Deconstructing dams and disease: predictions for salmon disease risk following Klamath River dam removals

The health of fish populations and the river systems they inhabit have broad ecological, cultural, recreational, and economic relevance. This is exemplified by the iconic anadromous salmonid fishes native to the West Coast of North America. Salmon populations have been constrained since the mid nineteenth century by dam construction and water reallocation. In the Klamath River (Oregon and California, USA), a series of dams built in the early-mid 20th century cut the basin in two and blocked anadromous fish access to more than 600 river kilometers. This dramatic loss of habitat, coupled with infectious diseases and resulting epizootics, have impacted the wellbeing of these salmonid populations. In 2023-2024, the Klamath River will undergo the largest river restoration project in US history. Removal of the four lowermost dams will cause profound physical changes to the river, including flow, water temperature, and channel geomorphology. The dam removals will reconnect the lower and upper portions of the basin, and provide fish passage after a century of segregation. Reestablishment of upstream and downstream fish movements will also alter the occupancy and abundance of the salmonid hosts and their pathogens. The increased habitat availability and longer migration routes will increase duration of pathogen exposure and potential impacts on juvenile survival and adult pre-spawn mortality. However, restoration of more natural flow and sediment regimes will decrease overall fish disease risk by disrupting complex parasite life cycles. To better understand these multifarious, competing factors, we review the salmonid species in the Klamath River, and provide an overview of their historical pathogen challenges and associated diseases and use this as a framework to predict the effects of dam removals on disease dynamics. Our review and predictions are a synthesis of expertise from tribal biologists, fish health specialists and fish biologists, many of whom have lived and worked on the Klamath River for decades. We conclude with recommendations for expansion of current pathogen monitoring and research efforts to measure changes in host-pathogen dynamics basin-wide

Fish Release Strategies at Iron Gate Hatchery: 2007 Fall Yearling Chinook Salmon Emigration Behavior

Abstract: Greater return rates and reduced competition with wild fish suggest that substantially increasing the proportion of Chinook salmon released from Iron Gate Hatchery as yearlings in the fall as opposed to sub-yearlings in the early summer is likely to be beneficial. However, prior to this study there was no information on the emigration behavior of yearling Chinook salmon released from Iron Gate Hatchery to help validate this management prescription. In order to address this lack of information, we surgically implanted a total of 30 Chinook salmon yearlings with radio transmitters and released them from Iron Gate Hatchery on November 13, 2007 as part of their typical yearling release. Of these, at least 23 (77%) emigrated to the first receiver station at rkm 282; by comparison a minimum of five of the 30 tagged fish reached the estuary, yielding an overall apparent survival rate of 17% with an average emigration duration of 44.6 d and an average travel rate of 6.6 km/d. No sources of mortality were documented. Apparent survival was calculated as opposed to known survival because the final fates of fish were not known with certainty. For example, fish that were that entered a reach but did not exit were assumed to be mortalities, but other possible reasons could include tag failure, tag loss, or fish holding until after tag battery exhaustion. None of these scenarios are considered likely for a variety of reason but it should be noted that apparent survival estimates can be consider the minimal actual survival experienced by tagged fish and would be adjusted upwards if any of the other reasons did indeed occur. All applicable fish exhibited slowed emigration and apparent holding at some point above the Salmon River confluence (rkm 107). The majority of fish emigrated from the vicinity of the hatchery at rkm 309 to below rkm 282 relatively quickly (mean of 1.7 d and 15.9 km/d; n=18). The most common area of slowed emigration was between the Shasta to the Scott rivers (mean of 23.5 d and 2.1 km/d; n=6). The fastest emigration rates (individual maximum of 149.2 km/d for a given reach) were observed in the lower Klamath River below its confluence with the Salmon River wherein the highest apparent survival rates were also observed. Depending of the precision of information required, another year of investigation may be warranted in which case the same study approach should be used with a greater sample size (e.g. 100 to 300 tags), and if funding permits, manual tracking and ultrasonic transmitter capabilities should be added