Freshwater fishes residing in a coastal embayment are largely buffered from thermal upwelling events.

Upwelling events are common in thermally stratified marine and freshwater systems and can lead to rapid changes in water temperature that presumably impact fishes. Here we examined the effect of rapid changes in water temperature (upwelling events) on the behaviour of two wild fish species from different thermal guilds: Northern Pike (Esox lucius), representing coolwater fishes, and Largemouth Bass (Micropterus nigricans), representing warmwater fishes. This study was conducted in Toronto Harbour, a coastal embayment on the north shore of Lake Ontario, which experiences frequent upwelling events during the summer. Acoustic transmitters implanted in fish were used to calculate detailed depth and temperature occupancy, while detailed water temperature information from logging stations was used to describe the thermal characteristics of the area. Observed changes in internal fish temperature were generally small. During upwelling events, behaviour was variable, but generally involved the occupancy of sheltered habitats and shallow depths, where the cooling effect of the upwelling was minimal. When vertical changes in depth were observed, small vertical movements to shallower depths were more typical for both species. Largemouth Bass were found within the first 2m of depth with small temperature variability, but Northern Pike showed larger variability in depth and temperature occupancy. Our study suggests that the fishes studied here were largely buffered from the effects of upwellings by selecting depths and regions that enable them to experience temperatures consistent with their thermal preferences. However, it is also possible that fish were selecting those thermally stable habitats for other reasons (e.g., food, physical habitat shelter).