Understanding How Northern North American Freshwater Fishes are Responding to Rapid Environmental Change

Many northern fishes are experiencing unprecedented environmental changes across their ranges. However, limited empirical evidence exists for understanding how these changes may combine and potentially interact to influence fish diversity, individual species loss and gains, and overall productivity at broad spatial scales relevant for informing evidence-based conservation and management efforts. \n \nThis dissertation investigated how northern fishes are being impacted by cumulative and potentially interacting environmental changes across broad spatial scales including climate change, water quality, and land use. I examined two main research questions including 1) what are the key environmental variables currently influencing northern freshwater fishes? And 2) how is climate interacting with other variables to produce non-additive effects (i.e., antagonistic or synergistic interactions). To do this, I used fish community data from small streams across relatively intact regions of Alaska, small boreal streams in a more developed region of Alberta, Canada, and subarctic lakes in a rapidly changing permafrost region in the lower Mackenzie River basin. \n \nWarming temperatures were linked to increased community diversity and abundance in small streams. However, this trend was not observed for fish communities in small subarctic lakes, where water quality degradation appeared to have a stronger role in regulating fish community health than direct warming. In all three chapters, there was evidence for localized gains and losses of individual fish species presence or relative abundance in association with mounting environmental changes that included warming, changes in precipitation, and land use. Some species may be temporarily benefiting from warmer conditions across their northern ranges, for example, Pacific salmon species including Coho Oncorhynchus kisutch, Chinook Oncorhynchus tshawytscha, and Sockeye Oncorhynchus nerka appeared to be experiencing a net distribution gain possibly due to warmer and longer growing seasons. In contrast, species declines associated with environmental changes were more likely to occur for Arctic specialists, species intolerant to environmental degradation, and large-bodied benthic species. Interactions between climate warming and land use were synergistic (i.e., the combined effect of both stressors was greater than the sum of their individual effects), highlighting the potential for amplified species declines under future warming and land use scenarios. Together, these results provide new information that can be used to inform improved northern conservation planning amid rapid environmental changes.

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