Annual Depth and Temperature Selection by Fishes in Toronto Harbour

Coastal wetlands represent important habitats for warmwater fishes in the Laurentian Great Lakes. Restoration efforts here seek to improve habitat conditions for key native species, but are often focused on spawning and nursery habitats.  Differences in habitat selection across seasons are poorly understood, particularly for winter.  Our objective was to track seasonal depth and temperature selection by key freshwater fish species in Toronto Harbour, a heavily modified coastal embayment where extensive restoration efforts are ongoing.  We used acoustic telemetry to monitor habitat selection for two top predators, largemouth bass (Micropterus salmoides) and northern pike (Esox lucius), and for common carp (Cyprinus Carpio), a nonative species comprising >50% of the harbour’s fish biomass.  All species demonstrated shifts in depth selection with changes in temperature.  Bass depths were relatively constant in summer (~1 m) and winter (~2 m), but were more variable in spring and fall.   Bass explored deeper waters in fall, when depth selection was most variable.  Carp also occupied shallow depths in summer but, in contrast to other species, were found in deepest depths in fall and early winter, then moved shallower mid-winter.  Pike were found in the deepest waters in winter (~5 m) and displayed highly variable depth selection in spring and fall.  Results will inform habitat creation efforts and timing windows for spring construction activities.