Science Advisory Report 2024/031

The Magnitude and Extent of Chinook Straying from Hatcheries in Southern British Columbia

Summary

• Hatchery-origin salmon provide significant socio-economic benefit as well as conservation value in some cases, but high levels of hatchery production can create fishery, ecological, and genetic risks for natural populations.
• The risks of hatchery-origin Chinook straying into non-natal rivers was the focus of this assessment. Straying was described from two distinct perspectives: from the populations in which strays originate, i.e., donor populations, and from the populations that receive them, i.e., recipient populations.
• This report describes first generation, hatchery-origin straying of 19 ocean-type populations and one stream-type Chinook Salmon population into non-natal rivers in Southern British Columbia using Coded Wire Tag (CWT) and otolith thermal mark data.
• Based on in-river samples, average hatchery donor stray rates were estimated at approximately 4% based on otolith thermal marks and 2% based on CWT samples.
• The donor stray rate to non-natal Conservation Units (CUs) was lower—less than 2% across populations and years—and was negligible between Stock Management Units (SMU).
• Lower stray rates from large hatchery releases can still result in a large magnitude of strays observed in smaller non-natal systems.
• Conuma Hatchery on the West Coast of Vancouver Island (WCVI) had a substantially higher donor stray rate; the highest among large-scale production hatcheries, which resulted in the greatest magnitude of strays to other rivers.
• Local and regional variability in environmental factors made separating their causal effect on stray rate from hatchery practices difficult. While the influence of hatchery practices from potential environmental factors were not statistically delineated in this work, the need for hatcheries to prioritize imprinting to reduce stray rates was emphasized.
• The contribution of strayed, hatchery-origin spawners (pHOS_stray) in the escapement was identified as a metric to assess the impact of strays on natural-origin populations; observed values were compared to a 0.03 benchmark established for wild populations.
• Average pHOS_stray was highest on the WCVI with most strays originating from Conuma Hatchery. Lower values, mostly below the 0.03 benchmark were observed on the East Coast of Vancouver Island (ECVI), and a negligible hatchery stray contribution was observed in Fraser River populations.
• Proportionate natural influence (PNI) is a metric used to evaluate and monitor the level of natural influence and genetic risk in integrated populations. On the WCVI, particularly in northern populations, PNI was low; ECVI had more populations with moderate to high PNI values; and the Fraser River contained many populations with consistently high PNIs.
• Genetic analysis indicated that genetic variation remained consistent for the three major hatchery populations within the WCVI from 1985–2015.
• On the WCVI, genetic results indicated that hatchery populations contributed significantly to natural spawners and resulted in increased genetic homogenization among non-natal rivers.
• The persistence of natal genetic signals in WCVI rivers that have experienced some level of homogenization suggests that enhanced contribution management, using PNI, is important to preserve wild influence and associated genetic diversity.
• Withler et al. (2018) and US Hatchery Scientific Review Group (HSRG; 2009) publications indicated a need for the adaptive and scientifically defensible management of hatcheries. The results presented in this report support this view, and suggest that the inclusion of a stray management framework would support these principles.

This Science Advisory Report is from the September 12–13 and December 14, 2023 regional peer review on Assessment of Hatchery Chinook Salmon Straying in Southern British Columbia. Additional publications from this meeting, including the full research document1, will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2024/031 (PDF, 1,510 KB)

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