Fishes play important roles in maintaining the resilience and recovery potential of coral reefs, yet our monitoring techniques rarely extend beyond counting the number of fishes in an ecosystem. The presence of fishes, however, does not mean that they are performing their functions in that location. The development of more sensitive tools, which complement traditional methods of monitoring coral reef fishes, may therefore reveal earlier signs of ecosystem changes, and provide an opportunity for pre-emptive responses. In this presentation I will reveal new, sensitive metrics of the ecosystem functions performed by fishes that allowed us to quantify subtle, yet destabilising, changes to the ecosystem on an inshore coral reef on the Great Barrier Reef. The findings clearly highlight that fish abundance does not equate to function on coral reefs, and that to fully understand the role played by fishes we should attempt to make direct process-based measurements. With herbivory proving to be among the most important functions performed by fishes on coral reefs, estimates of grazing and/or browsing rates appear to act as sensitive tools which can be used to detect subtle ecosystem degradation and may be critical in identifying the effects of disturbances prior to wide scale loss of fish species.