The roles of carrying capacity, immigration, and population synchrony on persistence of stream-resident cutthroat trout

Like many other species, cutthroat trout (Oncorhynchus clarki) have declined substantially relative to historic conditions, and many existing populations are isolated. To inform conservation and restoration efforts for stream-resident cutthroat trout, I built a stage-structured stochastic simulation model to explore the dynamics of populations in isolation or connected by immigration. Relatively small increases in carrying capacity substantially reduced extinction risk in small, isolated populations. In the sensitivity matrix, the two most important elements affecting population growth rate were the survival of subadults that became reproductively mature the next year and the survival of young-of-the-year fish. Increasing either of these should allow populations to recover more rapidly from disturbances and reduce extinction risk. When populations were allowed to interact with one another, relatively low immigration rates (=7 individuals/year) between populations substantially reduced extinction risk to those populations without sufficient space to maintain persistence when isolated. However, higher immigration rates were required to maintain persistence the more synchronous the dynamics of interacting populations became. The results highlight the importance of maintaining adequate space or increasing habitat capacity for isolated populations and the importance of connectivity and large population cores if remnant populations are to be maintained.