Columbia Basin fish and wildlife managers, policy makers, and researchers have expressed concern about differences in the conclusions and management implications of the following two publications on Columbia River Basin steelhead trout (Oncorhynchus mykiss): Avian predation on steelhead is consistent with compensatory mortality (Haeseker et al. 2020) and Measuring the additive effects of predation on prey survival across spatial scales (Payton et. al 2020).
Significant questions remain about the extent to which avian predation is additive or compensatory. At their extremes, (completely) additive means that changes in predation are reflected one-to-one in changes in the overall survival, whereas (completely) compensatory means that other life cycle factors operate to negate or counteract the effects of predation so that long-term survival is unaffected by the predation in question. More often in nature, mixtures of additivity and compensation are observed rather than the extremes of complete additivity or compensation (Haeseker et al. 2020; Payton et. al 2020). Results of analyses examining compensatory versus additivity in survival, such as the Haeseker et al. and Payton et al. papers, can strongly affect decisions about future regional management actions designed to reduce avian fish predators (i.e., hazing, re-locating, culling, etc.).
This ISAB report evaluates similarities and differences in data, analytical approaches, conclusions, and management implications of the two studies of avian predation on juvenile steelhead in the Columbia River Basin. Both studies focused on determining whether the effects of avian predation on overall survival were additive or compensatory. Haeseker et al. (2020) focused on bird predation in the Columbia River estuary of migrating smolts from the Snake River Basin and its effect on survivorship (return probability) of adults returning to Bonneville Dam. Haeseker et al. concluded that mortality from avian predation was consistent with full compensation.
In contrast, Payton et al. (2020) estimated the effects of bird predation both in the river and in the estuary on Upper Columbia Basin smolts at two life stages: from release at Rock Island Dam to Bonneville Dam as smolts, and from release to adult return to Bonneville Dam. Payton et al. concluded that predation mortality was super-additive for smolts between Rock Island and Bonneville, and partially additive from release to adult return to Bonneville Dam. Super-additive means causing more smolts to die than the number of smolts estimated to be consumed by bird predators. Sources for additional undetected mortality include deposition of PIT tags at locations other than the colony, smolts being stolen by gulls, wounding or injury that results in mortality but not capture by the predator, or consumption by birds that occupy areas other than the colonies monitored for PIT tags.
While the studies were conducted in different basins and employed only partially overlapping time series, the ISAB compared data and analyses from migration to adult return life stage to illuminate the nature of conclusions and interpretations. The ISAB concludes that despite these differences, the two studies are not inconsistent in their results.
Both studies looked at mortality during the estuarine/marine phase and concluded that predation is either largely (Payton et al.) or fully (Haeseker et al.) compensatory. Only Payton et al. assessed smolt survival during in-river migration, and there appears to be strong additivity of predation this life history phase. Results of both studies are consistent with the possibility of low-level partial additivity of predation effects on smolt-to-adult returns (SAR). For populations at risk, avian predation that is partially additive could affect population sustainability. Additional analyses and studies are needed to fully evaluate the relative importance of avian predation in a population conservation context.
A major question for management is whether an increase in SARs is worth the cost of suppressing avian predators or is critical to the support of ESA-listed salmonid species. Answering these questions requires estimates of the magnitude of avian predation effects rather than estimates of the degree of additivity or compensation, and also requires consideration of social concerns, cost effectiveness, and ecosystem consequences of avian control actions (ISAB 2019-1). Reconciling results from these studies in a side-by-side analysis, evaluating additional methods for obtaining predation effect size from tagging data, and incorporating these into life cycle models for different species and populations are the next steps toward understanding how avian predation fits into broader management strategies and goals for Columbia Basin salmonids. Until these additional steps can be implemented, the ISAB recommends that the finding of partial additivity/partial compensation over the entire life cycle of steelhead is the most prudent conclusion from a management perspective.