Growth an mortality risk of Fish are influenced by water chemistry, the physical nature of their habitat and a wide range of predators, prey, diseases and parasites. These factors are key determinants of the variations in population strength that ultimately influence the success of fisheries. Long-term monitoring, both of habitat and fish populations, has been extremely important for understanding and hence managing fisheries effectively. The value of such monitoring is further enhanced when combined with experimental studies that shed additional light on the processes that connect variation in habitat to growth and survival of fish.
Monitoring is of value only if it fullfils a number of important criteria. It should be strategically planned, with a view to providing temporal and spatial coverage appropriate to well-defined aims. It must also be of appropriate quality, otherwise much power of inference is lost. We term such activities "scientific monitoring" since the aim is to provide information for robust scientific analysis. Such monitoring ranges from very carefully deployed networks of precision instruments to less controlled, but nevertheless valuable, provision of rod catches by fisheries proprietors.