Science Advisory Report 2019/043

Recovery Potential Assessment – Interior Fraser Coho (Oncorhynchus kisutch)

Summary

• Interior Fraser Coho (IFC) are the Designatable Unit (DU) of Coho Salmon (Oncorhynchus kisutch) that spawns in the Fraser River watershed upriver from Hells Gate in British Columbia.
• IFC were designated as Endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 2002; however, they were not subsequently listed under the Species at Risk Act (SARA). COSEWIC reassessed the status of IFC in 2016 and revised the designation from Endangered to Threatened.
• Productivity was considerably higher during 1987-1993 (“historic” regime) than during 1994-2017 (“current” regime) return years; lowered smolt-to-adult survival was a major driver that reduced average productivity since return year 1994. The 3-year running average pre-fishery returns in the historic regime varied between 153,000 and 227,000 with an overall average of 199,000, while the current regime pre-fishery returns varied between 21,000 and 70,000 with an overall average of 38,000.
• Coho Salmon habitat (spawning ground, nursery, rearing, food supply, migration, and other areas) research has mostly been conducted in coastal river systems, which have different hydrological characteristics compared to the interior river system used by IFC.
• Redds, the spawning nests constructed by Pacific salmon and other species, meet the definition of a “residence” under SARA.
• IFC reside in the river systems of the BC interior where hydrological regimes are highly influenced by temperature extremes, snowpack and associated melt-water supply. This results in a different timing for spring freshet events when compared to rain-dominated coastal streams. The varying ability for juvenile IFC to access tributaries, side channels and isolated pools influences their behaviour and habitat use.
• The three highest ranked anthropogenic threats to IFC include modifications to catchment surfaces, linear development, and agricultural and forestry effluents. Several other threats including fishing, dams and water management/use, introduced genetic material, household sewage and urban waste water, and industrial and military effluents, were ranked as low-medium.
• The COSEWIC threats calculator utilized for this assessment integrates the additive cumulative impact of all threats and assessed the overall threat impact as High-Very High. Climate change is anticipated to exacerbate the impact of many anthropogenic and natural threats.
• A quantitative analysis of a dataset including natural-origin escapements from 1998-2016 was used to recommend a DU-level recovery target. The recommended recovery target for IFC is a 3-year geometric mean abundance of 35,935 natural-origin spawners within a 10 year timeframe.
• The probability that IFC will reach the DU recovery target was explored using stock-recruitment models and forward simulation. The results showed that the probability of reaching the recovery target in 10 years under current conditions is equally likely to reach the target as to not reach it. However, there is high uncertainty in the population trajectory projection (the 80% uncertainty interval spanned -29% to +29%).
• The simulation results showed that recovery of natural-origin IFC is unlikely (≤33%) under decreased smolt-to-adult survival conditions, regardless of the exploitation rate. Increased smolt-to-adult survival, however, increased the probability of recovery to a greater degree than decreasing the exploitation rate.
• IFC recovery is possible if human-induced mortality is minimized given current environmental conditions and variability, and if impacts from the identified threats are also mitigated. If the recent smolt-to-adult survival pattern continues and the models are correct, the population trajectory is most likely to be positive or stable at a zero exploitation rate and recovery may eventually occur. At an exploitation rate of ≤ 6% at current smolt-to-adult survival, model simulations indicated meeting the recovery target in 10 years was likely (≥ 66% chance) but not very likely (≥ 90% chance).
• Although not quantified here, there is a de facto human induced mortality rate on juvenile IFC in freshwater that is non-fisheries related. Therefore, impacts to freshwater environments should also be seriously considered in addition to exploitation rate when considering allowable harm.

This Science Advisory Report is from the May 22-24, 2019 regional peer review on Recovery Potential Assessment – Interior Fraser Coho (Oncorhynchus kisutch). Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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