What would reef fish communities look like without humans? Effective ecosystem management and con- servation requires a clear understanding of community structure and the processes that drive it. Relatively undisturbed reef fish communities appear to be inverted biomass pyramids (IBPs) with greater biomass of large-bodied predatory fishes compared to smaller fishes at lower trophic-levels. However, the processes that might give rise to IBPs are subject to debate. In this thesis I show that biomass pyramids and size spectra are equivalent and interchangeable representations of community structure. Key constraints on the slopes of size spectra – particularly mean community predator-to-prey-mass ratio (PPMR) – also constrain the shapes of biomass pyramids, meaning that IBPs are unlikely for closed communities. There are surprisingly few quantitative descriptions of biomass pyramids, and PPMR has not been estimated on reefs. I undertook a detailed case-study and quantify fish community size-structure using underwater vi- sual surveys and empirically estimate PPMR using stable isotopes at a relatively undisturbed island chain in Haida Gwaii, BC. I observe an IBP, but the PPMR estimate suggests that the community should be a stack or bottom-heavy. There is 4-5 times more biomass at the largest body-sizes than would be expected given observed PPMR. I hypothesise that the most plausible explanation is energetic subsidies. Using the same fish assemblage I show how two foundational components of habitat complexity (substrate rugosity and kelp canopy characteristics) shape fish community size-structure. Higher kelp canopy cover and den- sity leads to more biomass across all size classes, whereas higher substrate rugosity boosts the biomass of smaller-bodied fishes and leads to a more even distribution of biomass across size classes. Finally, I step back to the global scale and estimate baseline biomass spectra for the world’s reef fishes, accounting for local ecological variation. Current reef fish biomass is less than half of the baseline expectation and 90% of the largest (> 1 kg), most functionally-important, individuals are absent. In addition to providing the first global description of how humans have shaped reef biomass pyramids, my thesis gives new insight into how size-based processes underlie the structure and function reef fish communities
