The magnitude and extent of Chinook Salmon straying from hatcheries in Southern British Columbia

Abstract

Hatchery produced salmon have provided significant socio-economic benefit as well as conservation value in some cases. However, high levels of hatchery production also have the potential to create ecological and genetic risks for natural populations. In this report, we focus on one of those risks, straying of hatchery-origin, ocean-type Chinook Salmon into non-natal rivers in southern British Columbia. We compiled coded wire tag (CWT) and otolith thermal mark data to assess straying of first generation hatchery Chinook. The geographic scope included the west coast of Vancouver Island (WCVI), the east coast of Vancouver Island (ECVI), and the Fraser River. First we reviewed ‘donor’ stray rates. Based on otolith thermal marks we estimated an average hatchery ‘donor’ stray rate to non-natal rivers of approximately 4%. The donor stray rate to non-natal Conservation Units was lower, at less than 2% across populations and years. Straying between geographic regions of WCVI and ECVI was negligible. Conuma Hatchery on the WCVI had a significant stray rate, which resulted in the greatest magnitude of strays to other rivers. Even low stray rates from large hatcheries can result in significant interbreeding with non-natal populations leading to a homogenization of spawning populations. We identified the contribution of strayed, hatchery-origin spawners (pHOSstray) in the escapement, as a key metric to assess the impact of hatchery strays on natural-origin populations and compared observed values to a 0.03 benchmark proposed for wild populations by Withler et al. (2018). Average pHOSstray was highest on the WCVI, with most strays originating from Conuma Hatchery. Lower values, mostly below the 0.03 benchmark were observed on the ECVI, and a negligible stray contribution was observed in Fraser River populations. Genetic analysis of population composition from 1985–2015 indicated that the genetic variation within each of the three major WCVI hatchery populations remained relatively consistent from the late 1980s to 2015 (the last year of analysis). The same could not be said for most other spawning populations along the WCVI. Hatchery populations contributed significantly to natural spawners in the surrounding populations of the WCVI, which resulted in genetic homogenization. However, the analysis also showed that local population genetics are still visible in many WCVI rivers, which suggests there should be a rapid transition to managing hatchery abundance under a proportionate natural influence (PNI) framework. This would include increased use of mass marking to identify hatchery fish as well as marking techniques such as the use of parentage-based tags (or thermal marking) to assess and manage pHOSstray. Delineating the effect of environmental conditions and hatchery practices as causal factors influencing stray rate is difficult considering the multiple scales at which environmental factors act, and a dearth of local, fine-scale data. Local knowledge often provided logical explanations in understanding straying patterns. It is important that hatchery practices place an emphasis on imprinting and improved homing. In addition, monitoring both straying and key environmental conditions should become routine, especially in regions where there is considerable hatchery enhancement. The Hatchery Scientific Review Group (HSRG 2017, 2020), Anderson et al. (2020), and Withler et al. (2018) all indicate a need for the adaptive and scientifically defensible management of hatcheries, with improved annual planning, clear objectives, monitoring, assessment, and review. We support this view, and provide as a starting point, a data compilation and profile for each assessed river/Chinook population. These profiles include trends and data with respect to donor straying and recipient impacts. We hope these profiles will facilitate collection of local knowledge and support future efforts in the management of straying and genetic diversity, one river at a time.