Food supplies that drive fish productivity: Modeling prey flux in riverine networks

Physical characteristics of riverine ecosystems (e.g., wood abundance, pool geometry, flow conditions) have traditionally been thought to constrain fish production. How food quantity and quality control fish productivity has received far less attention, although food can be an important productivity driver. Classic food web illustrations suggest most prey consumed by fish are produced locally within the stream, but we propose the majority of food is derived from external and sometimes very distant sources - including subsidies from marine systems, fishless headwater tributaries, and from adjacent streamside vegetation and associated habitats. We developed a model that shows key trophic pathways and food supplies vary through time and space throughout watersheds, with marine food supplies commonly abundant in floodplains and headwater food sources more important further up drainages.  Climate change and resource management changes prey flux to rivers in many ways, including by changing riparian plant cover and associated invertebrate communities, and by affecting the amount of anadromous fish that return to fresh water. Insights into how food supplies affect fish species can help guide how we manage riverine ecosystems, since their structure and function appears to be determined by strong interactions with both marine and terrestrial systems.