Recovery potential assessment for southern British Columbian chinook populations, Fraser and southern mainland chinook designatable units (1, 6, 13, and 15)

Abstract

Southern British Columbian Chinook (SBCC) Designatable Units (DU) were assessed by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in November 2020. Of those in the Fraser River and Southern British Columbia (BC) mainland, one was designated as Threatened and three were designated as Endangered. Abundances of these Chinook salmon DUs have declined or remained at a low level since the COSEWIC assessment. The assessed DUs are: Southern Mainland Boundary Bay, Ocean, Fall – Boundary Bay (DU1) Lower Fraser (LFR) River Ocean Summer – Maria Slough (DU6) South Thompson (STh) Stream Summer 1.3 (DU13) Lower Thompson (LTh) Stream Spring 1.2 (DU15) All four DUs have experienced high levels of hatchery enhancement which is why they were not included in COSEWIC (2019, 2020) and DFO (2020, 2021). Redds, the spawning nests constructed by Pacific salmon and other fish species, meet the definition of a “residence” under the Species at Risk Act (SARA). The COSEWIC threats calculator was used to assess 33 ongoing and future threats limiting the population recovery of each DU over the next three generations. Natural systems modifications, climate change and severe weather were the highest ranked threat categories for these four DUs. Invasive and problematic species, pollution, and agriculture and aquaculture, and biological resource use were ranked high for two of the four DUs. The overall threat ranking for all DUs was Extreme based on their number and severity. This ranking suggests that a decline of 71-100% is anticipated over the next 3 generations or 10 years, unless threats are mitigated. Climate change is anticipated to negatively impact all DUs including through interactions with other threats. Habitat shifting and alteration has occurred and will continue to occur through changes to physical and biological processes in both marine and freshwater environments. Predicting future changes in salmon productivity and abundance is challenging in the current era of rapidly changing conditions, as there is significant uncertainty in both the future state of natural environments, and the ability to mitigate anthropogenic effects. Natural systems modification was ranked a medium-high or high threat risk for all DUs largely due to pervasive water management and anthropogenic land-based ecosystem modifications related to agriculture, forestry, resource extraction, and linear/urban development activities. Two recovery targets were proposed for each of the four DUs: A survival target that approximates conditions such that a DU would not be characterized as Endangered or Threatened by COSEWIC. A recovery target at which the DU would no longer be considered to be at risk under COSEWIC guidelines. Each target consists of two benchmarks: generational average wild spawner abundance and the three-generation trend in wild spawner abundance. Additionally, genetically-based targets associated with hatchery impacts on wild populations should align with targets recommended by DFO (2018a). There were insufficient data to conduct forward projections of population trajectories for these four DUs. However, given the number and severity of threats facing these DUs, it is unlikely that they will reach either their survival or recovery targets in three generations under current conditions. Some data to assess relative population abundances exist, but productivity could not be monitored for all of the DUs. Data limitations with absolute abundance and exploitation rate estimates, basic demographic attributes of spawning populations, and the inability to estimate hatchery contributions of Chinook Salmon existed in all but one system. A list of mitigation measures was developed to address identified threats. These measures may increase survival or productivity but information was not available to assess their efficacy, nor their potential to increase the probability of meeting the recovery targets. A mitigation survey provided expert opinion on the relative importance of mitigations for recovery of each DU. These DUs are at great risk based on the threats assessment and the qualitative assessments of population trajectories. Further harm may continue to jeopardize recovery and survival. Therefore, to promote the survival and recovery of these DUs, it is recommended that all future and ongoing human-induced harm should be prevented. It is important to note that some activities in support of survival or recovery could result in harm but may have a net positive effect on the population and should be considered