Characterizing the flow regime in Brook Trout (Salvelinus fontinalis) incubation habitats and the implications for management in a hydro-regulated river

Abstract

Hydropower accounts for more than one third of Ontario Power Generation’s electrical production. Hydroelectric development often occurs on rivers that also support recreational fisheries. The construction and operation of dams, diversions and generating facilities unavoidably influence the ecological function of rivers. The Aguasabon River is a northern Canadian Shield river with major developments for water diversion, storage, and power generation. This river offers opportunity to examine the importance of vertical flows through the substrate at a Brook Trout (Salvelinus fontinalis) spawning area. The vertical and horizontal hydraulic gradients and subsequent water temperature changes are the subject of this study. Piezometers were used to monitor the river and subsurface water levels near Brook Trout redds during the spawning and incubation period under normal and increasing discharge conditions. The Brook Trout spawning area in the Aguasabon River experienced upwelling conditions for the entire monitoring period (Oct 28th, 2016 – Jan 13th, 2017) before water release at the Long Lake Control Dam (LLCD). Hyporheic temperatures declined gradually, remaining >3.7 °C. The river temperature in the winter before water release was 1.5 °C. Rapid increase in water level after discharge from above the LLCD resulted in the reversal of flow in the hyporheic zone. Negatives values of vertical and horizontal gradients occurred for up to 30 h between surface water and hyporheic water up to 1.8 m below the river substrate. The water temperature in all piezometers decreased in unison with water release. Shallow inshore piezometers showed the greatest change, ˃53 h at <1 °C, compared to both shallow and deep offshore piezometers, which never fell <1 °C. A controlled experiment was used to monitor time to hatching, larval emergence and survival of Brook Trout in a lab setting. Historical data show redd temperature during the Brook Trout incubation period (October-April) on the Aguasabon River was maximal after a 2013 discharge, which lowered the water temperature of Brook Trout redds to 0.3 °C, a drop of 5.3 °C. Recovery to pre-discharge temperatures took 72 h, after 40 h at ˂1°C. This worst-case cold treatment was selected for an experiment at the Dorion Fish Culture Station, Ontario. The treatment had no effect on survival of incubating Brook Trout eggs (n = 1020) compared to control (n = 1020). Survival from fertilization to hatching was high for both treatment and control replicates (>90%), but was considerably lower for fertilization to emergence (55%). The cold treatment did not decrease development time from fertilization to larval emergence. The hatchery study implies that the redd temperature change experienced on the Aguasabon River in 2013 neither decreased the survival nor delayed hatching and emergence of Brook Trout. An overall recommendation is that a staged discharge at the LLCD would lessen the reversal of flow in the hyporheic zone and the magnitude of temperature changes monitored at Brook Trout redds during 2013 and 2017.