Science Advisory Report 2018/050

Recovery Potential Assessment for Chilcotin River and Thompson River Steelhead Trout (Oncorhynchus mykiss) designatable units

Summary

• This Recovery Potential Assessment (RPA) focuses on the Chilcotin and Thompson River Designatable Units (DU) of Steelhead Trout. Both of these DUs were assessed as Endangered by COSEWIC in an emergency assessment in January 2018.
• The estimated number of mature fish that returned to fresh water from the sea in the fall of 2017 and spawned in the spring of 2018 was 150 for the Thompson DU and 77 for the Chilcotin DU. The estimated decline of Steelhead Trout spawners over the last three generations has been 79% (over 15 years) for the Thompson DU, and 81% (over 18 years) for the Chilcotin DU.
• Given the shortened timelines required for an emergency assessment, the advice in this RPA only addresses a subset of the elements required in a full RPA. Outstanding elements will be addressed in the future as the Species at Risk Act processes continue.
• Threats and limiting factors identified to be most relevant to the survival and recovery of Steelhead Trout include changes in the marine environment, fishing mortality, degradation of freshwater and marine habitats, predation and competition. General categories of threats and limiting factors were agreed to, however the rationale and scoring for level of impact, causal certainty, and threat risk had greater uncertainty and will require further input and evaluation.
• Recommended Distribution Target is to retain the present level of occupancy in freshwater habitats, thereby avoiding contraction of freshwater range. Five spatial sub-areas within the spawning and juvenile rearing areas of the Thompson DU, including the main stem are recommended and two spatial subdivisions are recommended within the Chilcotin DU. These distribution targets are consistent with current level of occupancy in freshwater habitats, and are believed to be sufficient to avoiding contraction of freshwater range.
• Recommended Abundance Recovery Target for Thompson Steelhead Trout DU is 938 spawners. This value, which also meets the distribution target, is the total escapement to the DU that results in a 95% probability that a minimum of 100 spawners returns to each of its five sub-areas in the same year. Recommended Abundance Recovery Target for Chilcotin Steelhead Trout DU is 562–744 spawners, using a length-standardized requirement of 1.8–2.4 spawners/km. This also meets the distribution target for the Chilcotin DU.
• Model simulations suggest increases in future abundances of both DUs are conditional on improvements in natural productivity. Exploitation rate (fishing mortality) reduction has the potential to lessen rates of decline if the most recent productivities observed continue in the future. However, eliminating exploitation alone will not result in population recovery.
• Uncertainties regarding the exploitation rate estimates, unaccounted for fixed rate terminal harvest, and variations in escapement were identified as having the potential to affect the estimated productivity of each population.
• For the Thompson DU, simulations estimate that if productivity levels from the most recent year persist (recruits/spawner), recovery is not expected regardless of exploitation rate. If productivities double (10 and 5-year time periods), the estimated recovery probability exceeds 47% for all exploitation rates. However, if the 1-year time period productivity doubles, recovery probability estimates are 12% or less under all exploitation rates.
• For the Chilcotin DU, simulations estimate that recovery probability is zero at all exploitation rates if productivity levels from the most recent year persist, but recovery probability exceeds 39% at all exploitation rates if productivity increases to 5-year mean level. If the 5- and 10- year mean productivities double (10 and 5-year time periods), the estimated recovery probability exceeds 74% at all exploitation rates.
• Given the declining and very low abundances of both the Thompson and Chilcotin Steelhead DUs, any harm will inhibit or delay potential recovery and potentially result in further declines in abundance. Allowable harm should not be permitted to exceed current levels and should be reduced to the maximum extent possible. Preventing and mitigating habitat destruction, restoring damaged habitat, and reducing exploitation rates, to the extent possible, are immediate actions that will increase the likelihood that allowable harm will not exceed current levels and promote recovery if productivity increases.

This Science Advisory Report is from the September 20-21, 2018 regional peer review meeting on Recovery Potential Assessment – Chilcotin River and Thompson River Steelhead Trout (Oncorhynchus mykiss) Designatable Units. Additional publications from this Regional Peer Review will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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