High dominance renders reef fish trophic interactions vulnerable to human pressures in oceanic islands

Abstract

Human activities alter marine ecosystems by modifying their biodiversity, often disrupting trophic interactions that determine their structure and functioning. Human influence on trophic interactions mediated by fishes have traditionally been investigated through the lens of indirect proxies, such as the biomass of trophic groups. However, fish biomass alone may not adequately capture the nuanced responses of fish to different levels of human pressures, such as changes in the nature and intensity of trophic interactions. Here, we combined visual counts and remote video surveys to assess how human pressures influence spatial patterns in reef fish biomass and feeding pressure in an oceanic island in the eastern Atlantic Ocean. The biomass and feeding pressure of reef fish assemblages strongly declined across the human pressure gradient, by two- and five-fold, respectively. These patterns were primarily driven by fishery targeted species, which comprised 87 % of fish biomass and 93 % of fish feeding pressure. Despite this marked imprint of human pressures, we found distinct responses among trophic groups. The biomass of omnivores and herbivores declined by 19- and 3-fold respectively, while feeding pressure by these groups declined by 6- and 4-fold, respectively. In contrast, the effect of human pressures on piscivores, invertivores and planktivores was weak or negligible. Importantly, some trophic groups exhibited stronger declines in fish feeding pressure than biomass, whilst others exhibited lower declines. This highlights the nuanced responses of fishes to human exploitation, with compensatory mechanisms, and calls for more detailed studies to identify how humans disrupt trophic interactions and their potential flow on effects on energy and material fluxes.