Abstract
Research on regulatory mechanisms in biological populations often focuses on environmental covariates. An integrated approach that combines environmental indices with organismal-level information can provide additional insight on regulatory mechanisms. Survival of spring/summer Snake River Chinook salmon (Oncorhynchus tshawytscha) is consistently low whereas some adjacent populations with similar life histories experience greater survival. It is not known if populations with differential survival respond similarly during early marine residence, a critical period in the life history. Ocean collections, genetic stock identification, and otolith analyses were combined to evaluate the growth-mortality and match-mismatch hypotheses during early marine residence of spring/summer Snake River Chinook salmon. Interannual variation in juvenile attributes, including size at marine entry and marine growth rate, was compared with estimates of survival and physical and biological metrics. Multiple linear regression and multi-model inference were used to evaluate the relative importance of biological and physical metrics in explaining interannual variation in survival. There was relatively weak support for the match-mismatch hypothesis and stronger evidence for the growth-mortality hypothesis. Marine growth and size at capture were strongly, positively related to survival, a finding similar to spring Chinook salmon from the Mid-Upper Columbia River. In hindcast models, basin-scale indices (Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO)) and biological indices (juvenile salmon catch-per-unit-effort (CPUE) and a copepod community index (CCI)) accounted for substantial and similar portions of variation in survival for juvenile emigration years 1998–2008 (R2>0.70). However, in forecast models for emigration years 2009–2011, there was an increasing discrepancy between predictions based on the PDO (50–448% of observed value) compared with those based on the NPGO (68–212%) or biological indices (CPUE and CCI: 83–172%). Overall, the PDO index was remarkably informative in earlier years but other basin-scale and biological indices provided more accurate indications of survival in recent years.
Highlights
One focus of population ecology is the identification of environmental indices that are related to variation in population size or productivity [1,2,3]
We evaluated the ability of juvenile attributes and environmental indices during early marine residence to account for interannual variation in survival
We selected this metric because: (1) all smolt-to-adult return ratios (SARs) estimates for this population were highly correlated from 1998–2008 (r = 0.986 for SARs with and without jack; r = 0.740 for SARs for wild and hatchery composite; and r = 0.978 for SARs from LGD to Bonneville Dam and from LGD-LGD; (2) the available estimates for the proportion of jacks within a brood year were not significantly correlated with SARs (r = 0.455, 2000–2010); and (3) we were interested in making inference about the survival of the naturally-spawned portion of the population, which is of primary conservation concern
Summary
One focus of population ecology is the identification of environmental indices that are related to variation in population size or productivity [1,2,3] Such relationships are often based on hypothesized mechanisms, such as a ‘‘stable ocean’’ [4] or ‘‘optimal upwelling window’’ [5], but the relationships fail to hold up over time [6,7]. If the critical period is successfully identified, the abundance or condition of a cohort during, or shortly after, this critical period should provide a robust indication of relative survival This approach is not necessarily based on a mechanistic understanding of mortality but can focus research efforts by identifying the critical period(s) in a species’ life history. A combined approach can identify relevant local or regional environmental factors and provide insight on the timing and mechanisms of major mortality events
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