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Ecosystem indicators of marine survival in Puget Sound steelhead trout
Abstract
Understanding ecosystem drivers of fish stock performance is essential to improving conservation and management. We used a hypothesis-driven approach to identify potential ecosystem indicators and developed a retrospective analysis using regression models of steelhead trout marine survival in Puget Sound, an urban estuary in Washington State, USA, over a 30-year time series. Indicators related to predator abundance, the presence of hatchery Chinook salmon in the system, salinity of marine waters, and timing of cumulative river discharge explained the most variance in smolt survival. Seal abundance was the strongest predictor, showing a strong negative relationship with steelhead marine survival, indicating that predation pressure may influence marine survival. Several other predictors were supported in our models, but with much less individual explanatory power. Our results support the conclusion that a combination of factors with differing mechanisms have likely contributed to declining survival of steelhead in inland marine waters. This study emphasizes the importance of collecting long-term survival and environmental data, taking a hypothesis-based approach to understanding ecosystem drivers related to marine survival, and evaluating the mechanisms associated with survival for species with complex life histories like anadromous steelhead trout.
... Ecosystem indicators show promise for providing insight into Pacific salmonid trends and environmental and anthropogenic constraints that shape these trends (Logerwell et al., 2003;Burke et al., 2013;Sobocinski et al., 2020). Ecosystem indicators are defined as quantitative measurements that reflect the structure, composition, or functioning of a complex system (Niemeijer and de Groot, 2008;Kershner et al., 2011;Otto et al., 2018). ...
... We developed multiple hypotheses related to potential mechanisms for coho and Chinook salmon marine survival ( Table 1) building off of the approach developed by Sobocinski et al., (2020) for steelhead trout and informed by other salmon indicators efforts (Burke et al., 2013;Mills et al., 2013;Peterson et al., 2014;Litzow et al., 2018;Satterthwaite et al., 2019). We posited seven overarching explanations for declines in salmon marine survival, capturing a variety of ecosystem processes; we summarize these below. ...
... Lastly, for H7 we evaluate anthropogenic impacts. We describe these hypotheses briefly below (we refer readers to Sobocinski et al., 2020 for additional background), with supporting references in Table 1. ...
Efforts to understand causes of declines in productivity of species of concern often involve retrospective evaluation of multiple possible causes based on trends in relevant ecological indicators. We describe a hypothesis testing framework for examining declines in marine survival for coho and Chinook salmon in the Salish Sea. Independent populations of both anadromous species have declined over the last 50 years, prompting extensive examination of mortality in different life stages. Previous studies have identified declining trends in marine survival, and we re-evaluated these trends in light of a number of possible hypotheses for declines. We laid out seven potential explanations for declines: changes in predator buffering related to abundance and timing, density-dependent or-independent food availability, water quality, timing of freshwater delivery to Puget Sound, and anthropogenic impacts. We compiled ecosystem indicators relevant to these hypotheses and used generalized additive models (GAMs) to examine multivariate relationships with survival from multiple coho and Chinook salmon stocks. We also developed additional models using the most informative indicators based on variable importance weighting (VIW) from the seven hypothesis groups. We examined how these models explained overall trends in marine survival, as well as survival in three temporal stanzas (before, during, and after a major decline, based on statistical breakpoint analysis). Across the entire time series, best fitting models explained 30-40% of the variation in the survival data. Best fitting models were from multiple hypotheses, including predation (abundance and timing), competition, water quality, and anthropogenic impacts; the freshwater delivery hypothesis was the least supported. Different models performed best (lowest error) during different stanzas of the coho salmon marine survival time series and the two VIW models were generally the top performing models, but performance varied in different years. Indicators with the strongest support included seal abundance, herring abundance, timing of hatchery salmon releases, and indicators related to water properties like stratifi-cation and temperature. These findings suggest that multiple processes embedded in several of our hypotheses influence marine survival but that an ecological "smoking gun" for Salish Sea salmon declines will remain elusive.
... 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 Outside the USA © The US Government 2021. ...
... 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. ...
... Generally, cool Pacific Ocean temperatures (negative PDO indices) are associated with lower marine mortality rates for Pacific salmon (Mantua et al. 1997, Daly & Brodeur 2015. However, ocean temperature effects on Puget Sound steelhead smolt-to-adult return (SAR) rate appear to be opposite in sign, with warm-phase PDO years corresponding to lower steelhead mortality in marine waters, although largescale ocean indicators are not strong indicators of Puget Sound survival trends (Sobocinski et al. 2020). Recent evidence suggests that conditions within Puget Sound have a strong influence on steelhead SAR trends (Kendall et al. 2017), including a strong negative association of harbor seal population abundance and weak positive associations of the PDO and sea surface temperature in the Salish Sea (in the year of ocean entry) with Puget Sound smolt-to-adult steelhead survival rates (Sobocinski et al. 2020). ...
Steelhead Oncorhynchus mykiss smolts 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 seal Phoca vitulina haulouts 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 anchovy Engraulis mordax in 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.
... Although most steelhead research focuses on freshwater systems (Daly et al., 2014), declining smolt-to-adult return (SAR) rates have motivated research into factors contributing to estuarine and marine survival (Friedland et al., 2014;Kendall et al., 2017;Losee et al., 2021;Moore et al., 2015;Thalmann et al., 2020). Broad-scale patterns of ocean circulation, described by indices such as the Pacific Decadal Oscillation (PDO), the El Niño-Southern Oscillation (ENSO), and the North Pacific Gyre Oscillation (NPGO), and finer-scale predator prey interactions appear to influence steelhead marine survival (Kendall et al., 2017;Moore et al., 2021;Sobocinski et al., 2020). However, research on non-stationary relationships between varying ocean indicators and salmon populations (Litzow et al., 2018) indicates that survival-related factors drive ecological relationships at multiple spatiotemporal scales. ...
... Scientists and managers are aware that poor marine survival of Pacific Northwest steelhead may contribute to their declining abundance and lack of recovery (Kendall et al., 2017;Losee et al., 2021;Moore et al., 2015;Moore et al., 2021;Sobocinski et al., 2020;Ward, 2000). Steelhead stocks are often monitored less than their neighboring salmon stocks, including for marine survival estimates. ...
Dear Chairs, I am writing to provide you with the Washington Department of Fish and Wildlife's report to the legislature titled the Coastal Steelhead Proviso Implementation Plan (CSPIP). Funding and the proviso language requires a report to the relevant committees of the legislature per language in the 2021-23 operating budget proviso (37), which reads as follows: (37) $200,000 of the general fund-state appropriation for fiscal year 2022 and $100,000 of the general fund-state appropriation for fiscal year 2023 are provided solely for the department to develop a plan to protect native and hatchery produced steelhead for each river system of Grays harbor, Willapa bay, and coastal Olympic peninsula. The plan must adequately protect those fisheries for healthy runs year-after-year as well as provide reasonable fishing opportunities. The plan must include active stakeholder input and include an outreach strategy sufficient to keep conservation and angler interests well informed of proposed changes in advance of annual fishing seasons. The plan must be reported to the appropriate committees of the legislature by December 1, 2022. Declines in coastal steelhead population abundance and associated reductions in angling opportunity have highlighted the need to design and fund fisheries management strategies that provide sustainable angling opportunities and support the long-term viability of steelhead populations through science-based conservation objectives. In pursuit of that goal, the Washington Department of Fish and Wildlife (WDFW) has developed the CSPIP, which advances steelhead fishery management in the river systems of Grays Harbor, Willapa Bay, and the coastal Olympic Peninsula. The CSPIP aligns with and facilitates the implementation of existing policy in the Statewide Steelhead Management Plan (SSMP), the Anadromous Salmon and Steelhead Hatchery Policy C-3624 (2021), and Joint Comanager Hatchery Policy, which is currently under development. Although the state engaged tribal co-managers and Olympic National Park in the development of this plan, the CSPIP only applies to state steelhead management on Washington's Pacific coast. The science based CSPIP incorporates ecological knowledge of the target species while considering the history of harvest and management, state and federal mandates, and socioeconomic implications that underpin coastal steelhead management. The Plan lays out an adaptive management strategy that assigns appropriate management actions pertaining to monitoring and evaluation, fisheries regulations, hatchery operations based on steelhead population viability and the level of monitoring that is available to inform management. The CSPIP also provides guidelines pertaining to habitat, human dimensions and public communications and includes an implementation timeline with benchmarks, budget projections, critical research needs, and a vision for next steps. To this point, limited resources have resulted in a lack of crucial data needed to inform steelhead management decisions. Data gaps cause uncertainty around fishery impacts and in some cases lead to fishery closures when managers do not have enough information to ensure that angling opportunities remain within sustainable impact limits. Increased monitoring and research would address those problems, not only by enabling sustainable angling opportunity through high-precision sport fishery monitoring, but also by collecting the data required to evaluate and update long-term management strategies and conservation objectives. Thus, the CSPIP creates a win-win situation for multiple stakeholder groups and steelhead-related interests. WDFW aims to increase the two-way flow of information between those steelhead stakeholders and resource managers by providing accurate and consistent information about coastal steelhead, strengthening community partnerships, and increasing opportunities for the public to engage in the fisheries management process. Implementing the CSPIP requires an estimated biennial budget of $5.9 million (including indirect costs) above current appropriations, with most of this amount dedicated to freshwater sport fishery monitoring. WDFW intends to implement the CSPIP during the 2023-2025 biennium budget period. Among other implications, failure to fund this Plan would: (1) result in continued uncertainty regarding coastal steelhead fishery impacts, which could lead to fishery closures, (2) hinder the development of long-term steelhead management strategies and conservation objectives and (3) slow the pace of critical scientific research needed to inform fishery management. In accordance with the CSPIP, adaptive management, community engagement, and the refinement of quantitative tools will persist in perpetuity, with the understanding that reevaluation and adaptation are inherent elements of this new paradigm.
