Effect of Cowichan Lake water management on Vancouver lamprey in British Columbia and identification of possible enhancement opportunities

Abstract

Vancouver Lamprey (also known as Cowichan Lake Lamprey; Entosphenus macrostomus) is listed as a threatened species under the Species at Risk Act (SARA). This species is endemic to three interconnected lakes on Vancouver Island, British Columbia. Cowichan Lake, one of the lakes in which this species resides, is a water reservoir managed to meet the downstream conservation and anthropogenic needs of the watershed. Currently Cowichan Lake water levels and outflows to the Cowichan River are controlled in part by the water control weir at the outlet of Cowichan Lake. Recent droughts in the Cowichan River watershed have resulted in lower than normal summer water levels in the lake which has resulted in reduced outflows to the river as well as dewatering of alluvial fans and lake margins. These dewatered areas include Vancouver Lamprey critical habitat, which is protected under SARA. Protection of Vancouver Lamprey’s critical habitat from destruction was accomplished in February 2020 through a SARA critical habitat order made under subsections 58(4) and (5), which invoked the prohibition in subsection 58(1) against the destruction of the identified critical habitat. It is worth noting that protections under SARA apply to all aquatic species listed as extirpated, endangered or threatened regardless of whether the species is on federal or provincial lands. Decreased lake water levels potentially destroy critical habitat and negatively impact spawning and rearing for this species. Fisheries and Oceans Canada’s (DFO) Fish and Fish Habitat Protection Program (FFHPP) has requested that Science Branch provide science advice on the effect water management has on Vancouver Lamprey spawning and rearing habitat, as well as identification of other conservation issues and enhancement opportunities for critical habitat. The assessment and advice arising from this Canadian Science Advisory Secretariat (CSAS) Science Response (SR) process will be used to inform resource managers on the water management of Cowichan Lake and support the development of current and new water use policy. It may also inform future water management decisions related to the design/operation of the new weir. The specific objectives of this review are to: 1. Evaluate what range of water levels are required to provide maximum protection to Vancouver Lamprey and its critical habitat by life stage for adults (spawning), eggs (incubation), and ammocoetes (feeding and rearing), and when these water levels are required by the species for these life stages (i.e. biologically significant periods). 2. Evaluate how much critical habitat (in square meters or hectares) is being affected by lake draw downs, and at what lake elevations critical habitat starts being affected within the normal range of current water management (full supply level and zero storage with minimum release of 7.08 m3/s into the Cowichan River). 3. Identify conservation issues for Vancouver Lamprey and the types of potential enhancement or rehabilitation opportunities that may exist for this species in the Cowichan Lake watershed. 4. Examine and identify uncertainties in the data and methods. This Science Response results from the regional peer review of January 18, 2023 on the effects of Cowichan Lake water management on Vancouver Lamprey in British Columbia and identification of possible enhancement opportunities.