1. Implementing an ecosystem approach to fisheries management requires an effective ecosystem monitoring programme, the utility of which depends upon its ability (measured by the statistical power) to detect effects that trigger management action. 2. Using data from a long-term ecosystem monitoring programme of the predators of Antarctic krill Euphausia superba at South Georgia together with a krill population model to simulate natural and fisheries induced variability in krill abundance, the power to detect the effects of different levels of fishing was examined. 3. The power to detect the effects of fishing using either the krill population or a combined predator response index was low (20–40% power after 20 years with the probability of a type I error (α) = 0.05). The power increased to >50% when α was increased to 0.2 when the ability to detect change was greater with the predator response index than using the krill population itself. 4. The results indicate that although this monitoring programme has a proven ability to detect the effects of natural variability in krill abundance, its ability to detect the effects of fishing may be limited if there is a requirement for statistical significance at the 95% level. A situation where changing α produces a marked increase in statistical power, and the difference in the relative ecological costs of making type I and type II errors is likely to be high, may require a more flexible approach to choosing significance levels required to trigger management action. 5. Although long-term monitoring provides a wealth of basic ecological information it is essential to evaluate, the ability to detect specific changes in order that management action is not delayed because of an inability to detect an effect rather than the lack of an effect of the fishery.