“Integrating genetic and demographic effects of connectivity on population stability: the case of hatchery trucking in salmon”
Allison G. Dedrick and Marissa L. Baskett (Aug 2018)
Just as investing money in multiple stocks can provide more stable returns than investing in one stock, having multiple sub-populations that act independently can lead to a more stable overall population in a natural system, a phenomenon known as the “portfolio effect”. The ability of a population to behave as a portfolio, however, might decrease with increased connectivity (exchange) between sub-populations due to two processes. First, connectivity might directly couple sub-populations. Second, connectivity and increased interbreeding between sub-populations might make sub-populations more genetically similar in traits that influence the potential for independent responses to changes in the environment in time. An example of the potential for increased connectivity to decrease the portfolio effect is Chinook salmon in the California Central Valley. Salmon populations are distinct in freshwater, where fish hatch and rear, then combine in the ocean before separating again when adult salmon return to rivers to breed. The salmon populations once showed a strong portfolio that has become weaker in recent decades, including a recent fishery collapse due to all sub-populations declining simultaneously. One possible reason for the lost portfolio is the practice of trucking juvenile hatchery fish downstream for release. Trucking juvenile fish increases survival by bypassing the high-mortality migration route from rivers to the ocean, but it also increases connectivity among rivers because the fish do not have the experience to know which river to return to as breeding adults. Using a model, the authors find that trucking can lead to a tradeoff between an increased average population size and a weakened portfolio. This tradeoff is substantially stronger, and the capacity for a weakened portfolio greater, when accounting for the genetic effects of increased connectivity than when ignoring genetic effects. Therefore, restoration of genetic diversity might be central to restoring the portfolio effect in Central Valley Chinook salmon.
Connectivity among populations can have counteracting effects on population stability. Demographically, connectivity can rescue local populations but increase the synchrony across populations. Genetically, connectivity can counteract drift locally but homogenize genotypes across populations. Population independence and diversity underlies system-level buffering against environmental variability, termed the portfolio effect. The portfolio effect has declined in California fall-run Chinook salmon, possibly in part due to the trucking of juvenile hatchery-reared fish for downstream release, which reduces juvenile mortality but increases the connectivity between rivers. We use a dynamical population model to test whether this increased connectivity can explain the loss of the portfolio effect and quantify the relative demographic and genetic contributions to portfolio effect erosion. In the model, populations experience different within-population environmental conditions and the same time-variable ocean conditions, the response to which can depend on a quantitative genetic trait. We find that increased trucking for one population's hatchery can lead to a loss of the portfolio effect, with a system-level trade-off between increased average abundance and increased variability in abundance. This trade-off is much stronger when we include the effects of genetic homogenization than when we consider demographic synchronization alone. Therefore, genetic homogenization can outweigh demographic synchrony in determining the system-level effect of connectivity.