The abundance of herbivorous fishes is known to vary strongly with latitude, yet our understanding of this pattern is often based on the examination of nominally herbivorous fishes (both herbivores and detritivores) as a single group. We do not know how this collective classification could have confounded our understanding of distribution patterns, nor how different trophic pathways function across latitudes and associated temperature gradients. This constrains our ability to predict how tropicalising reefs may function as oceans warm and taxa extend their ranges. To address this, we explore the productivity of seven groups of nominally herbivorous fishes across ~3800 km in eastern and ~2600 km in western Australia, with specific consideration of the herbivore versus detritivore dichotomy. By separating detritivores from herbivores, we reveal that detritivore productivity dominates the tropics but declines abruptly in temperate regions, with this decline closely correlated with decreasing temperature in a synchronous manner along both coastlines. No such synchronous relationship was observed between herbivore productivity and temperature, with herbivore productivity dominating in subtropical regions. We emphasise the importance of the herbivore versus detritivore division in understanding reef trophodynamics, and the importance of understanding fish-based detritivory when predicting how reef trophodynamics may change along warming coastlines.