https://cpw.cvlcollections.org/files/original/b42853a72473033ec451693bbb3174c4.pdf 57c29972cdb8f74bdb39b824134d4376 PDF Text Text C O L O R A D O P A R K S & W I L D L I F E Rainbow Trout Fry Investigations FRY SAMPLING INFORMS SURVIVAL, DISEASE ISSUES, AND HABITAT RESTORATION IN LARGE RIVER SYSTEMS Early Life Stages of Rainbow Trout in Colorado’s Rivers A B C D Rainbow Trout eggs (A) and sac fry (B). After swimming up from the gravel, fry are available for sampling (C and D). Rainbow Trout are spring spawners, and deposit their eggs in gravel at sites known as redds. The eggs hatch in 4-7 weeks depending on water temperature. After hatching, fish spend up to two weeks as sac fry, remaining in the gravel and absorbing their yolk sac. Once the yolk sac is absorbed, the fry emerge from the gravel to begin feeding. Fry spend the first several months of their life in shallow areas found along the river margins, using vegetation and interstitial spaces between rocks as cover from predators. During this time, they are also easily sampled to estimate population size. Changes in the fry population over time can indicate whether factors such as water quality, flow, temperature, or presence of parasites are affecting survival. Additionally, fry estimates conducted later in the year can be indicative of the number of fish that will recruit to the juvenile and adult populations. Finding ways to increase fry survival in rivers has been the focus of many CPW aquatic research projects. Conducting Fry Population Estimates Fry population estimates are conducted using one to three backpack electrofishing units and making three passes through a 50-foot site along the river margins. Fish are removed on each pass, and kept separate until they can be counted and measured after all three passes have been completed. Flows, habitat complexity, and differences in how the electricity affects each individual causes some fish to be missed on each pass. Equations used to calculate a population estimate account for this imperfect Electrofishing for Rainbow Trout fry detection between passes to generate a population estimate. This estimate per 50 feet of riverbank is then standardized to number of fry per mile to allow comparison between sites or sections of the river. The example below produced an estimate of 70 (± 1) salmonid fry per 50 feet or 7,376 (± 1,978) fry per mile. Pass 1 Pass 2 Pass 3 Forty-five salmonid fry were removed from this 50-foot site on pass 1. An additional 13 were removed on pass 2 and eight removed on pass 3. A total of 66 fry were collected across the three passes. COLORADO PARKS & WILDLIFE • 317 West Prospect Road, Fort Collins, CO 80526 • (970) 472-4436 • cpw.state.co.us �Fry as Disease and Management Indicators The disappearance of Rainbow Trout fry from many of Colorado’s large rivers was one of the first indicators of a problem after whirling disease (caused by Myxobolus cerebralis) first arrived in these systems in the early 1990s. Fry are most susceptible to infection, and susceptible Rainbow Trout fry exposed to the parasite rarely survive past their first year of life. With no fry available to recruit to the adult population, these losses resulted in the collapse of Rainbow Trout populations across Colorado. Whirling disease resistant Rainbow Trout developed and produced by CPW have been stocked throughout the state to reestablish these lost Rainbow Trout populations. 250 Adult Rainbow Trout per Mile CPW started stocking whirling disease resistant Rainbow Trout fry in the upper Colorado River to 200 increase post-stocking survival in 2013. At that time, there were only three adult Rainbow Trout per mile in the river. The first strain CPW stocked was the 150 Hofer strain crossed with Colorado River Rainbow Trout (H×C), which was stocked in 2013-2015. H×C 100 fry survived well, with over 1,100 H×C fry remaining in October of each year. In response, the adult population showed an exponential increase, with up 50 to 212 Rainbow Trout per mile in 2017. Unfortunately, a separate hatchery disease outbreak 0 in the brood stock (bacterial kidney disease) caused 2013 2014 2015 2016 2017 2018 2019 2020 2021 that strain to be discontinued in 2015. Pure Hofer strain fry were stocked in the upper Colorado River in 2016-2018, but they did not survive as well in this river as the H×Cs. Only 250 Hofer fry per mile remained in October of each year, not enough to sustain