Grays Harbor Fall Chum Abundance and Distribution, 2017

Background

The goals of this project are to improve estimates of Grays Harbor Fall Chum spawner abundance and describe the distribution and spawning habitats of this species throughout the sub-basins of Grays Harbor. Washington Department of Fish and Wildlife (WDFW) and the Aquatic Species Enhancement Plan Technical Committee of the Chehalis Basin Strategy (Aquatic Species Enhancement Plan Technical Committee 2014) identified abundance, distribution, and key spawning habitats of Grays Harbor Fall Chum salmon as key information gaps in the Chehalis River basin. The gap occurs because the existing methodology for estimating Chum spawner abundance, developed in the 1980s, is based on surveys of spawning habitat that have substantially changed or degraded over time and on a spawning distribution for Chum that requires additional documentation.

In 2015, a pilot study identified which sub-basins within the Chehalis River basin contained Chum, developed a survey frame within the Wynoochee and Satsop sub-basins, and identified index reaches within those sub-basins with high densities of Chum spawners. In 2016, a new survey design was developed and implemented. The new survey design resulted in an overall abundance estimate of 55,000 to 56,000 Chum within the tributaries of the Satsop and Wynoochee sub-basins and the main stems of the Satsop sub-basin, but was not successful in completing an estimate for the main stem of the Wynoochee sub-basin due to poor survey conditions throughout the season. Our 2016 estimates from the Wynoochee and Satsop sub-basins only were equivalent to those derived by the existing methodology for the entire Grays Harbor Chum spawning distribution. However, several inconsistencies with the 2016 data collection were identified for improvement to ensure quality of the final estimate. These findings justified the purpose of this study as well as further efforts to improve information on Grays Harbor Chum spawner abundance and distribution.

Methods

Data collected for this study include distribution inside versus outside index reaches, area-under-the-curve estimates within index reaches, carcass tagging estimates of abundance in select index reaches, survey life estimates, and total spawner abundance on Chum salmon. Distribution inside versus outside index reaches was based on live counts during a one-time survey conducted throughout the Chum survey frame during the peak spawning period. Area-under-the-curve estimates within the index reaches were based on live counts obtained during weekly surveys. Carcass tagging estimates of abundance were based on a Jolly-Seber abundance estimator for open populations. Survey life was calculated in selected index reaches from the combination of area-under-the-curve and carcass tagging estimates of abundance. The index reaches selected to estimate survey life represented variable stream size classes - side channel, small/medium, and large - defined a priori for the purpose of analysis. Abundance in all index reaches was based on area-under-the-curve calculations and the survey life of the corresponding stream size classification. Total spawner abundance was the abundance in index reaches expanded by the proportion of spawning that occurred inside versus outside index reaches. Live count data used in the analysis were partitioned between ‘spawners' (i.e., actively spawning) and ‘holders' (i.e., holding in pools and potentially passing through the spawning area) to ensure we understood the sensitivity of the final estimate to these two different types of live counts. This distinction will be important when considering how to apply the results of this work to historical live counts from the index reaches. In 2017, we continued focus in the Wynoochee and Satsop sub-basins and implemented several changes to the data collection protocol to ensure the quality of the final abundance estimates.

