Can Spatial Refuges Mediate the Effects of Altered Thermal Regimes for Juvenile Salmon in Streams?

Temperature is a key driver of many ecological processes in streams, and controls growth and development rates in juvenile salmon. Climate change is expected to alter stream thermal regimes in the Pacific Northwest by increasing water temperatures and by altering temporal trends. A warmer climate will likely decrease connections among cool water refuges. Although fish can seek cooler water, behavioral thermoregulation becomes challenging as distance between refuges increases. In addition, altered thermal regimes influence biological rates like growth and metabolism. This may either directly decrease survival of juvenile salmon or it may indirectly affect fitness through altered behavior such as outmigration timing. Given that water temperature is patchy in streams, we hypothesized that certain network topologies may afford greater opportunities for fish to seek cool water refuges in streams experiencing climate change. Here, we describe a framework for assessing (1) how network topology influences the way that connectivity of cool water refuges is expected to change as the climate warms, and (2) whether survival and outmigration timing of juvenile salmon might respond to climate change differently in different types of networks. To address the first objective, we use a spatially explicit statistical approach to evaluate what features of stream networks best predict distances between cool water refuges. For instance, does inter-refuge distance decrease when networks are more highly branched or compact? For the second objective, we use an individual-based simulation approach to evaluate the response of egg-to-smolt survival and outmigration timing to thermal regimes expected under climate change in stream networks having different spatial properties.  Results suggest hypotheses that can be tested experimentally, and identify parameters that should be monitored in the field. More immediately, this approach characterizes which network topologies maximize protection for fish from predicted effects of climate change, and can help to prioritize conservation and restoration opportunities.