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Impacts of climate change on marine ecosystems and fisheries
K. M. Brander
DTU Aqua, Denmark
[email protected]
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.
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