Size-based insights into fish community structure: a kelp forest case study and Southern Ocean applications

Reducing uncertainty in ecosystem management and conservation is a great challenge, particularly if community structure and the processes that drive it are poorly understood. Size-based analyses and modelling offer new... [ view full abstract ]

Reducing uncertainty in ecosystem management and conservation is a great challenge, particularly if community structure and the processes that drive it are poorly understood. Size-based analyses and modelling offer new opportunities for simplifying and resolving key uncertainties in the structure and function of food webs, and how they might be affected by indirect effects arising from perturbations. We provide an overview of results from a rocky reef kelp forest fish community at a remote island chain off the north-western coast of British Columbia, Canada. In this system, visual surveys and stable isotope analyses indicated a strongly positive biomass spectrum slope (i.e. a trend for more biomass at large vs. size-classes – an inverted biomass pyramid) and a predator-to-prey mass ratio (PPMR) in the order of 1650. These results present a paradox, because size spectra theory predicts that bottom-heavy biomass pyramids or ‘stacks’ should predominate in real world communities if trophic-level increases with body-size (mean PPMR >1). We hypothesise that this mismatch could arise from energetic subsidies in the form of movement of mobile consumers across habitats, and from seasonal pulsed production inputs at small body-sizes. Such discontinuities between empirical pattern and expectations from ecological processes provide a measure of the magnitude of cross-boundary subsidies. Such size-based insights into the structure of marine communities can be applied more broadly. In the Southern Ocean, mesopelagic fish and squid play a key role transferring energy from zooplankton to higher predators, and several species of fish are of commercial importance. However, the sensitivities of mesopelagic fish and squid to the combined impacts of fishing and climate change are poorly understood. Model representations of these groups – which will inform our understanding of their current status and potential future trends – are currently lacking. We provide an overview of a new program of research that will use size-based models related to pelagic habitat variables to advance our understanding of the role of mesopelagic species in Southern Ocean ecosystems and inform the development of indicators for ecological change.