenarios of climate change? Do we have monitoring systems in place which will warn of impending changes and provide information which can be used to respond? Are there ways in which we can adapt to climate change and mitigate? The effects of climate change can be detected at individual, populations and ecosystem level. We require better understanding of the processes which act at all these levels, from experimental and field work in order to provide credible responses to the questions posed above. Climate change is not the only pressure which humans impose on the marine environment and in the short term it is probably not the one with the greatest impact. Fishing, habitat degradation, pollution and species introductions also have many undesirable consequences and they also interact with each other. In coastal and inland waters there are also effects of changes in land use, coastal defence, damming, flood control and alteration of waterways. 1 D:\841166211.doc Climate change can act directly on productivity of commercial fish species by altering growth, reproduction and other aspects of life history either positively or negatively. It can also act indirectly by enhancing or suppressing prey, predators and pests, with consequences for the structure and resilience of the whole ecosystem. The processes and pathways by which biological production is transferred through marine foodchains are longer and more complex than in terrestrial agriculture and the scope for experiments and research is far less. Fisheries production depends on net primary production (NPP) and the transfer of this production through the food chain to human consumption. Future NPP may increase in some high latitude regions due to warming and decreased ice cover, but the dynamics in low latitude regions are governed by different processes and production may decline due to reduced vertical mixing of the water column and hence reduced recycling of nutrients. Recent changes in distribution and productivity of a number of fish species can be ascribed with high confidence to regional climate variability, such as ENSO. The effects of fishing and of climate interact, because fishing reduces the age, size and geographic diversity of populations and the biodiversity of marine ecosystems, making both more sensitive to additional stresses, such as climate change. The frequency and intensity of extreme climate events is likely to have a major impact on future fisheries production in both inland and marine systems. Reducing fishing mortality in the majority of fisheries, which are currently fully exploited or overexploited, is the principal feasible means of reducing the impacts of climate change. 2