Disease in Central Valley Salmon: Status and Lessons from Other Systems Brendan M
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Sponsored by the Delta Science Program and the UC Davis Muir Institute POLICY & PROGRAM ANALYSIS Disease in Central Valley Salmon: Status and Lessons from Other Systems Brendan M. Lehman,*1,2 Rachel C. Johnson,2,3 Mark Adkison,4 Oliver T. Burgess,5 Richard E. Connon,6 Nann A. Fangue,7 J. Scott Foott,8 Sascha L. Hallett,9 Beatriz Martínez–López,10 Kristina M. Miller,12 Maureen K. Purcell,13 Nicholas A. Som,14,15 Pablo Valdes Donoso,11 Alison L. Collins16 ABSTRACT has been linked to declining numbers of Chinook Salmon (Oncorhynchus tshawytscha) salmonids, and identified as a key stressor are increasingly vulnerable to anthropogenic that results in mortality. In the Central activities and climate change, especially at Valley, targeted studies have revealed their most southern range in California’s extremely high prevalence of infectious Central Valley. There is considerable agents and disease. However, there has been interest in understanding stressors that insufficient monitoring to understand the contribute to population decline and in effect that disease may have on salmon identifying management actions that populations. To inform future research, reduce the effects of those stressors. Along monitoring, and management efforts, a the west coast of North America, disease two-day workshop on salmon disease was SFEWS Volume 18 | Issue 3 | Article 2 https://doi.org/10.15447//sfews.2020v18iss3art2 * Corresponding author: [email protected] 9 Department of Microbiology, Oregon State University 1 Physical and Biological Sciences, Univ. of California, Corvallis, Oregon 97331-3804, USA Santa Cruz, Santa Cruz, CA 95060 USA 10 Center for Animal Disease Modeling and Surveillance, 2 Southwest Fisheries Science Center, NMFS / NOAA Dept. of Medicine and Epidemiology, Santa Cruz, CA 95060 USA School of Veterinary Medicine, Univ. of California, Davis 3 Center for Watershed Sciences, Univ. of California, Davis, California 95616, USA Davis Davis, CA 95616 USA 11 University of California Agricultural Issues Center 4 Fish Health Lab, California Dept. of Fish and Game Davis, California 95616, USA Rancho Cordova, CA 95670, USA 12 Pacific Biological Station, Fisheries and Oceans Canada 5 Bay-Delta Office, US Bureau of Reclamation Nanaimo, British Columbia, V9T 6N7, Canada Sacramento, CA 95814 USA 13 Western Fisheries Research Center, US Geological Survey 6 School of Veterinary Medicine, Univ. of California, Seattle, WA 98115 USA Davis Davis, CA 95616, USA 14 US Fish and Wildlife Service 7 Department of Wildlife, Fish, and Conservation Arcata, California 95521 USA Biology, University of California, Davis 15 Dept. of Fisheries Biology, Humboldt State University Davis, CA 95616 USA Arcata, CA 95519 USA 8 California–Nevada Fish Health Center, US Fish and 16 The Metropolitan Water District of Southern California Wildlife Service, Anderson, CA 96007 USA Sacramento, CA 95814 USA 1 SAN FRANCISCO ESTUARY & WATERSHED SCIENCE VOLUME 18, ISSUE 3, ARTICLE 2 held at the University of California, Davis because of difficulties observing and (UC Davis) on March 14-15, 2018. This paper sampling diseased fish in the wild (Hedrick summarizes the science presented at this 1998). Diseased fish functionally disappear workshop, including the current state of from existing monitoring programs because knowledge of salmonid disease in the Central they suffer disease-associated mortality such Valley, and current and emerging tools to as being eaten by predators (Miller et al. better understand its effects on salmon. We 2014). Therefore, by only sampling survivors, highlight case studies from other systems researchers are often left with incomplete where successful monitoring programs have and inadequate information, making it been implemented. First, in the Klamath difficult to implement sound, scientifically River where the integration of several data- accurate management decisions. Despite the collection and modeling approaches led to challenges, substantial progress has been the development of successful management made toward understanding the effects of actions, and second in British Columbia infectious agents on Pacific Salmon at both where investment in researching novel the individual and population levels. For technologies led to breakthroughs in the example, novel research techniques (Hallett understanding of salmon disease dynamics. and Bartholomew 2006; Miller et al. 2014) Finally, we identify key information and and the implementation of comprehensive knowledge gaps necessary to guide research multi-agency monitoring networks (e.g., for and management of disease in Central Valley Ceratonova shasta in the Klamath River) have salmon populations. led to integration of pathogen monitoring