... Interestingly, surveyed steelhead anglers did not identify predation pressure as a large threat to steelhead populations in contrast to interviewees. However, in DFO's recent recovery potential assessment for Chilcotin River and Thompson River steelhead trout ( DFO, 2018 ), inshore predation from seals, especially harbor seals ( Phoca vitulina ), was identified as the single largest predictor of steelhead declines (see also Melnychuk et al., 2014 ;Berejikian et al., 2016 ;Sobocinski et al., 2020 ). Our research suggests anglers are generally unaware of the magnitude of this threat. ...
Inland fisheries are complex social-ecological systems that can generate important nutritional, economic, cultural, and recreational benefits. Effective management of these systems for multiple user-groups requires an understanding of the complex natural and human dimensions interactions within them. We examine the perceptions of stakeholders, Indigenous rightsholders, and regulatory/governance groups on the current and future status of Oncorhynchus mykiss (including their resident form – rainbow trout – and their anadromous form – steelhead) populations and fisheries in British Columbia (BC), Canada from 65 qualitative interviews and 1029 quantitative survey responses. Participants generally did not believe resident rainbow trout were threatened at the provincial level but were definitive in assessing anadromous steelhead trout as threatened. Habitat alterations, water temperature extremes, and climate change, were key threats identified for all forms of O. mykiss while bycatch in commercial fisheries and predation pressure from pinnipeds were specifically identified threats for steelhead trout. Anglers did not perceive recreational fishing pressure as a key threat in contrast to regulatory and governance groups. Fisheries managers were praised for stocking programs and managing small lakes fisheries but criticized for not doing enough to protect fish populations, for an unwillingness to confront or challenge commercial and Indigenous interests and rights which infringe on conservation, and for a lack of aquatic monitoring. Three factors identified by participants contribute to fishery mismanagement, inaction, and decision paralysis: (1) insufficient resources (funding, staff, time), (2) confusion in jurisdictional authority between provincial and federal governments, and (3) organizational structure of natural resource management agencies which are not autonomous from competing commercial and industrial objectives and directions. Despite conservation being purported as the highest priority of fisheries managers, economic, social, and political drivers are perceived as increasingly influencing conservation decisions and actions. These findings can inform fisheries management and conservation decisions, policies and practices to ensure that they are more salient, robust, legitimate, and effective.
... Steelhead (Oncorhynchus mykiss) populations listed as Threatened under the US ESA (72 FR 26722, 11 May 2007) experience low survival rates as they migrate through Puget Sound, limiting productivity and impeding recovery (Moore et al., 2015;Moore & Berejikian, 2017;National Marine Fisheries Service, 2019). Puget Sound steelhead smolt-to-adult marine survival rates are strongly, negatively related to harbor seal (Phoca vitulina) abundance (Sobocinski et al., 2020), and pinniped (i.e., seals and sea lions) predation is thought to be an important factor limiting the marine survival of other imperiled salmonid populations as well (Chasco et al., 2017;Nelson et al., 2019;Wargo Rub et al., 2019). ...
Fundamental movements of migratory species can be substantially influenced by marine habitat disruptions caused by coastal infrastructure. The Hood Canal Bridge (HCB) spans the northern outlet of Hood Canal in the Salish Sea, extends 4.6 m (15 ft) underwater, and forms a partial barrier for steelhead migrating from Hood Canal to the Pacific Ocean. Spatial mark–recapture survival models using acoustic telemetry data indicate that only 49% (2017; 95% confidence interval (CI) = 40%–58%) and 56% (2018; 95% CI = 48%–65%) of the steelhead smolts encountering the HCB survived past the bridge and 7 km to the next array. We studied fine‐scale movements of more than 300 steelhead smolts to understand how migration behavior was affected across the entire length of the HCB and to quantify spatial and temporal patterns of mortality. Individually coded acoustic telemetry transmitters implanted in juvenile steelhead were used in conjunction with an extensive array of acoustic receivers surrounding the HCB to obtain approximations of the path each steelhead took as they encountered the bridge structure. Steelhead survival past the HCB appeared unaffected by tidal stage, population‐of‐origin, approach location, current velocity, or time of day, but was influenced by week of bridge encounter. Behavioral data from transmitters with temperature and depth sensors ingested by predators are consistent with high levels of marine mammal predation. This study confirms the considerable impact of the HCB on Endangered Species Act‐listed steelhead smolt survival, and provides detailed information on the behavior of steelhead smolts and their predators at the HCB for use in planning recovery actions.
... Lastly, given the magnitude of threats to steelhead trout, many study participants are in favour of adopting COSEWIC's recommendation and listingThompson River and Chilcotin River populations of steelhead under the federal Species at Risk Act(Government of Canada 2018).Interestingly, steelhead anglers did not identify pinniped predation pressure as a large threat to steelhead populations in contrast to interviewees. However, in DFO's recent recovery potential assessment for Chilcotin River andThompson River steelhead trout (2018), inshore predation from seals, especially harbour seals (Phoca vitulina), was identified as the single largest predictor of steelhead declines (see alsoMelnychuk et al. 2014;Berejikian et al. 2016;Sobocinski et al. 2020). Seal abundance has steadily increased due to marine mammal protection and DFO (2018) extrapolated seal consumption estimates fromThomas et al. (2017) for 2012 and 2013 estimating 360,000 steelhead smolts were consumed per year. ...
Wildlife managers are faced with decisions and issues that are increasingly complex, spanning natural and human dimensions. A strong evidence base that includes multiple forms and sources of knowledge would support these complex decisions. However, a growing body of literature demonstrates that environmental managers are far more likely to draw on intuition, experience, or opinion to inform important decisions rather than empirical evidence.
In 2018, I interviewed members from natural resource management branches of Indigenous (n = 4) and parliamentary (n = 33) governments, as well as nongovernmental stakeholder groups (n = 28) involved in wildlife management and conservation in British Columbia, Canada. I set out to: assess how interviewees perceive and use western-based scientific, Indigenous and local knowledge and the extent to which socio-economic and political considerations challenge the integration of evidence [Chapter 2]; examine perceptions on the current and future status of rainbow trout (Oncorhynchus mykiss) populations and fisheries [Chapter 3] (supplemented with n = 1029 online survey responses from rainbow trout anglers); and identify perceived benefits and existing barriers supporting or limiting the use of a particular type of evidence, conservation genomics [Chapter 4]. Then in 2019, I facilitated fuzzy cognitive mapping workshops with n = 12 participants from four groups of fisheries managers, detailing their perceptions on the evidence influencing freshwater fish and fisheries decisions [Chapter 5].
Collectively, this research suggests that wildlife management issues and decisions are time-sensitive and value-laden. Interviewees relied heavily on personal contacts with internal colleagues and institutional information to inform decisions and practices. Evidence which may influence decisions is within a closed social network, centralized to a handful of decision-making organizations and their partners. A lack of time and information overload were major barriers to external evidence use. A lack of trust and hesitancy to share were major barriers to Indigenous and local knowledge use. Abundant environmental evidence may not be immediately ‘actionable’ and relevant to known problems faced by decision-makers due in part to poor communication and dissemination. Participants perceived a diminishing role of evidence in decisions due to increases in socio-economic and political influence that may supersede conservation.
... In the inland waters of the Salish Sea, predation by harbor seals has become increasingly evident through new techniques to identify prey in scat [16,66] and other recently developed techniques to detect predation events through different tag technologies [67,68]. Declines in some anadromous salmonids and marine survival rates have coincided with increased harbor seal abundance over the past 40-50 years, although these associations and estimates based on bioenergetics and fisheries data do not provide a mechanistic link [17,69]. Harbor seals are estimated to consume more Chinook salmon than other marine mammal predators (e.g., Steller and California sea lions, and killer whales), largely because they consume more numerous smolts migrating to the Pacific Ocean, and resident salmon that remain in Puget Sound yearround [15]. ...
Harbor seals (Phoca vitulina richardii) in the inland waters of Washington were reduced by predator control programs in the twentieth century, but stocks have rebounded since being protected in the 1970s. Three management stocks are recognized, but there is little information on their current abundance. We conducted 38,431 km of aerial line-transect surveys throughout the range of these stocks in 2013-2016, sighting a total of 4,678 groups of harbor seals. Line-transect analysis with Beaufort sea state as a covariate provided estimates of the number of seals in the water. We then incorporated tagging data from 15 instrumented seals to develop correction factors, both for seals missed in the water while diving, and those that were on shore. Tagging data were modeled with generalized linear mixed models to provide estimates of the proportions diving and hauled out. After applying these correction factors, we estimated that the Hood Canal stock contained 1,368 seals (CV = 16.8%), the Southern Puget Sound stock contained 1,976 seals (CV = 20.5%), and the Washington Northern Inland Waters stock contained 7,513 seals (CV = 11.5%). This study presents a non-traditional approach to estimating the size of Washington inland waters harbor seal stocks, which may also be applicable to other species for which survey and tagging data are available.
