Erik Olsen, member of the Working Group on Integrated Assessments of the North Sea (WGINOSE)
'When studying how we humans affect the marine environment or how the environment responds to natural changes, it is not practical to measure everything. We therefore often choose a more manageable number of factors to measures, and ensure that these are representative of the whole ecosystem. We coin these 'indicators'. We measure what the current status of the ecosystem is; e.g. how large the various fish stocks are, the average temperature in March, water-inflow into the North Sea. These are all measures of the state of the system, and are hence called state indicators. Similarly we're also interested in monitoring and measuring human activities in the marine environment, e.g. time spent fishing, the number of windmills placed, kilometres of pipeline laid out. These measures of human use reflect the pressure our activities exert on the environment and are hence called pressure indicators.'