into adaptive management strategies. Along the west coast of the United States and KEY WORDS Canada, pathogens and associated disease pathogen, infectious disease, Ceratonova shasta are increasingly identified as a stressor (previously Ceratomyxa shasta), Pacific salmon that contributes to mortality, and are hypothesized to be one potential factor in declining salmonid populations (St-Hilaire et INTRODUCTION al. 2002; Belchik et al. 2004; Fujiwara et al. Salmon populations along the west coast 2011; Jeffries et al. 2014a; Bass et al. 2019; of North America have been in decline Teffer et al. forthcoming [2020]). since the early 1900s. Pacific Salmon are culturally iconic and economically Improving the condition and survival important, and there is considerable interest of both rearing and migrating Chinook in understanding factors that contribute Salmon is necessary for their recovery in to their decline as well as in identifying California’s Central Valley (NMFS 2014). management tools to facilitate population To implement effective management actions to recovery. Salmon have adapted to persist increase survival, we need to understand the through extreme environmental conditions, individual and interactive effects of factors such predation pressures, variable resource as temperature, flow, food availability, habitat availability, and disease. However, humans quality, predator-prey interactions, and disease. have altered freshwater systems, reducing the Several salmon monitoring programs in the amount of spawning, rearing, and migratory Central Valley already exist to monitor these habitat required for abundant and persistent factors. There have also been recent efforts to populations of anadromous fishes. improve the efficacy of these programs and their ability to inform relevant management questions Infectious agents likely play a role in (Johnson et al. 2017). To date, however, pathogen salmonid population dynamics. Yet, and disease monitoring in the Central Valley as quantifying this role remains challenging important components of fish condition have 2 SEPTEMBER 2020 been omitted from status and trend monitoring some pathogens are highly virulent, many more frameworks, resulting in a lack of information for opportunistic pathogens only cause disease when practitioners to incorporate disease into robust the host becomes compromised for other reasons, management plans (Johnson et al. 2017). such as unfavorable environmental factors. Even under favorable conditions, salmon are constantly On March 14-15, 2018, UC Davis hosted a Salmon exposed to a variety of pathogens. Whether or not Disease Symposium aimed at understanding infection develops into a disease state in a fish the present state of knowledge of Central Valley depends on pathogen exposure levels (external salmon disease ecology, defining past and current agent), the susceptibility of the individual (host), monitoring efforts, and identifying key knowledge and the environment in which host and agent are gaps. The goal of this review is to summarize brought together. Collectively, this is known as the information presented by the attendees of the epidemiological triad (Box 1). this workshop. Additionally, we draw on research and case studies from other systems to present It is important to distinguish the differences potential future research opportunities that could among the presence of infectious agents in the provide fisheries and water managers with better environment, whether or not those agents have information for managing salmon populations. infected the host, and whether that infection Last, we provide recommendations for designing has led to disease. Dozens of infectious agents a monitoring framework for salmon disease in the known to infect Pacific salmonids persist in the Central Valley.1 environment at all times. For example, many microparasites such as the myxozoans Ceratonova shasta and Parvicapsula minibicornis are present INFECTIOUS AGENTS AND DISEASE OVERVIEW in freshwater environments year-round, but Infectious agents that cause disease in salmon the density and virulence of spores fluctuates include viral, bacterial, fungal, protozoan, and through time and space (Bartholomew et al. myxozoan microparasites (Table 1) as well as 2007; Hallett et al. 2012). Host organisms have macroparasites such as sea lice. They are innately developed immune system coping mechanisms part of the ecosystems in which salmon live to deal with constant exposure. Often, fish host and have co-evolved with their salmon hosts. multiple pathogens and parasitic organisms at However, rapid environmental changes can upset any given time without exhibiting any noticeable the balance of host-pathogen interaction. While disease or adverse effect. For bacterial or viral infections, an infected fish only becomes 1. To address the