Understanding the drivers of mortality during critical life history periods is an important part of increasing our capacity to rebuild depressed salmonid populations. For threatened steelhead Oncorhynchus mykiss in Puget Sound, Washington, early marine predation has been implicated as a key source of mortality. Yet, the agents that mediate predation pressure are poorly understood. In this study, we characterize abundances of juvenile Coho Salmon O. kisutch and Chinook Salmon O. tshawytscha in Puget Sound and relate these abundance patterns to weekly steelhead survival to better understand whether pulses of hatchery‐released salmonids mediate steelhead survival. We found that weekly abundances of hatchery Coho Salmon and Chinook Salmon smolts vary by several orders of magnitude across weeks, indicating that large resource pulses are available to salmonid predators. We further found that weekly steelhead survival was significantly negatively related to abundances of hatchery‐released Coho Salmon but not Chinook Salmon, which had considerably smaller body sizes than both Coho Salmon and steelhead smolts. Together, our results suggest that releases of Coho Salmon into Puget Sound mediate mortality of steelhead smolts, possibly via increased predation pressure by shared predators.
Each year, juveniles of eight salmonid species enter the Salish Sea - the inland marine waters between northwestern Washington, USA and British Columbia, Canada. These species vary in the proportions remaining there and migrating to feed in the Pacific Ocean. Such differential migration affects their growth rates, and exposure to habitat alteration, predators, fisheries, and contaminants. We review these diverse migration patterns and present data from Puget Sound illustrating the variation in downstream migration timing, residency in the Salish Sea, and upriver return timing. Recreational catch records indicate that proportionally fewer remain in the Salish Sea than in past decades for several species, and the declines began after peaks in the late 1970s – early 1980s. These declines resist easy explanation because the factors controlling residency are poorly understood, and the Salish Sea has changed over the past decades. Regardless of the cause, the diversity of migration patterns is important to the ecology of the salmon and trout species, and to the humans and other members of the Salish Sea community with which they interact.
This report synthesizes information on past, current, and emerging stressors within the Salish Sea estuarine ecosystem. The Salish Sea is a complex waterbody shared by Coast Salish Tribes and First Nations, Canada, and the United States. It is defined by multiple freshwater inputs and marine water from the Pacific Ocean that mix in two primary basins, Puget Sound and the Strait of Georgia. Human impacts are multifaceted and extensive within the Salish Sea, with a regional population of almost 9 million people. Population growth has driven urbanization and development, which in turn has triggered structural changes to the landscape and seascape. Meanwhile, the growing effects of climate change are fundamentally altering physical and biological processes. The report describes the most pervasive and damaging impacts affecting the transboundary ecosystem, recognizing that some are generated locally while others are the locally realized impacts from global-scale changes in climate, oceans, land use, and biodiversity. The Salish Sea is under relentless pressure from an accelerating convergence of global and local environmental stressors and the cumulative impacts of 150 years of development and alteration of our watersheds and seascape. Some of these impacts are well understood but many remain unknown or are difficult to predict. While strong science is critical to understanding the ecosystem, the report provides a spectrum of ideas and opportunities for how governments, organizations, and individuals can work together to meet the needs of science and science-driven management that will sustain the Salish Sea estuarine ecosystem.
Understanding how protected species influence the population dynamics of each other is an essential part of ecosystem‐based management. Chinook salmon (Oncorhynchus tshawytscha) are critical prey for endangered southern resident killer whales (SRKWs; Orcinus orca), and increasing releases of hatchery Chinook salmon has been proposed to aid SRKW recovery. We analyzed 30 yr of data and found that density‐dependent survival of hatchery Chinook salmon released into the central and southern parts of the Salish Sea (Washington, USA; and British Columbia, Canada) may be associated with the presence of naturally produced pink salmon (O. gorbuscha), which are highly abundant as juveniles only in even‐numbered years. We first modeled hatchery Chinook salmon marine survival as a function of the numbers of juvenile Chinook released and the presence of emigrating juvenile pink salmon between 1983 and 2012. Then, we related reconstructed numbers of hatchery Chinook salmon returning to Puget Sound to the abundance of juvenile Chinook released in even (pink emigration) and odd (non‐pink emigration) years from 1980 to 2010. We found that in some regions of the Salish Sea, both hatchery Chinook salmon marine survival and adult Chinook returns varied depending on the number of hatchery Chinook released and the presence of juvenile pink salmon. Specifically, in some regions survival of hatchery Chinook salmon decreased when greater numbers of juveniles were released into the Salish Sea in even years, when large numbers of pink salmon were present, but increased or remained stable when pink salmon were not present in large numbers (in odd years). This suggests lower, density‐dependent survival of juvenile Salish Sea Chinook salmon during even outmigration years. Our analyses suggest that scientists and managers should further investigate potential mechanisms for density‐dependent survival of hatchery Chinook salmon from Salish Sea hatcheries when designing strategies to maximize adult returns.
For over a century, hatchery programs have been used to subsidize natural salmon populations in order to increase fisheries opportunities and, more recently, to conserve declining natural populations. While an extensive literature has described the impacts of large‐scale hatchery operations on freshwater ecosystems, less attention has been given to ecosystem interactions within the marine environment. We analyzed records of hatchery‐released Chinook salmon in the Salish Sea to assess temporal and spatial changes in hatchery practices since 1950, with the goal of identifying potential implications for ecosystem dynamics and conservation efforts in the region. Over the past 65 yr, we found significant changes in the size and time at which juvenile salmon are released, resulting in decreased diversity of these traits. Research suggests that predation on juvenile salmon by other fish, avian, and marine mammal species could be size‐dependent, and our results indicate that current hatchery practices are releasing Chinook salmon in the size range preferred by these predators. With current marine survival rates at chronically low levels, and increasing demand for hatchery subsidies, it is important to consider how modifying existing hatchery programs intended to reduce homogenization may promote more natural marine food web dynamics, with potential benefits to both hatchery and natural Chinook populations.
Pacific salmon and trout (Oncorhynchus spp., Salmonidae) of the Puget Sound region of Washington State, USA, have experienced recent and longer‐term (multidecadal) variability in abundance while supporting robust fisheries. As part of the post‐season salmon management process, population‐specific estimates of total adult abundance to Puget Sound (Strait of Juan de Fuca) for pink (O. gorbuscha), chum (O. keta), coho (O. kisutch), sockeye (O. nerka), and Chinook (O. tshawytscha) salmon and steelhead trout (O. mykiss) are calculated annually. We compiled annual estimates of body mass, abundance and survival of hatchery‐ and naturally produced salmon from 1970 to 2015 to compare spatial and temporal patterns across species. Average weights of adult salmon and steelhead returning to Puget Sound, with the exception of coho salmon, have decreased since the 1970s. Temporal trends in abundance, survival and productivity varied by species and origin (hatchery vs. naturally produced). Generally, abundance and survival rates of natural‐origin species decreased whereas those of hatchery‐produced species did not, which is in contrast with other studies' general conclusions of decreasing survival among Puget Sound salmonids. Species diversity has decreased in recent years, with salmonids that rely on a short freshwater rearing phase in the natural environment (hatchery‐produced fish and naturally produced pink and chum) representing >90% of total returns in most years. This new information reveals patterns of body size, abundance, survival and productivity across species, life history and rearing type over the past 45 years and, in doing so, demonstrates the strength in multidecadal, multifactor time series to critically evaluate salmonid species.
Studies of climate effects on ecology often account for non-stationarity in individual physical and biological variables, but rarely allow for non-stationary relationships among variables. Here, we show that non-stationary relationships among physical and biological variables are central to understanding climate effects on salmon (Onchorynchus spp.) in the Gulf of Alaska during 1965- 2012. The relative importance of two leading patterns in North Pacific climate, the Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO), changed around 1988/1989 as reflected by changing correlations with leading axes of sea surface temperature variability. Simultaneously, relationships between the PDO and Gulf of Alaska environmental variables weakened, and long-standing temperature-salmon and PDO-salmon covariance declined to zero. We propose a mechanistic explanation for changing climate-salmon relationships in terms of non-stationary atmosphere-ocean interactions coinciding with changing PDO-NPGO relative importance. We also show that regression models assuming stationary climate-salmon relationships are inappropriate over the multidecadal time scale we consider. Relaxing assumptions of stationary relationships markedly improved modelling of climate effects on salmon catches and productivity. Attempts to understand the implications of changing climate patterns in other ecosystems might also be aided by the application of models that allow associations among environmental and biological variables to change over time. © 2018 The Author(s) Published by the Royal Society. All rights reserved.
