Nonrepresentative fish and ocean migration assumptions confound inferences in Rechisky et al.

Close examination of the methods, assumptions, and results of Rechisky et al. (1) indicate that their results are confounded by nonrepresentative tagging, rearing, and release factors, and that critical assumptions are inconsistent with available data. Thus, the authors’ conclusions regarding hydrosystem-related delayed mortality are overreaching and unsupportable.

Nonrepresentative fish with acoustic tags were 10–20 mm longer, were released 21–83 d later, and were released 55–249 rkm further downriver than their corresponding hatchery populations of inference. Length at tagging, timing of release (2), and migration distance have all been shown to influence survival rates of Chinook salmon at multiple life stages. Any of these factors alone confound comparisons with the populations of inference, let alone the combination of all three.

Rechisky et al. (1) report that estimation of detection probabilities for the Lippy Point subarray was not possible because of too few detections of tagged smolts at the distant Alaska subarray. This assumption weakens the reliability of survival estimates used to draw conclusions concerning delayed mortality. The sensitivity analysis used to explore the effects of alternative assumptions is narrow in view of the large uncertainty in detection probability.

Rechisky et al. (1) assume that all fish migrated North on the continental shelf at depths shallower than 200 m and through the Lippy Point subarray. If this assumption is not valid, the reported survival estimates will be biased low. Studies by McMichael et al. (3) and Schreck et al. (4) indicate that this assumption is likely violated. The degree of bias is unknown.

Contrary to Rechisky et al. (1), in-river survival varies between 25% and 83% and is influenced by hydrosystem conditions (2). Ocean survival rates and smolt-to-adult survival rates are also influenced by hydrosystem conditions (2, 5). These studies demonstrate that hydrosystem management actions influence survival at multiple life stages.

Rechisky et al. (1) found no evidence that Snake River hatchery Chinook smolts experienced lower survival rates in the early ocean than those from the Yakima River that migrated through fewer dams. The authors acknowledge these estimates represented tagged groups whose size, holding, and timing of release had been significantly manipulated to accommodate acoustic tags. As a result, tagged fish were not representative of the hatchery populations of inference. Similarly, the size-distribution of the hatchery study fish was larger than all but a small fraction of the wild individuals, concurrent with differences in migration timing between study fish and wild fish. The study was short term (3 y) and the migration conditions that study fish experienced were different from migration conditions experienced by most wild and hatchery fish. Because of low sample sizes and poor detection efficiency, untested, critical assumptions about detection probabilities and ocean migration patterns were required. Thus, the findings of Rechisky et al. on delayed hydrosystem mortality for wild or hatchery fish are highly questionable.