Brook trout occupancy in rivers and streams of the Mixedwood Plains Ecozone, Ontario

Brook trout (Salvelinus fontinalis) are an iconic freshwater salmonid native to northeastern North America. They prefer cold, clean rivers and lakes, and have been culturally, socially and economically valued for decades. Overharvest and deforestation associated with European settlement in southern Ontario, Canada led to notable declines in brook trout abundance in rivers and streams by the late 1800s. Today, they are threatened by further habitat degradation, non-native species, and climate change. In this study, we document brook trout occupancy in rivers and streams of the Mixedwood Plains Ecozone in southern Ontario, between 1970-1980 and 2000-2010. We used generalised linear models to determine occupancy variation between the two time periods at five spatial scales spanning multiple watershed levels. We detected a decline in occupancy between time periods for the tertiary (from 23% to 17%) and quaternary (from 35% to 22%) watershed scales. In the Credit River-Sixteen Mile Creek watershed, mean occupancy decreased to a third of past estimate (from 31% to 10%). For a subset of 86 sites with overlapping sampling in both periods no temporal trend was detected, but a post-hoc analysis indicated low statistical power (0.07), which suggested that more intensive sampling would be needed to better quantify brook trout occupancy changes at this scale. The occupancies quantified for the past period in our study may serve as benchmarks for brook trout restoration efforts.

Automated summary

Brook trout, an iconic freshwater salmonid native to northeastern North America, are facing threats including habitat degradation, non-native species, and climate change. A study conducted in southern Ontario showed a decline in brook trout occupancy in rivers and streams between 1970-1980 and 2000-2010. This decline was observed at different watershed scales, with some areas experiencing a significant decrease in brook trout population. The findings of this study can serve as benchmarks for future restoration efforts.