Numerical abundance and biomass values presented here for Pink Salmon Oncorhynchus gorbuscha, Chum Salmon O. keta, and Sockeye Salmon O. nerka in the North Pacific Ocean span 90 years (1925–2015), representing the most comprehensive compilation of these data to date. In contrast to less populous species of salmon, these species are more abundant now than ever, averaging 665 × 10⁶ adult salmon each year (1.32 × 10⁶ metric tons) during 1990–2015. When immature salmon are included, recent biomass estimates approach 5 × 10⁶ metric tons. Following an initial peak during 1934–1943, abundances were low until the 1977 regime shift benefited each species. During 1990–2015, Pink Salmon dominated adult abundance (67% of total) and biomass (48%), followed by Chum Salmon (20%, 35%) and Sockeye Salmon (13%, 17%). Alaska produced approximately 39% of all Pink Salmon, 22% of Chum Salmon, and 69% of Sockeye Salmon, while Japan and Russia produced most of the remainder. Although production of natural‐origin salmon is currently high due to generally favorable ocean conditions in northern regions, approximately 60% of Chum Salmon, 15% of Pink Salmon, and 4% of Sockeye Salmon during 1990–2015 were of hatchery origin. Alaska generated 68% and 95% of hatchery Pink Salmon and Sockeye Salmon, respectively, while Japan produced 75% of hatchery Chum Salmon. Salmon abundance in large areas of Alaska (Prince William Sound and Southeast Alaska), Russia (Sakhalin and Kuril islands), Japan, and South Korea are dominated by hatchery salmon. During 1990–2015, hatchery salmon represented approximately 40% of the total biomass of adult and immature salmon in the ocean. Density‐dependent effects are apparent, and carrying capacity may have been reached in recent decades, but interaction effects between hatchery‐ and natural‐origin salmon are difficult to quantify, in part because these fish are rarely separated in catch and escapement statistics. The following management changes are recommended: (1) mark or tag hatchery salmon so that they can be identified after release, (2) estimate hatchery‐ and natural‐origin salmon in catches and escapement, and (3) maintain these statistics in publicly accessible databases.
Ecosystem approaches to natural resource management are seen as a way to provide better outcomes for ecosystems and for people, yet the nature and strength of interactions among ecosystem components is usually unknown. Here we characterize the economic benefits of ecological knowledge through a simple model of fisheries that target a predator (piscivore) and its prey. We solve for the management (harvest) trajectory that maximizes net present value (NPV) for different ecological interactions and initial conditions that represent different levels of exploitation history. Optimal management trajectories generally approached similar harvest levels, but the pathways toward those levels varied considerably by ecological scenario. Application of the wrong harvest trajectory, which would happen if one type of ecological interaction were assumed but in fact another were occurring, generally led to only modest reductions in NPV. However, the risks were not equal across fleets: risks of incurring large losses of NPV and missing management targets were much higher in the fishery targeting piscivores, especially when piscivores were heavily depleted. Our findings suggest that the ecosystem approach might provide the greatest benefits when used to identify system states where management performs poorly with imperfect knowledge of system linkages so that management strategies can be adopted to avoid those states.
Small pelagic fish are key planktivores and prey in marine ecosystems, and their population abundances undergo strong temporal and spatial variability. Top-down (predator controlled) and bottom-up (prey-driven) processes during early life history are important for determining forage fish survival and recruitment. We examined biological and environmental factors hypothesized to influence age-0 Pacific herring Clupea pallasi in the Strait of Georgia (SOG), British Columbia, Canada. Primarily bottom-up processes affected interannual variability in age-0 herring abundance and condition, with some evidence of top-down effects on condition. Age-0 herring abundance increased with increasing adult spawning biomass and peaked when most adults spawned about 20 d prior to the peak spring primary production bloom. This timeline would temporally align first-feeding herring larvae with their prey, such as small copepods. Age-0 herring abundance also increased with increasing juvenile salmon abundance, indicating that conditions favourable for herring were also favourable for their predators and competitors. Age-0 herring condition decreased with increasing spawning biomass, increased when most adults spawned closer to the peak spring bloom, increased with increasing temperatures above 8.2°C, and increased then stabilized with increasing prey zooplankton density. Age-0 herring condition had a dome-shaped relationship with predator abundance, indicating that high predator abundances negatively affected fish condition. Study results suggest that density-dependent processes, such as intraspecific competition, may be important in the SOG. A positive correlation between age-0 herring abundance and subsequent age-3 recruit abundance may provide a leading indicator of low recruitment years. © Fisheries and Oceans Canada, Terrance J. Quinn, and outside the USA, The US Government 2019. Open Access under Creative Commons by Attribution Licence. Use, distribution and reproduction are unrestricted. Authors and original publication must be credited.
Coho salmon ( Oncorhynchus kisutch ), Chinook salmon ( Oncorhynchus tshawytscha ) and steelhead ( Oncorhynchus mykiss ) in Puget Sound and the Strait of Georgia have exhibited declines in marine survival over the last 40 years. While the cause of these declines is unknown, multiple factors, acting cumulatively or synergistically, have likely contributed. To evaluate the potential contribution of a broad suite of drivers on salmon survival, we used qualitative network modelling (QNM). QNM is a conceptually based tool that uses networks with specified relationships between the variables. In a simulation framework, linkages are weighted and then the models are subjected to user-specified perturbations. Our network had 33 variables, including: environmental and oceanographic drivers (e.g., temperature and precipitation), primary production variables, food web components from zooplankton to predators and anthropogenic impacts (e.g., habitat loss and hatcheries). We included salmon traits (survival, abundance, residence time, fitness and size) as response variables. We invoked perturbations to each node and to suites of drivers and evaluated the responses of these variables. The model showed that anthropogenic impacts resulted in the strongest negative responses in salmon survival and abundance. Additionally, feedbacks through the food web were strong, beginning with primary production, suggesting that several food web variables may be important in mediating effects on salmon survival within the system. With this model, we were able to compare the relative influence of multiple drivers on salmon survival.
Experiences of migratory species in one habitat may affect their survival in the next habitat, in what is known as carryover effects. These effects are especially relevant for understanding how freshwater experience affects survival in anadromous fishes. Here, we study the carryover effects of juvenile salmon passage through a hydropower system (Snake and Columbia rivers, northwestern United States). To reduce the direct effect of hydrosystem passage on juveniles, some fishes are transported through the hydrosystem in barges, while the others are allowed to migrate in-river. Although hydrosystem survival of transported fishes is greater than that of their run-of-river counterparts, their relative juvenile-to-adult survival (hereafter survival) can be less. We tested for carryover effects using generalized linear mixed effects models of survival with over 1 million tagged Chinook salmon, Oncorhynchus tshawytscha (Walbaum) (Salmonidae), migrating in 1999–2013. Carryover effects were identified with rear-type (wild vs. hatchery), passage-type (run-of-river vs. transported), and freshwater and marine covariates. Importantly, the Pacific Decadal Oscillation (PDO) index characterizing cool/warm (i.e., productive/nonproductive) ocean phases had a strong influence on the relative survival of rear- and passage-types. Specifically, transportation benefited wild Chinook salmon more in cool PDO years, while hatchery counterparts benefited more in warm PDO years. Transportation was detrimental for wild Chinook salmon migrating early in the season, but beneficial for later season migrants. Hatchery counterparts benefited from transportation throughout the season. Altogether, wild fish could benefit from transportation approximately 2 weeks earlier during cool PDO years, with still a benefit to hatchery counterparts. Furthermore, we found some support for hypotheses related to higher survival with increased river flow, high predation in the estuary and plume areas, and faster migration and development-related increased survival with temperature. Thus, pre- and within-season information on local- and broad-scale conditions across habitats can be useful for planning and implementing real-time conservation programs.
Time series analysis is an essential method for decomposing the influences of density and exogenous factors such as weather and climate on population regulation. However, there has been little work focused on understanding how well commonly collected data can reconstruct the effects of environmental factors on population dynamics. We show that analogous to similar scale issues in spatial data analysis, coarsely sampled temporal data can fail to detect covariate effects when interactions occur on timescales that are fast relative to the survey period. We propose a method for modeling mismatched time series data that couples high-resolution environmental data to low-resolution abundance data. We illustrate our approach with simulations and by applying it to Florida's southern snail kite population. Our simulation results show that our method can reliably detect linear environmental effects and that detecting nonlinear effects requires high-resolution covariate data even when the population turnover rate is slow. In the snail kite analysis our approach performed among the best in a suite of previously used environmental covariates explaining snail kite dynamics and was able to detect a potential phenological shift in the environmental dependence of snail kites. Our work provides a statistical framework for reliably detecting population-environment interactions from coarsely surveyed time series. An important implication of this work is that the low predictability of animal population growth by weather variables found in previous studies may be due, in part, to how these data are utilized as covariates.
Examination of population abundance and survival trends over space and time can guide management and conservation actions with information about the spatial and temporal scale of factors affecting them. Here, we analyzed steelhead trout (anadromous Oncorhynchus mykiss) adult abundance time series from 35 coastal British Columbia and Washington populations along with smolt-to-adult return (smolt survival) time series from 48 populations from Washington, Oregon, and the Keogh River in British Columbia. Over 80% of the populations have declined in abundance since 1980. A multivariate autoregressive statespace model revealed smolt survival four groupings: Washington and Oregon coast, lower Columbia River, Strait of Juan de Fuca, and Puget Sound – Keogh River populations. Declines in smolt survival rates were seen for three of the four groupings. Puget Sound and Keogh River populations have experienced low rates since the early 1990s. Correlations between population pairs’ time series and distance apart illustrated that smolt survival rates were more positively correlated for proximate populations, suggesting that important processes, including those related to ocean survival, occur early in the marine life of steelhead.
