Recovery potential assessment of lake whitefish (Coregonus clupeaformis), Lake Opeongo large-bodied designatable unit and Lake Opeongo small-bodied designatable unit

Abstract

The Lake Opeongo large-bodied and small-bodied designatable units (DUs) of Lake Whitefish (Coregonus clupeaformis) were assessed as Threatened by COSEWIC (Committee on the Status of Endangered Wildlife in Canada) as a unique species pair found only in Lake Opeongo, Algonquin Provincial Park, Ontario (Figure 1). The large-bodied and small-bodied DUs have likely evolved to use different ecological niches in the lake. Lake Opeongo is the only known lake where a small-bodied DU of Lake Whitefish persists despite the presence of Cisco (Coregonus artedi; introduced in 1948). The greatest threat to both DUs is the introduction of new invasive species that could disrupt the ecological processes that maintain divergence in the species pair. Invasive zooplankton and fishes occur near Lake Opeongo and could be introduced by human-mediated mechanisms. Other threats include existing introduced species, climate change, and possibly human disturbances. The impact of these threats is not well known; however, in other lakes, introduced species are implicated in the loss of other Lake Whitefish species pairs. Habitat features required for the adult life stage of both DUs include areas of deep, cold water (not exceeding 20°C) in the hypolimnion and nearshore areas over hard substrates < 10 m in depth for spawning activities; eggs, in general, require hard substrates and cold overwinter temperatures with extensive ice cover for proper development; larvae require warming, productive surface waters initially for feeding and growth. Lake Opeongo has a total area of 5,860 ha. The minimum area required to support the large-bodied DU was determined to be ~4,900 ha and ~1,200 ha for the small-bodied DU, suggesting Lake Opeongo has sufficient habitat to support both DUs. Two population modelling scenarios were considered: one where the two DUs are reproductively isolated, and one where a single population is characterized by two alternative life-history strategies. When the two DUs are reproductively isolated, the impact of harm was sensitive to a DU’s population trajectory. The large-bodied DU is most sensitive to the juvenile stage when experiencing population growth, and is most sensitive to the adult stage when the population is stable or declining. The small-bodied DU is similarly sensitive across life stages under most population growth scenarios, but is less sensitive for the adult life stage when experiencing population growth. The minimum viable population (MVP) size was estimated as ~1,400 to ~2,300 adult females for the large-bodied DU, and ~4,100 to ~8,700 adult females for the small-bodied DU depending on catastrophe rate. Several knowledge gaps exist around the biological nature of the DUs. Key knowledge gaps also exist around the population size and trajectory, particularly for the small-bodied DU, and the impacts of existing and future invasive species.