Research Document 2016/110

Exploration of data and methods for developing estimates of a biologically-based spawning goal and biological benchmarks for Little Tahltan Chinook (Stikine River drainage)

By Pestal, G., Etherton, P., Boyce, I., Richards, P., and Jaecks, T.

Abstract

This paper evaluates the quantitative basis for establishing a biological frame of reference for spawner abundances of Chinook Salmon (Oncorhynchus tshawytscha) in the Little Tahltan River, which is a tributary of the Stikine River in northwestern British Columbia. Stikine River Chinook Salmon are a transboundary stock aggregate, and are managed cooperatively by Canada and the US under the Pacific Salmon Treaty.

The project originally focused on fitting various Spawner-Recruit (SR) models and estimating biological benchmarks for each model, but during the peer-review process for this work, hosted by DFO’s Canadian Science Advisory Secretariat (CSAS), reviewers and participants raised serious concerns regarding the available data. The main concerns were

1. indications that the counting weir may have affected Chinook spawning distribution and increased mortality,
2. Little Tahltan may be an opportunistic spawning site in years with large abundance rather than a persistent spawning population, and
3. observed productivity dropped dramatically starting with the 2001 brood year.

Based on these concerns, the peer-review process discussed two alternative interpretations of the available data:

1. if one takes the weir counts at face value, then biological benchmark estimates are consistent with estimates published in 2000, and recent spawner abundances are substantially below various estimates of SMSY and declining further.
2. Alternatively, if one considers the potential data concerns strong enough to invalidate recent weir counts, then biological benchmarks cannot be updated, current status of Little Tahltan is unknown, the current weir does not provide a useful indicator for total Stikine Chinook, and the drainage-wide assessment program needs to be reviewed.

After intensive debate, the consensus conclusion was that biological benchmarks based on spawner-recruit data cannot be estimated with the data currently available, but that it would be nevertheless informative for decision-makers to document our benchmark estimates and summarize the data concerns identified during the peer-review.

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