Steelhead trout (Onchorhynchus mykiss) smolts suffer high mortality rates during their rapid migration through the Salish Sea. Among-population variability in mortality rates may reflect (1) genetic fitness variation among populations, (2) freshwater environmental effects on fish condition, or (3) differences in local marine conditions upon seawater entry. A reciprocal transplant experiment was conducted to separate the influence of freshwater effects (combined effects of population and freshwater environment) from effects of local marine conditions on survival of two Puget Sound steelhead populations. Steelhead smolts from the Green River in Central Puget Sound (urbanized and hatchery-influenced) and the Nisqually River in South Puget Sound (less urbanized; no hatchery influence) were tagged with acoustic telemetry transmitters and released back into their natal river or transported and released into the other river. Population of origin had little influence on probability of surviving the migration through Puget Sound. However, smolts released into the Green River had higher survival through Puget Sound (17%) than smolts released into the Nisqually River (6%); the extra 64-km migration segment for the Nisqually-released fish accounted for most of the difference between the two release locations. Neither fork length nor translocation influenced survival, though release date did affect survival of Nisqually population smolts regardless of their release location. Residence time and behavior in the two estuaries were similar, and no effects of population of origin or release date were evident. Marine travel rates also did not differ between populations, release dates, or release locations. This study indicates that mortality occurring in the Salish Sea is likely driven by processes in inland marine environments, more so than intrinsic effects of population or freshwater-rearing environments.
We describe sharp thermal limits to the ocean distribution of steelhead trout (Oncorhynchus mykiss) which restrict the distribution of steelhead to only a small fraction of the area of the North Pacific otherwise available for grazing. Maximum likelihood estimates indicate that: (1) steelhead exhibit a strong response to temperature in all regions of the North Pacific ocean and in all seasons of the year; (2) both upper and lower thermal thresholds exist; (3) although the thermal limits have been largely stable over the last 40 years, the temperature defining the southern edge of the steelhead distribution exhibits statistically significant fine-scale diWerences between regions (<2°C) and between decades (<0.5°C). As a result of this strong thermal control, steelhead are distributed in a narrow north-south band stretching across much of the width of the Pacific. Projections of the degree of ocean warming resulting from greenhouse gas induced climate change suggest that the ocean distribution of steelhead may shift far enough north under a 2xCO2 climate that large areas of currently occupied ocean habitat could lie outside the range of these thermal limits by the middle of the next century.
Conflicts can arise when the recovery of one protected species limits the recovery of another through competition or predation. The recovery of many marine mammal populations on the west coast of the United States has been viewed as a success; however, within Puget Sound in Washington State, the increased abundance of three protected pinniped species may be adversely affecting the recovery of threatened Chinook salmon (Oncorhynchus tshawytscha) and endangered killer whales (Orcinus orca) within the region. Between 1970 and 2015, we estimate that the annual biomass of Chinook salmon consumed by pinnipeds has increased from 68 to 625 metric tons. Converting juvenile Chinook salmon into adult equivalents, we found that by 2015, pinnipeds consumed double that of resident killer whales and six times greater than the combined commercial and recreational catches. We demonstrate the importance of interspecific interactions when evaluating species recovery. As more protected species respond positively to recovery ...
The Strait of Georgia, British Columbia, provides important feeding and rearing habitat for forage fish, such as Pacific Herring Clupea pallasii and Eulachon Thaleichthys pacificus as well as all species of North American Pacific salmon Oncorhynchus spp. during their juvenile out-migration. In recent decades, this region has undergone large-scale physical and biological changes. Pacific Herring and Pacific salmon populations have experienced dramatic population fluctuations, while Eulachon have failed to recover from precipitous declines in the 1990s. Archival records of stomach content data from the 1960s, collected primarily from juvenile Pacific salmon, Pacific Herring, and Eulachon, allowed us to investigate diet variability in these species 60 years ago. Consistent with contemporary reports, we found that all species except Eulachon had generalist diets. In contrast to recent studies finding that Pacific Herring are the most important fish prey, Eulachon were the most frequently consumed fish, occurring in 28% of all piscivorous fish stomachs. This suggests that Pacific Herring are an important component of some Pacific salmon diets now, but only because lipid-rich Eulachon are no longer available. Chinook Salmon O. tshawytscha and Coho Salmon O. kisutch had the most similar diets, in part because of their greater piscivory. Species, length, and month and year of capture showed some explanatory power in differentiating the diets of the fish, although they explained less than 10% of total diet variation. Historical data, such as those presented here, offer a unique opportunity to investigate temporal differences in foraging ecology, informing management on how changes in the Strait of Georgia ecosystem may impact the trophic interactions between species.
Received September 30, 2015; accepted July 5, 2016
The complexity of ecosystem-based management (EBM) of natural resources has given rise to research frameworks such as integrated ecosystem assessments (IEA) that pull together large amounts of diverse information from physical, ecological, and social domains. Conceptual models are valuable tools for assimilating and simplifying this information to convey our understanding of ecosystem structure and functioning. Qualitative network models (QNMs) may allow us to conduct dynamic simulations of conceptual models to explore natural–social relationships, compare management strategies, and identify tradeoffs. We used previously developed QNM methods to perform simulations based on conceptual models of the California Current ecosystem's pelagic communities and related human activities and values. Assumptions about community structure and trophic interactions influenced the outcomes of the QNMs. In simulations where we applied unfavorable environmental conditions for production of salmon (Oncorhynchus spp.), intensive management actions only modestly mitigated declines experienced by salmon, but strongly constrained human activities. Moreover, the management actions had little effect on a human wellbeing attribute, sense of place. Sense of place was most strongly affected by a relatively small subset of all possible pair-wise interactions, although the relative influence of individual pair-wise interactions on sense of place grew more uniform as management actions were added, making it more difficult to trace effective management actions via specific mechanistic pathways. Future work will explore the importance of changing conceptual models and QNMs to represent management questions at finer spatial and temporal scales, and also examine finer representation of key ecological and social components.
This study was designed to assess the occurrence and concentrations of a broad range of contaminants of emerging concern (CECs) from three local estuaries within a large estuarine ecosystem. In addition to effluent from two wastewater treatment plants (WWTP), we sampled water and whole-body juvenile Chinook salmon (Oncorhynchus tshawytscha) and Pacific staghorn sculpin (Leptocottus armatus) in estuaries receiving effluent. We analyzed these matrices for 150 compounds, which included pharmaceuticals, personal care products (PPCPs), and several industrial compounds. Collectively, we detected 81 analytes in effluent, 25 analytes in estuary water, and 42 analytes in fish tissue. A number of compounds, including sertraline, triclosan, estrone, fluoxetine, metformin, and nonylphenol were detected in water and tissue at concentrations that may cause adverse effects in fish. Interestingly, 29 CEC analytes were detected in effluent and fish tissue, but not in estuarine waters, indicating a high potential for bioaccumulation for these compounds. Although concentrations of most detected analytes were present at relatively low concentrations, our analysis revealed that overall CEC inputs to each estuary amount to several kilograms of these compounds per day. This study is unique because we report on CEC concentrations in estuarine waters and whole-body fish, which are both uncommon in the literature. A noteworthy finding was the preferential bioaccumulation of CECs in free-ranging juvenile Chinook salmon relative to staghorn sculpin, a benthic species with relatively high site fidelity.
Estimating diet composition is important for understanding interactions between predators and prey and thus illuminating ecosystem function. The diet of many species, however, is difficult to observe directly. Genetic analysis of fecal material collected in the field is therefore a useful tool for gaining insight into wild animal diets. In this study, we used high-throughput DNA sequencing to quantitatively estimate the diet composition of an endangered population of wild killer whales (Orcinus orca) in their summer range in the Salish Sea. We combined 175 fecal samples collected between May and September from five years between 2006 and 2011 into 13 sample groups. Two known DNA composition control groups were also created. Each group was sequenced at a ~330bp segment of the 16s gene in the mitochondrial genome using an Illumina MiSeq sequencing system. After several quality controls steps, 4,987,107 individual sequences were aligned to a custom sequence database containing 19 potential fish prey species and the most likely species of each fecal-derived sequence was determined. Based on these alignments, salmonids made up >98.6% of the total sequences and thus of the inferred diet. Of the six salmonid species, Chinook salmon made up 79.5% of the sequences, followed by coho salmon (15%). Over all years, a clear pattern emerged with Chinook salmon dominating the estimated diet early in the summer, and coho salmon contributing an average of >40% of the diet in late summer. Sockeye salmon appeared to be occasionally important, at >18% in some sample groups. Non-salmonids were rarely observed. Our results are consistent with earlier results based on surface prey remains, and confirm the importance of Chinook salmon in this population's summer diet.
Ecologists are increasingly interested in quantifying consumer diets based on food DNA in dietary samples and high-throughput sequencing of marker genes. It is tempting to assume that food DNA sequence proportions recovered from diet samples are representative of consumer's diet proportions, despite the fact that captive feeding studies do not support that assumption. Here, we examine the idea of sequencing control materials of known composition along with dietary samples in order to correct for technical biases introduced during amplicon sequencing, and biological biases such as variable gene copy number. Using the Ion Torrent PGM©, we sequenced prey DNA amplified from scats of captive harbour seals (Phoca vitulina) fed a constant diet including three fish species in known proportions. Alongside, we sequenced a prey tissue mix matching the seals’ diet to generate Tissue Correction Factors (TCFs). TCFs improved the diet estimates (based on sequence proportions) for all species and reduced the average estimate error from 28 ± 15% (uncorrected), to 14 ± 9% (TCF corrected). The experimental design also allowed us to infer the magnitude of prey-specific digestion biases and calculate Digestion Correction Factors (DCFs). The DCFs were compared to possible proxies for differential digestion (e.g., fish% protein,% lipid,% moisture) revealing a strong relationship between the DCFs and percent lipid of the fish prey, suggesting prey-specific corrections based on lipid content would produce accurate diet estimates in this study system. These findings demonstrate the value of parallel sequencing of food tissue mixtures in diet studies and offer new directions for future research in quantitative DNA diet analysis.
