Potential Population Level Consequences of Network Structure for Headwater Fishes

 Many species of stream fishes are confined to headwater of streams. Headwaters are located at the upstream terminus of streams, but can be found throughout the stream network and range from headwaters that flow into increasingly larger streams (headwater tributaries) to headwaters that flow directly into larger river channels (adventitious tributaries). I explored the potential population level consequences of the overall network location of headwater streams for headwater fishes using a simulation modeling approach that combined movement and density dependence. Simulation results indicate that populations occupying adventitious tributaries can be less than half the size of those found in headwater tributaries, even when movement direction is random and movements are limited. Upstream bias and limiting movements offset some of the population size difference between headwater and adventitious tributaries. Downstream bias and increased movement increased differences in population size between headwater and adventitious tributaries. My results suggest that headwater tributaries may be more important than adventitious tributaries for the conservation of headwater fishes. More studies that quantify movements by all life stages of stream fishes are needed to fully understand the overall effects of stream network structure on population size and persistence of headwater stream fishes.