Prioritizing Which Salmon Populations to Re-Establish: We Opened It, Now How Long Will It Take?

Managers are faced with difficult choices about which extirpated populations of anadromous salmonids to try to re-establish, often by removing migration barriers. As habitats that have blocked access are being re-opened (e.g., the Elwha and Cedar rivers, WA), questions commonly asked include (1) How long will it take for newly opened habitats to be colonized by extant populations from downstream? (2) Should we wait or should we manually introduce fish upstream? and (3) What effect will these populations have on neighboring populations? We evaluated these questions using a graph theoretical approach for ‘metapopulations’ of Chinook salmon (spring run) in the Lower Columbia, Upper Willamette, Upper Columbia and Snake Rivers. We evaluated the influence of stray rate and the size of source populations on the potential time it takes for newly colonized populations to reach abundances estimated to have occurred before the barriers were erected. Using hypothetical scenarios, we compared how quickly a population grew when fish were allowed to recolonize naturally and when helped by manual introductions. We also evaluated how the recolonized population altered the spatial structure of the larger metapopulation over time.  We found that recolonization processes differed among these geographically distinct areas, but that in general, recolonization rates depended strongly on stray rates and on the size of source populations. When stray rates were low or source populations were small, demographic stochasticity limited the rate of recolonization possible. Recolonizing extirpated populations usually increased connectivity within the metapopulation. The degree of influence on connectivity was related to the size of recolonized populations, which was limited by the capacity of opened habitat to support populations. Our results can help identify which of a suite of candidate populations is most likely to rebuild quickly and to contribute the most to the stability of the metapopulation. This information will be invaluable for prioritizing conservation opportunities.