Changes in the Puget Sound ecosystem over the past 3 decades include increases in
harbor seal (Phoca vitulina) abundance and declines in many of their preferred prey species.
Harbor seals were outfitted with acoustic telemetry receivers and GPS tags to investigate spatial
and temporal interactions with steelhead trout Oncorhynchus mykiss smolts implanted with
acoustic transmitters. A total of 6846 tag detections from 44 different steelhead trout smolts (from
an initial group of 246 smolts released into 2 rivers) were recorded by the 11 recovered sealmounted
receivers. Central Puget Sound seal receivers detected a greater proportion of smolts
surviving to the vicinity of the haul-out locations (29 of 51; 58%) than Admiralty Inlet seal
receivers (7 of 50; 14%; p < 0.001). Detection data suggest that none of the tagged smolts were
consumed by the 11 monitored seals. Nine smolts were likely consumed by non-tagged harbor
seals based partly on detections of stationary tags at the seal capture haul-outs, although tag
deposition by other predators cannot be ruled out. Smolts implanted with continuously pinging
tags and smolts implanted with tags that were silent for the first 10 d after release were detected
in similar proportions leaving Puget Sound (95% CI for the difference between proportions:
−0.105 to 0.077) and stationary at harbor seal haul-outs (95% CI: −0.073 to 0.080). This study suggests
that harbor seals contribute to mortality of migrating steelhead smolts, and we hypothesize
that documented changes in the Puget Sound ecosystem may currently put steelhead smolts at
greater risk of predation by harbor seals and possibly other predators.
Until recently, research on mortality of anadromous fishes in the marine environment was largely limited to estimates of total mortality and association with group characteristics or the environment. Advances in sonic transmitter technology now allow estimates of survival in discrete marine habitats, yielding important information on species of conservation concern. Previous telemetry studies of steelhead Oncorhynchus mykiss smolts in Puget Sound, Washington, USA indicated that approx. 80% of fish entering marine waters did not survive to the Pacific Ocean. The present study re-examined data from previous research and incorporated data from additional Puget Sound populations (n = 7 wild and 6 hatchery populations) tagged during the same period (2006?2009) for a comprehensive analysis of steelhead early marine survival. We used mark-recapture models to examine the effects of several factors on smolt survival and to identify areas of Puget Sound where mortality rates were highest. Wild smolts had higher survival probabilities in general than hatchery smolts, with exceptions, and wild smolts released in early April and late May had a higher probability of survival than those released in early and mid-May. Steelhead smolts suffered greater instantaneous mortality rates in the central region of Puget Sound and from the north end of Hood Canal through Admiralty Inlet than in other monitored migration segments. Early marine survival rates were low (16.0 and 11.4% for wild and hatchery populations, respectively) and consistent among wild populations, indicating a common rather than watershed-specific mortality source. With segment-specific survival information we can begin to identify locations associated with high rates of mortality, and identify the mechanisms responsible.
Abstract Indicator species (IS) are used to monitor environmental changes, assess the efficacy of management, and provide warning signals for impending ecological shifts. Though widely adopted in recent years by ecologists, conservation biologists, and environmental practitioners, the use of IS has been criticized for several reasons, notably the lack of justification behind the choice of any given indicator. In this review, we assess how ecologists have selected, used, and evaluated the performance of the indicator species. We reviewed all articles published in Ecological Indicators (EI) between January 2001 and December 2014, focusing on the number of indicators used (one or more); common taxa employed; terminology, application, and rationale behind selection criteria; and performance assessment methods. Over the last 14 years, 1914 scientific papers were published in EI, describing studies conducted in 53 countries on six continents; of these, 817 (43%) used biological organisms as indicators. Terms used to describe organisms in IS research included "ecological index", "environmental index", "indicator species", "bioindicator", and "biomonitor," but these and other terms often were not clearly defined. Twenty percent of IS publications used only a single species as an indicator; the remainder used groups of species as indicators. Nearly 50% of the taxa used as indicators were animals, 70% of which were invertebrates. The most common applications behind the use of IS were to: monitor ecosystem or environmental health and integrity (42%); assess habitat restoration (18%); and assess effects of pollution and contamination (18%). Indicators were chosen most frequently based on previously cited research (40%), local abundance (5%), ecological significance and/or conservation status (13%), or a combination of two or more of these reasons (25%). Surprisingly, 17% of the reviewed papers cited no clear justification for their choice of indicator. The vast majority (99%) of publications used statistical methods to assess the performance of the selected indicators. This review not only improves our understanding of the current uses and applications of IS, but will also inform practitioners about how to better select and evaluate ecological indicators when conducting future IS research.
Changes in density-independent mortality can alter the spatial extent of populations through patch extinction and colonization, and spatial contraction may alter population productivity and compensatory capacity. Here, we analyze a time series of steelhead (Oncorhynchus mykiss) abundance and examine the hypothesis that spatial contraction can decrease compen- satory capacity. Over the last 20 years, steelhead in the Keogh River have declined by an order of magnitude because of a period of poor smolt-to-adult survival. Low abundance has been associated with more depressed production of out-migrating smolts than would be expected based on traditional models of compensatory dynamics. Patterns of juvenile density over time show changes in the spatial distribution of the population. We developed a spatially explicit population model to explore spatial structure and juvenile recruitment under varying marine survival. Results suggest that spatial contraction during a period of poor marine survival can strengthen density-dependent population regulation, reducing smolt production at the watershed scale. Our results highlight that spatial contraction can alter the fundamental density-dependent relationships that define population dynamics, recovery trajectories, and sustainable harvest levels of spatially structured populations. © 2015, National Research Council of Canada. All rights reserved.
Although Pacific salmon never swim within 2,000 km of the equatorial Pacific Ocean, this does not mean they are beyond the reach of El Nino and the Southern Oscillation (ENSO). Pacific salmon habitats are subject to robust long-distance climate linkages; tropical ENSO indicators are well correlated with the dominant pattern of North Pacific climate variations, the Pacific Decadal Oscillation (PDO), and with the dominant pattern of North America’s 20th-century salmon landings (1, 2). As such, might a change in the dominant flavor of ENSO variability and its link with North Pacific climate also affect patterns of Pacific salmon population dynamics? Kilduff et al. (3) show that, indeed, a shift in Pacific climate has a strong and previously unidentified signature in West Coast salmon survival rates; they also suggest that this shift increased the synchrony of survival rate variations for dozens of salmon populations across ∼2,000 km of coastline and destabilized the delivery of services these salmon provide to people and ecosystems.
Significance
Historically, many Pacific salmon species were thought to be influenced by the Pacific Decadal Oscillation (PDO), an indicator of ocean conditions, associated with El Niños. As the nature of El Niños has changed recently, another ocean indicator, the North Pacific Gyre Oscillation (NPGO) has become more important. By comparing time series, we find that coho and Chinook salmon survival rates along the west coast of North America are more strongly connected to the NPGO than the PDO. This was accompanied by increased similarity in survival rates of both species. Society seeks to maintain biodiversity among ecosystem components to provide a stable supply of ecosystem services through a portfolio effect. Increasingly similar salmon survivals implies reduced stability of these ecosystem services.
Understanding the factors contributing to declining smolt-to-adult survival (hereafter “smolt survival”) of Coho Salmon Oncorhynchus kisutch originating in the Salish Sea of southwestern British Columbia and Washington State is a high priority for fish management agencies. Uncertainty regarding the relative importance of mortality operating at different spatial scales hinders the prioritization of science and management activities. We therefore examined spatial and temporal coherence in smolt survivals for Coho Salmon based on a decision tree framework organized by spatial hierarchy. Smolt survival patterns of populations that entered marine waters within the Salish Sea were analyzed and compared with Pacific coast reference populations at similar latitudes. In all areas, wild Coho Salmon had higher survival than hatchery Coho Salmon. Coherence in Coho Salmon smolt survival occurred at multiple spatial scales during ocean entry years 1977–2010. The primary pattern within the Salish Sea was a declining smolt survival trend over this period. In comparison, smolt survival of Pacific coast reference populations was low in the 1990s but subsequently increased. Within the Salish Sea, smolt survival in the Strait of Georgia declined faster than it did in Puget Sound. Spatial synchrony was stronger among neighboring Salish Sea populations and occurred at a broader spatial scale immediately following the 1989 ecosystem regime shift in the North Pacific Ocean than before or after. Smolt survival of Coho Salmon was synchronized at a more local scale than reported by other researchers for Chinook Salmon O. tshawytscha, Pink Salmon O. gorbuscha, Chum Salmon O. keta, and Sockeye Salmon O. nerka, suggesting that early marine conditions are especially important for Coho Salmon in the Salish Sea. Further exploration of ecosystem variables at multiple spatial scales is needed to effectively address linkages between the marine ecosystem and Coho Salmon smolt survival within the Salish Sea. Since the relative importance of particular variables may have changed during our period of record, researchers will need to carefully match spatial and temporal scales to their questions of interest.
