Consequences of Salmon Hatch Timing in Streams with Variable Thermal Regimes in the Copper River Delta, Alaska

Temperature is an important factor influencing the timing of Pacific salmon life history events because it directly affects development rates.  Salmon have complex lifecycles and phenology is a crucial determinant of individual fitness because transition timing to new life stages and habitats will place constraints on subsequent growth opportunities. Although we know that temperature and phenology interact to affect salmon fitness, little work has been done to elucidate these mechanisms empirically. We tested hypotheses relating stream thermal regimes to Coho Salmon (Oncorhynchus kisutch) hatch timing and subsequent juvenile growth.  Hatch timing and growth was measured in five streams with variable thermal regimes on the Copper River Delta, Alaska from May-October in 2013 and 2014.  Thermal regimes ranged from groundwater streams with relatively constant annual temperatures (3-5°C), to surface-water streams with much greater thermal variation (0- 17°C). Coho Salmon hatched 2-4 weeks earlier in groundwater streams, likely due to greater accumulated thermal units during egg incubation in winter. Juvenile growth rates were nearly twice as fast in groundwater streams, suggesting these systems offer better growth opportunities for developing salmon.  Understanding the interactive effects of temperature, phenology and fitness is important as climate change continues to alter thermal regimes worldwide.