Evaluation of Chinook salmon (Oncorhynchus tshawytscha) fry survival at Lookout Point Reservoir, western Oregon, 2017

Abstract

A field study was conducted to estimate survival of fry-sized juvenile Chinook salmon ( Oncorhynchus tshawytscha ) in Lookout Point Reservoir, western Oregon, during 2017. The field study consisted of releasing three groups of genetically marked fish in the reservoir and monthly fish sampling. Fish were released during April 18–19 (43,950 fish), May 30–June 2 (44,145 fish), and on June 28, 2017 (3,920 fish). Reservoir sampling began in May and occurred monthly through October, consisting of 5-day events where juvenile Chinook salmon were collected using various gear types (electrofishing, shoreline traps, gill nets). Data were analyzed using two models: (1) a staggered release-recovery model (SRRM), and (2) a parentage-based tagging (PBT) N -mixture model. The SRRM provided survival estimates from two periods: (1) mid-April to late May (S SRRM1 ), and (2) late May to late June (S SRRM2 ). Multiple estimates of survival were possible for each period using different combinations of recovery data from the three groups of fish that were released. Survival estimates for S SRRM1 ranged from 0.470 to 0.520. Estimates for S SRRM2 ranged from 0.968 to 0.969; cumulative survival from mid-April to late June (S SRRM2 ) was estimated at 0.870. We suspect that issues with the third release group led to biased survival results using the SRRM. The PBT N -mixture model provided survival estimates from six periods: (1) mid-April to mid-May (S NMIX1 ), (2) mid-May to mid-June (S NMIX2 ), (3) mid-June to mid-July (S NMIX3 ), (4) mid-July to mid-August (S NMIX4 ), (5) mid-August to mid-September (S NMIX5 ), and (6) mid-September to mid-October (S NMIX6 ). Survival estimates from the PBT N -mixture model were lowest for S NMIX1 (0.461) and increased monthly to a high of 0.970 for S NMIX6 . Cumulative survival from mid-April to mid-July was 0.233 and overall survival from mid-April to mid-October was 0.188. This suggests that most mortality occurred early in the study when juvenile Chinook salmon were small. This could be because these fish were most vulnerable to predation in the reservoir at that time. We determined that mortality of juvenile Chinook salmon was high in the reservoir during this study and similar estimates of parr-to-smolt survival have been observed in other systems. Additional analyses are required, including results from the second year of study (2018), and potentially similar evaluations will need to be made at other locations to determine if reservoir mortality is a limiting survival factor for Chinook salmon in the Middle Fork Willamette River.