Portfolio Effects for Resident Trout: Implications for Management

The portfolio effect suggests that non-concordance in the impact of environmental variation across habitats and populations is fundamental to long-term persistence and resilience.  Using data from a long-term (>15 year) individual-based study of a resident brook trout population, we assessed the potential for spatial variation in environmental effects (season-specific  annual variation in stream flow and stream temperature) on population vital rates  to support a portfolio effect.  We found that the effects of flow and temperature differed systematically in magnitude and in some cases in direction between headwater tributary (1^st order) and mainstem (3^rd order) habitats.  Similar, the success of individuals that were mainstem-resident, tributary-resident, or that moved between tributaries and the mainstem  also differed markedly between years characterized by different flow and temperature regimes.  As required for a portfolio effect, ‘good’ years in some habitats and for some life-history strategies, were ‘bad’ years for others.  Given that some of the differences among tributaries and mainstems we observed are likely to be broadly applicable, we suggest that management strategies should consider the maintenance of longitudinal diversity when identify and prioritizing conservation and restoration actions for resident trout.