Limited effects of management and ecological subsidies on the size-spectra of kelp forest fish communities

Abstract

Animal body size influences key ecological processes across biological hierarchies. For instance, densities (N) and community biomass (B) are allometric functions of body mass (M). Energetic equivalence predicts that density scales with body size as N ∝ M-0.75 and that biomass scales with body size as B ∝ M0.25. However, the way fish size-spectra are influenced by external processes, such as ecological subsidies (e.g. nutrients from upwelling zones) and fisheries management, is not well understood. We investigated the relationship of body size with density and biomass of reef fishes associated with subtidal kelp forests of Lessonia trabeculata that were influenced by the separate and interactive effects of management (Territorial Use Rights for Fisheries [TURF] or open access) and upwelling regimes (upwelling or non-upwelling zones). Fish densities and lengths were recorded using underwater visual censuses. Within each of 4 locations, paired TURF and open-access sites were surveyed. We surveyed 18 fish species, encompassing 1511 individuals ranging between 2 and 6639 g. We observed that fish size-spectra deviated from energetic equivalence, as N ∝ M-0.32 and B ∝ M2.2, indicating that the contribution of large-sized fishes to community density and biomass was greater than that predicted by energetic equivalence. Multi-model inference suggested that TURF and upwelling scenarios had weak effects on fish size-spectra. Results indicated that fish communities may have access to external food sources beyond local kelp forests. In addition, size-spectra may be a spatially persistent attribute of these fish communities.