Coastal ecosystems face a variety of natural and anthropogenic influences, raising questions about mechanisms by which species abundance and composition change over time. We examined these questions by synthesizing 6 surface-trawling efforts in greater Puget Sound, Washington (USA), spanning 40 yr, and then determining changes in forage fish abundance and composition and jellyfish prevalence. We also assessed whether patterns were associated with potential anthropogenic pressures (human population density and commercial harvest) as well as large-scale climate signals. We found evidence for trends in abundance of all forage species in 4 sub-basins of Puget Sound. Cumulative distribution functions of catch per unit effort indicate that the historically dominant forage fishes (Pacific herring and surf smelt) have declined in surface waters in 2 sub-basins (Central and South Puget Sound) by up to 2 orders of magnitude. However, 2 other species (Pacific sand lance and three-spine stickleback) increased in all 4 sub-basins. Consequently, species composition diverged among sub-basins over the last 40 yr. In addition, jelly - fish-dominated catches increased 3- to 9-fold in Central and South Puget Sound, and abundance positively tracked human population density across all basins. The strongest predictors of forage fish declines were human population density and commercial harvest. Climate signals offered additional explanatory power for forage fish but not jellyfish catch. These patterns suggest possible linkages between coastal anthropogenic activities (e.g. development, pollution) and the abundance of forage fish and jellyfish in pelagic waters. Our findings also provide a basis for improving indicators for assessment, monitoring, and spatial planning to rehabilitate pelagic ecosystems.
Pursuit of the triple bottom line of economic, community and ecological sustainability has increased the complexity of fishery management; fisheries assessments require new types of data and analysis to guide science-based policy in addition to traditional biological information and modeling. We introduce the Fishery Performance Indicators (FPIs), a broadly applicable and flexible tool for assessing performance in individual fisheries, and for establishing cross-sectional links between enabling conditions, management strategies and triple bottom line outcomes. Conceptually separating measures of performance, the FPIs use 68 individual outcome metrics-coded on a 1 to 5 scale based on expert assessment to facilitate application to data poor fisheries and sectors-that can be partitioned into sector-based or triple-bottom-line sustainability-based interpretative indicators. Variation among outcomes is explained with 54 similarly structured metrics of inputs, management approaches and enabling conditions. Using 61 initial fishery case studies drawn from industrial and developing countries around the world, we demonstrate the inferential importance of tracking economic and community outcomes, in addition to resource status.
For widely distributed species at risk, such as Pacific salmon (Oncorhynchus spp.), habitat monitoring is both essential and challenging. Only recently have widespread monitoring programs been implemented for salmon habitat in the Pacific Northwest. Remote sensing data, such as Landsat images, are therefore a useful way to evaluate trends prior to the advent of species-specific habitat monitoring programs. We used annual (1986-2008) land cover maps created from Landsat images via automated algorithms (LandTrendr) to evaluate trends in developed (50-100% impervious) land cover in areas adjacent to five types of habitat utilized by Chinook salmon (O. tshawytscha) in the Puget Sound region of Washington State, U.S.A. For the region as a whole, we found significant increases in developed land cover adjacent to each of the habitat types evaluated (nearshore, estuary, mainstem channel, tributary channel, and floodplain), but the increases were small (<1% total increase from 1986 to 2008). For each habitat type, the increasing trend changed during the time series. In nearshore, mainstem, and floodplain areas, the rate of increase in developed land cover slowed in the latter portion of the time series, while the opposite occurred in estuary and tributary areas. Watersheds that were already highly developed in 1986 tended to have higher rates of development than initially less developed watersheds. Overall, our results suggest that developed land cover in areas adjacent to Puget Sound salmon habitat has increased only slightly since 1986 and that the rate of change has slowed near some key habitat types, although this has occurred within the context of a degraded baseline condition.
Strongly positive temperature anomalies developed in the NE Pacific Ocean during the boreal winter of 2013–14. Based on a mixed layer temperature budget, these anomalies were caused by lower than normal rates of the loss of heat from the ocean to the atmosphere, and of relatively weak cold advection in the upper ocean. Both of these mechanisms can be attributed to an unusually strong and persistent weather pattern featuring much higher than normal sea level pressure over the waters of interest. This anomaly was the greatest observed in this region since at least the 1980s. The region of warm SST anomalies subsequently expanded and reached coastal waters in spring and summer 2014. Impacts on fisheries and regional weather are discussed. It is found that sea surface temperature anomalies in this region affect air temperatures downwind in Washington state.
The Puget Sound Steelhead Technical Recovery Team (PSS TRT) convened in March 2008 to review information relevant to the identification of historical demographically independent populations (DIPs) of steelhead (Oncorhynchus mykiss) in the Puget Sound steelhead distinct population segment (DPS). The PSS TRT identified three major population groups (MPGs) containing a total of 32 steelhead DIPs in Puget Sound.
Steelhead in the Puget Sound DPS exhibit two distinct life history strategies: summer-run and winter-run migrations. Winter-run steelhead, also known as ocean-maturing steelhead, return to freshwater during the winter and early spring months and spawn relatively soon after entering freshwater. Alternatively, summer-run (stream-maturing) steelhead return to freshwater during late spring and early summer in a relatively immature state and hold there until spawning in the following winter/spring. Generally, but not necessarily, summer-run steelhead return-timing is coordinated with river flow patterns that allow access past barriers to headwater spawning areas. Presently and historically, winter-run steelhead numerically represent the predominant life history type in Puget Sound.
Steelhead exhibit considerable diversity in age at smoltification, age at return or maturation, and spawning timing and repeat spawning (iteroparity). Overall, there were few clear trends in these life history traits across the Puget Sound DPS. Steelhead in lowland, rain-dominated streams tended to spawn earlier than fish in upland or headwater, snowfall-dominated streams. Information on life history characteristics is limited for all but a few DIPs and completely absent for others, especially for summer-run populations. Additionally, there is little information available on ocean migratory patterns outside of Puget Sound and, until recently, steelhead tagging studies have not been undertaken to any great degree.
The PSS TRT reviewed available information on Puget Sound steelhead, which included life history and genetic data. This information was not universally available for all populations and, in many cases, ecological information was used to estimate life history characteristics. In the absence of historical demographic information (e.g., abundance, spatial structure), the TRT also used basin characteristics to estimate the potential historical size and level of interaction between prospective populations. The TRT initially utilized an expert panel system to develop criteria for establishing DIP criteria, but ultimately incorporated these criteria into a decision support system to identify DIPs. DIPs were in turn organized in MPGs. These larger scale units delineate DPS-wide spatial structure. The TRT identified MPGs based on the geographic and ecological characteristics of the DPS and the genetic clustering of existing steelhead populations in Puget Sound.
As a preliminary filter for putative DIPs, the TRT only considered basins with intrinsic productivity (based on stream area) equal to or greater than that estimated for Snow Creek, an apparently self-sustaining, small, wild population located on the northeastern corner of the Olympic Peninsula. The decision support system relied on basin intrinsic potential, basin elevation, snow cover, distances between potential DIPs, genetic differences between potential DIPs, life history differences between potential DIPs, and the presence of temporal migrational barriers between potential DIPs. The decision support system, or gatekeeper model, required that the TRT estimate for each factor a threshold value that indicated populations were demographically independent with a very high certainty. One of the benefits of this system was that missing information did not bias the outcome.
The boundaries for historical DIPs were in part established using information related to two isolating mechanisms: homing fidelity and migration timing. Homing fidelity was examined to estimate the extent of adult exchange among putative spawning populations. Analysis of the terminal recoveries of adult marked hatchery fish indicates that less than 10% of the recoveries occur more than 50 km from the mouth of their natal stream (stream of release). Within a basin, temporal differences in return migration and spawn timing provided mechanisms for establishing demographically and reproductively isolated populations. Adult run and spawn timing are often coordinated with stream hydrology and temperature, which in turn are strongly affected by basin elevation. Major run-timing (e.g., summer and winter) differences were used as one criterion for distinguishing DIPs in the gatekeeper decision support system, especially where temporal barriers provided a reproductive barrier between presumptive DIPs.
In the Puget Sound DPS, three MPGs were identified: Northern Cascades, Central and South Puget Sound, and Hood Canal and Strait of Juan de Fuca. Within the Northern Cascades MPG, 16 DIPs (8 winter run, 3 summer/winter run, 5 summer run) were identified as historically present. In the Central and South Puget Sound MPG, 8 winter-run DIPs were historically present. There was some discussion regarding the presence of an additional historical summer-run DIP in the Green River or, alternatively, that the Green River winter-run DIP should be designated as a mixed summer/winter-run DIP, although the information available was not considered compelling. Additionally, while there are no known native-origin summer-run steelhead currently in the Green River (i.e., the summer-run steelhead currently released into and naturally spawning in the Green River originated from the Skamania Hatchery in the Columbia River basin), it is possible that resident O. mykiss above Howard Hansen Dam may contain the genetic legacy of a summer run. The Hood Canal and Strait of Juan de Fuca MPG historically contained 8 DIPs (1 summer/winter run and 7 winter run, with 2 of these winter runs possibly historically including summer-run components).
Where steelhead population information was available, especially genetic information, it was possible to identify steelhead DIPs with a relatively high degree of certainty. In other cases, ecological information provided a reasonable proxy for population data. The TRT strongly recommends further life history and genetics sampling and evaluation, especially in those areas currently less well studied. For some populations, basic abundance data are still lacking and need to be collected. It is likely that, in the process of collecting additional information on these populations, some revision in the DPS population structure will be necessary and should be undertaken.
