Abstract
SteelheadOncorhynchus mykisssmolts must migrate through distinct freshwater, estuarine, and coastal habitats, each with unique biological and physical characteristics, on their route from natal streams to the ocean to complete their life cycle. Fewer than 15% of steelhead smolts survived their migration from the Nisqually River through Puget Sound to the Pacific Ocean from 2006 to 2009 and in 2014. Rapid smolt migration coupled with evidence of smolt mortality at harbor sealPhoca vitulinahaulouts during 2014 indicated that predation by pinnipeds was an important source of mortality. When Puget Sound temperatures increased with the Northeast Pacific heat wave from late 2014 to 2016, steelhead smolt survival probabilities increased to 38% in 2016 and 2017, then decreased again as Puget Sound water temperatures cooled. Many ecological changes accompanied the increased marine temperatures, including the increased abundance of northern anchovyEngraulis mordaxin Puget Sound. The years of lowest smolt mortality coincided with abundant larval and post-larval anchovy during the previous year; and we provide data from telemetered harbor seals and steelhead indicating that the resulting high abundance of age-1+ anchovy provided an alternative prey source for predators of migrating steelhead smolts. Identification of ecological mechanisms that drive patterns in the survival of Endangered Species Act-threatened steelhead provides critical understanding of the systems within which management strategies must operate.
Highlights
Marine ecosystems are fundamentally influenced by broad-scale climate patterns
We modeled fish presence π and counts when present μ from net tow i in year t, month m, and volume filtered w as: logit(πi,t,m,w) = λtX + f1 (m) + w logit(μi,t,m,w) = βtX + f2 (m) + w where the former is fit using a binomial error distribution, the latter is fit using a zero-truncated negative binomial error distribution, f(.) represents a second-order random walk used to parameterize a nonlinear function that accounts for phenology, and X is a binary variable describing whether an observation did (1) or did not (0) occur during a particular year t
To determine whether temperature during or prior to migration might affect observed steelhead mortality patterns, we modeled instantaneous steelhead mortality (M) in each year (t) as a linear function of the Puget Sound temperature (PST) during the year prior to migration (Model 1) and M as a function of PST in the year of migration (Model 2) using the base function ‘lm’ in R version 3.6.2 (R Core Team 2019)
Summary
Marine ecosystems are fundamentally influenced by broad-scale climate patterns. Indices of ocean climate variability, such as the Pacific Decadal Oscillation (PDO), El Niño−Southern Oscillation, and North Pacific Gyre Oscillation, have been linked to fluctuations in marine species at various trophic levels (Brodeur et al 1985, Clark & Hare 2002, Menge et al.2009, Keister et al 2011), including marine survival patterns of anadromous Pacific salmon (Oncorhynchus spp.; Mantua et al 1997, Hare et al 1999, Malick et al 2017) and steelhead O. mykiss (Welch et al 2000, Sobocinski et al 2020). Patterns of association between salmon survival and ocean environmental conditions are inherently difficult to characterize, because relationships between physical and biological ecosystem components can shift over time. A single salmonid cohort encounters diverse marine ecosystems (i.e. estuary, nearshore, continental shelf, open ocean), each with its own suite of physical and biological processes and inherent climate−survival linkages. Ecosystem dynamics involving both bottom-up and top-down control of survival patterns near the point of ocean entry are increasingly being considered as important components of the marine survival equation (Sydeman et al 2013, Wells et al 2017, Sobocinski et al 2020). Understanding the influence of climate variation on interactions among living marine resources is vital to proper consideration of ecosystem-based management strategies
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