Science Advisory Report  2016/006

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

Summary

• The Little Tahltan River flows into the Tahltan River, a major tributary of the Stikine River. The Stikine River originates in the province of British Columbia and flows to the sea through the state of Alaska. Little Tahltan River Chinook Salmon have historically been major contributors to the total Stikine Chinook production, but have declined in recent years, both in abundance and relative contribution to the total Stikine Chinook run.
• Under the Pacific Salmon Treaty, a number of bilaterally agreed-to spawning goals have been established for transboundary river salmon. A drainage-wide spawning goal for Stikine River Chinook Salmon was established 15 years ago. Due to concern over the recent decline in abundance of Little Tahltan Chinook, the Canadian caucus of the Trans-boundary River Panel of the Pacific Salmon Commission expressed interest in a project to develop reference points that could be used as the basis for a biological spawning goal.
• The analytical process began with a review of size-specific spawner abundance estimates for Little Tahltan Chinook for the brood years 1985 to 2007. This was necessary as Transboundary management focuses on adult or “large” Chinook. To address variability in the estimates, three different spawner datasets were compiled. Similarly, due to variability in recruitment estimates, two different recruit datasets were developed. The three spawner abundance datasets and two recruit datasets were combined to provide four datasets for stock-recruitment analysis.
• The primary approach taken for the development of biological benchmarks was that of stock-recruitment modeling. Three model forms were explored: Ricker, Ricker AR1, and Beverton-Holt. As a consistency check on the results, a data-poor analysis technique known as the percentile method for biological reference ranges was also conducted.
• Little Tahltan weir counts and total Stikine mark-recapture abundance estimates follow a similar trend from 1996 (first year of mark-recapture data) through 2007. Following the 2007 return year, the weir counts show a decreasing trend while the mark-recapture estimates show an increasing trend.
• Beginning in the 2001 brood year (corresponding to the 2007 return year), and continuing until the 2006 brood year, the Little Tahltan Chinook exhibited a period of productivity below replacement. This pattern was consistent across the four datasets used for stock-recruitment analysis. The datasets seem to fit a classic Ricker-type relationship. However, separating the data into two time periods reveals two clusters of points, both of which lack any clear pattern. Prior to brood year 2001, estimates are for highly variable recruitment across a narrow range of spawner abundances, but for brood year 2001 and later there was a wide range of spawner abundances all with very poor recruitment.
• Data quality concerns confound the results of the stock-recruitment analyses. The potential productivity changes for Little Tahltan Chinook violate one of the assumptions for stock-recruitment analyses. It is also possible that the fish counting weir may have influenced the apparent spawner abundance used in the datasets. Therefore, science advice for the establishment of an escapement goal cannot be provided based on the results of this assessment.
• In order to estimate an escapement goal, the appropriate stock unit for analysis must first be determined; this may require additional field work. A review of the two Wild Salmon Policy Conservation Unit delineations for Stikine Chinook is recommended. In addition, alternative assessment approaches to the fish counting weir should be considered.

This Science Advisory Report is from the November 12-13, 2015 regional peer review on Estimates of a Biologically-Based Spawning Goal and Biological Benchmarks for Little Tahltan Chinook (Stikine River Drainage). 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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