Predation risk and competition among conspecifics significantly affect survival of juvenile salmon, but are rarely incorporated into models that predict recruitment in salmon populations. Using densities of harbour seals (Phoca vitulina) and numbers of hatchery-released Chinook salmon (Oncorhynchus tshawytscha) smolts as covariates in spatially structured Bayesian hierarchical stock–recruitment models, we found significant negative correlations between seal densities and productivity of Chinook salmon for 14 of 20 wild Chinook populations in the Pacific Northwest. Changes in numbers of seals since the 1970s were associated with a 74% decrease (95% CI: −85%, −64%) in maximum sustainable yield in Chinook stocks. In contrast, hatchery releases were significantly correlated with Chinook productivity in only one of 20 populations. Our findings are consistent with recent research on predator diets and bioenergetics modeling that suggest there is a relationship between harbour seal predation on juvenile Chinook and reduced marine survival in parts of the eastern Pacific. Forecasting, assessment, and recovery efforts for salmon populations of high conservation concern should thus consider including biotic factors, particularly predator– prey interactions.
Marine strategy framework directive (MSFD) Generalized additive modelling (GAM) State space approach Marine food web indicators Baltic sea Zooplankton Pelagic fish A B S T R A C T Finding suitable state indicators is challenging and cumbersome in stochastic and complex ecological systems. Typically, a great focus is given to criteria such as data availability, scientific basis, or measurability. Features associated with the indicator's performance such as sensitivity or robustness are often neglected due to the lack of quantitative validation tools. In this paper, we present a simple but flexible framework for selecting and validating the performance of food web indicators. In specific, we suggest a 7-step process in which indicator performances at a regional scale are quantified and visualized allowing for the selection of complementary indicator suites. We demonstrate its application by comparing the performance of pelagic food web indicators for three basins of the Baltic Sea and by assessing the food web status based on selected indicator suites. Our analysis sheds light on spatial differences in indicator performances with respect to direct and indirect pressures, the role of non-linearity and non-additivity in pressure responses, as well as relationships between indicators caused by species interactions. Moreover, our results suggest that the present food web states in the Bornholm and Gotland basins of the Baltic Sea deviate distinctly from an earlier reference period. We advocate the use of our quantitative framework as decision-support tool for selecting suites of complementary indicators under given management schemes such as the EU Marine Strategy Framework Directive.
Climate change is affecting arctic and subarctic ecosystems, and anadromous fish such as Pacific salmon Oncorhynchus spp. are particularly susceptible due to the physiological challenge of spawning migrations. Predicting how migratory timing will change under Arctic warming scenarios requires an understanding of how environmental factors drive salmon migrations. Multiple mechanisms exist by which environmental conditions may influence migrating salmon, including altered migration cues from the ocean and natal river. We explored relationships between interannual variability and annual migration timing (2003–2014) of Sockeye Salmon O. nerka in a subarctic watershed with environmental conditions at broad, intermediate, and local spatial scales. Low numbers of Sockeye Salmon have returned to this high-latitude watershed in recent years, and run size has been a dominant influence on the migration duration and the midpoint date of the run. The duration of the migration upriver varied by as much as 25 d across years, and shorter run durations were associated with smaller run sizes. The duration of the migration was also extended with warmer sea surface temperatures in the staging area and lower values of the North Pacific Index. The midpoint date of the total run was earlier when the run size was larger, whereas the midpoint date was delayed during years in which river temperatures warmed earlier in the season. Documenting factors related to the migration of Sockeye Salmon near the northern limit of their range provides insights into the determinants of salmon migrations and suggests processes that could be important for determining future changes in arctic and subarctic ecosystems.
Received October 24, 2016; accepted March 2, 2017
Knowing the species and life stages of prey that predators consume is important for understanding the impacts that predation may have on prey populations, but traditional methods for determining diets often cannot provide sufficient detail. We combined data from two methods of scat analysis (DNA metabarcoding and morphological prey ID) to quantify the species and life stages of salmon (Oncorhynchus spp.) consumed by harbour seals (Phoca vitulina) in the Strait of Georgia, Canada, where juvenile Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon survival is poor. Harbour seals primarily consumed adult salmon of lesser conservation concern in the fall (August–November): chum (Oncorhynchus keta: 18.4%), pink (Oncorhynchus gorbuscha: 12.6%), sockeye (Oncorhynchus nerka: 7.4%), Chinook (7.1%), and coho (1.8%). However, the opposite species trend occurred during the spring when seals preferred juvenile salmon of greater conservation concern (April–July): coho (2.9%), Chinook (2.9%), sockeye (2.5%), pink (1.4%), and chum (0.8%)—percentages that can equate to many individuals consumed. Our data suggest that harbour seals select juveniles of salmon species that out-migrate at ages >1 year and provide evidence of a potential causal relationship between harbour seal predation and juvenile salmon survival trends.
Marine ecosystems are complex adaptive systems with physical and biological processes operating on multiple spatial and temporal scales. Here, we present an operational regional indicator for California's continental shelf system and investigate its skill in predicting a variety of biological responses across trophic levels. This updated Multivariate Ocean Climate Indicator (MOCI) version 2 includes data that are readily available from the Internet so the indicator can be automatically updated and shared regularly. MOCIv.2 is a simplified version of MOCIv.1, but it captures ocean-climate variability similarly. MOCIv.2 illustrates all major ENSO events that occurred over the past 25 years as well as the phasing and magnitude of the most recent North Pacific marine heat wave, dubbed ‘The Blob’. It also shows differences in the magnitude and timing of ocean-climate variability in different regions off California. MOCIv.2 has skill in nowcasting marine ecosystem dynamics, from zooplankton to top predators, and therefore may be useful in establishing bio-physical relationships important to ecosystem-based fisheries and wildlife management in California.
Model choice is usually an inevitable source of uncertainty in model-based statistical analyses. While the focus of model choice was traditionally on methods for choosing a single model, methods to formally account for multiple models within a single analysis are now accessible to many researchers. The specific technique of model averaging was developed to improve predictive ability by combining predictions from a set of models. However, it is now often used to average regression coefficients across multiple models with the ultimate goal of capturing a variable's overall “effect.” This use of model averaging implicitly assumes the same parameter exists across models so that averaging is sensible. While this assumption may initially seem tenable, regression coefficients associated with particular explanatory variables may not hold equivalent interpretations across all of the models in which they appear, making explanatory inference about covariates challenging. Accessibility to easily implementable software, concerns about being criticized for ignoring model uncertainty, and the chance to avoid having to justify choice of a final model have all led to the increasing popularity of model averaging in practice. We see a gap between the theoretical development of model averaging and its current use in practice, potentially leaving well-intentioned researchers with unclear inferences or difficulties justifying reasons for using (or not using) model averaging. We attempt to narrow this gap by revisiting some relevant foundations of regression modeling, suggesting more explicit notation and graphical tools, and discussing how individual model results are combined to obtain a model averaged result. Our goal is to help researchers make informed decisions about model averaging and to encourage question-focused modeling over method-focused modeling. This article is protected by copyright. All rights reserved.
The first edition of this book has established itself as one of the leading references on generalized additive models (GAMs), and the only book on the topic to be introductory in nature with a wealth of practical examples and software implementation. It is self-contained, providing the necessary background in linear models, linear mixed models, and generalized linear models (GLMs), before presenting a balanced treatment of the theory and applications of GAMs and related models. The author bases his approach on a framework of penalized regression splines, and while firmly focused on the practical aspects of GAMs, discussions include fairly full explanations of the theory underlying the methods. Use of R software helps explain the theory and illustrates the practical application of the methodology. Each chapter contains an extensive set of exercises, with solutions in an appendix or in the book’s R data package gamair, to enable use as a course text or for self-study.
Yukon River Chinook salmon (Oncorhynchus tshawytscha) populations are declining for unknown reasons, creating hardship for thousands of stakeholders in subsistence and commercial fisheries. An informed response to this crisis requires understanding the major sources of variation in Chinook salmon productivity. However, simple stock–recruitment models leave much of the variation in this system’s productivity unexplained. We tested adding environmental predictors to stock–recruitment models for two Yukon drainage spawning streams in interior Alaska — the Chena and Salcha rivers. Low productivity was strongly associated with high stream discharge during the summer of freshwater residency for young-of-the-year Chinook salmon. This association was more consistent with the hypothesis that sustained high discharge negatively affects foraging conditions than with acute mortality during floods. Productivity may have also been reduced in years when incubating eggs experienced major floods or cold summers and falls. These freshwater effects — especially density dependence and high discharge — helped explain population declines in both rivers. They are plausible as contributors to the decline of Chinook salmon throughout the Yukon River drainage.
In the coastal British Columbia stream adult runs averaged 922 (range 209-2730) with c10% repeat spawning incidence. Females repeat spawned more than males and were more abundant as kelts, but maiden run adults were equally male and female. The proportion of males returning after 1, 2 and 3 yr in the ocean averaged 3, 62 and 35%, respectively; 58 and 42% of females returned after 1, 2 and 3 yr, respectively. Adult age structure, smolt number, and smolt size varied biennially. Adult size decreased with freshwater age, but increased with ocean age of returns. Males were larger at each ocean age. Smolts were 2-5 yr old, and freshwater age 3 was most prevalent (average 56%). Mean survival from smolt to adult was 16% (7% from 1978 cohorts to 26% from 1982 cohorts). Survival was positively correlated with smolt length and weight, but 1982 cohorts had twice the survival of other cohorts, possibly related to El Nino. On average, freshwater age was inversely related to ocean age. -from Authors