Interconnected effects of water temperature and habitat on relative abundance of juvenile salmonids during the summer months in coastal urban streams

Juvenile Pacific salmon in urban streams face the double threat of habitat degradation and the effects of climate change. Increased water temperature and habitat alterations may compound to limit salmon distribution in fresh water. My thesis investigates which biotic and abiotic variables effect the abundance of anadromous and non-anadromous salmonid species in a dozen urbanized Metro Vancouver streams. I used Partial Least Squared Regression (PLSR) to elucidate which of the 31 habitat metrics measured were most strongly associated with relative abundance of coho (Oncorhynchus kisutch) and trout (O. mykiss, clarkii). Water temperature, even within an optimal range for growth, had a negative impact on relative abundance of coho and trout. Other key habitat metrics included a negative impact of fine sediments on coho, smaller channel sizes favored trout, and all juvenile salmonid abundance responded positively to a higher proportion of cobble. Non-anadromous trout may be disproportionally impacted by higher water temperatures and changes in substrate composition due to their resident life history tactic that limit their distributions options to their natal freshwater watershed. There was also a positive relationship between the percent impervious surface area of a watershed and higher values of temperature, canopy openness, and percent fines indicating that urbanization is likely a driver behind the lower relative abundance of these species in certain stream reaches. A better understanding of the interconnected mechanisms driving salmonid relative abundance in high-order urban streams has implications for prioritizing conservation strategies towards improving key habitat metrics relevant to urban ecosystems and for predicting salmonid species responses to future climate change.