Results

• Distribution inside versus outside index reaches: In the Wynoochee tributaries, 92-93% of Chum spawning occurred within the index reaches with the highest densities observed in Schafer and Neil creeks. In the Wynoochee main stem, 10-11% of Chum spawning occurred within the index reaches with the highest densities of Chum observed between river miles 29.1 and 39.5. In the Satsop tributaries, 44-45% of Chum spawning occurred within the index reaches with the highest densities of spawning observed in Decker Creek. In the Satsop main stem, 70-69% of Chum spawning occurred within the index reaches with the highest densities observed between river mile 12.4 and 14.7. The range in proportions represents the range in values provided by the different types of live counts (spawners only versus total live).
• Area-under-the-curve in index reaches: In the Wynoochee sub-basin, fish-day calculations summed across 8 index reaches ranged between 24,656 (spawners only) and 32,211 (total live counts). In the Satsop sub-basin, fish-day calculations summed across 16 index reaches ranged between 49,086 (spawners only) and 55,384 (total live counts).
• Abundance in carcass tagging index reaches: Chum spawner abundance was estimated to be 186 (173-208 95% C.I.) in the side channel index (Satsop Tributary 0462), 721 (605-885 95% C.I.) in the medium stream channel index (Schafer Creek) and 3,408 (2,402-4,755 95% C.I.) in the large stream channel index (EF Satsop River).
• Survey life: In this study, survey life represented BOTH the number of days a live Chum is present AND the observer efficiency within an index reach. For the side channel index (Satsop Tributary 0462), survey life was 8.98 days (±0.43) using counts of spawners only and total lives. The estimate did not differ by count type because no ‘holders' were observed in the side channel index. For the medium stream channel index (Schafer Creek), survey life was 9.00 days (±0.93) with spawners only and 12.50 days (±1.30) with total live counts. For the large stream channel index (EF Satsop River), survey life was 1.52 days (±0.31) for spawners only and 1.83 days (±0.37) for total live counts.
• Abundance in all index reaches: In the Wynoochee sub-basin, abundance within the 8 index reaches was estimated between 2,780 (spawners only) and 2,606 (total live counts). In the Satsop sub-basin, abundance within the 16 index reaches was estimated between 8,631 (spawners only) and 8,329 (total live counts).
• Spawner abundance: The 2017 Chum spawner abundance for the Wynoochee sub-basin was estimated to be 16,728 (±1,422) using spawner only counts and 13,852 (±1,084) using total live counts. Chum spawner abundance for the Satsop sub-basin was estimated to be 15,161 (±806) using spawner counts only and 14,460 (±791) using total live counts.

Conclusions

The overall estimates of Chum spawner abundance differed slightly based on the type of counts (spawners only, total live counts including spawners and holders) used for analysis. However, our estimates were consistently higher than those derived using the existing methodology for Grays Harbor Chum. All together, we estimated a 2017 Chum spawner abundance of approximately 28,000 to 32,000 fish for the sub-basins included in our study. Our estimate in the Satsop and Wynoochee sub-basins alone was 9,000 to 13,000 fish more than the number of spawners estimated for the entire Grays Harbor basin using the existing methodology (n = 18,627). Similar to our findings in 2016, these results suggest that the existing methodology likely underestimates the abundance of Grays Harbor Chum salmon.

Tributary estimates for the Wynoochee and Satsop sub-basins were derived in both 2016 and 2017. The 2017 tributary estimates were 80-83% lower in the Wynoochee and 45-51% lower in the Satsop than the 2016 tributary estimates. In addition to differences in abundance, flow regimes in 2017 varied from 2016. The fall of 2016 was characterized by high flows by mid-October that were maintained throughout the spawning reason whereas the fall of 2017 had lower sustained flows until mid-November when the river flows increased. The difference in flows did not greatly affect distribution in the Satsop sub-basin where tributary and main stem estimates were available for both years. Chum spawners using tributaries of the Satsop sub-basin were 46-53% of the total sub-basin estimate in 2016 and 48-51% of the total sub-basin estimate in 2017. A corresponding comparison for spawning distribution in the Wynoochee sub-basin was not available due to the lack of information from main stem areas in 2016.

Survey life estimates have a far greater influence on the final abundance than the type of live counts. In this study, survey life represented BOTH the number of days a live Chum was present AND the observer efficiency within an index reach. Estimates of survey life in 2016 and 2017 ranged between 5.7 and 12.5 days, with a much lower estimate of 1.52 days in the large stream channel index (EF Satsop River) in 2017. The low value in the EF Satsop River was likely influenced by low observer efficiency as surveyors consistently encountered low visibility and high angler activity in this index reach (and throughout the main stem Satsop survey reaches). Additional years of study are needed to better understand the variability in survey life and the consequences of this variability for the final estimates of Chum spawner abundance.