the adult population, which declined between 2018 and 2020. By that time, CPW had created a replacement strain for the H×C, a cross between the Hofer and Gunnison River Rainbow (H×G). H×G fry were stocked in the upper Colorado River in 2019-2021 and showed excellent survival, with over 1,500 H×G fry per mile remaining in October of each year. The H×G fry also showed good recruitment, with an increase of 50 adult Rainbow Trout per mile between 2020 and 2021, and a continued increase in the adult population expected in 2022 and 2023. Fry Habitat Associations Data obtained from fry evaluations show how important fry survival is for recruitment to and management of adult populations. Although habitat restoration projects typically focus on adult fish habitat, creating ideal fry habitat will help to establish self-sustaining whirling disease resistant Rainbow Trout populations. A recent study in the upper Colorado River showed that Brown Trout and Rainbow Trout fry overlap in their habitat associations, but there are opportunities for both single- and multi-species management. Brown Trout and Rainbow Trout fry were most often found together in sites with pebble sizes of 151 mm, velocities between 0.20 and 0.23 m/s, and depths of 0.18 m. However, more Rainbow Trout fry were found in sites with velocities ≥ 0.23 m/s, and depths ≤ 0.17 m. Larger particle sizes and higher velocities in near-shore habitats can reduce fine sediment habitats for the intermediate host of whirling disease, the Tubifex worm, and fry infection severity. Overall, interspecific competition, fry stocking and strain type stocked, and the presence of pathogens such as M. cerebralis, cause deviations from published habitat suitability indices typically used to design restored habitat reaches. Evaluating system-specific differences in these factors on fry habitat use may allow future restoration activities to be more effective. Associated Literature Fetherman, E. R., and B. W. Avila. 2022. Habitat associations of Rainbow Trout Oncorhynchus mykiss and Brown Trout Salmo trutta fry. Journal of Fish Biology 100:51-61. COLORADO PARKS & WILDLIFE • 317 West Prospect Road, Fort Collins, CO 80526 • (970) 472-4436 • cpw.state.co.us � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Fact Sheets Type The nature or genre of the resource Fact Sheet Format The file format, physical medium, or dimensions of the resource application/pdf Description An account of the resource CPW Fact Sheets Text A resource consisting primarily of words for reading. Examples include books, letters, dissertations, poems, newspapers, articles, archives of mailing lists. Note that facsimiles or images of texts are still of the genre Text. Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Rainbow trout fry investigations Description An account of the resource Rainbow Trout are spring spawners, and deposit their eggs in gravel at sites known as redds. The eggs hatch in 4-7 weeks depending on water temperature. After hatching, fish spend up to two weeks as sac fry, remaining in the gravel and absorbing their yolk sac. Once the yolk sac is absorbed, the fry emerge from the gravel to begin feeding. Fry spend the first several months of their life in shallow areas found along the river margins, using vegetation and interstitial spaces between rocks as cover from predators. During this time, they are also easily sampled to estimate population size. Changes in the fry population over time can indicate whether factors such as water quality, flow, temperature, or presence of parasites are affecting survival. Additionally, fry estimates conducted later in the year can be indicative of the number of fish that will recruit to the juvenile and adult populations. Finding ways to increase fry survival in rivers has been the focus of many CPW aquatic research projects. Creator An entity primarily responsible for making the resource Colorado Parks & Wildlife Subject The topic of the resource Rainbow trout Spawning Fry Population management Extent The size or duration of the resource. 2 pages Date Created Date of creation of the resource. 2022 Rights Information about rights held in and over the resource <a href="http://rightsstatements.org/vocab/InC-NC/1.0/" target="_blank" rel="noreferrer noopener">In Copyright - Non-Commercial Use Permitted</a> Type The nature or genre of the resource Fact Sheet Format The file format, physical medium, or dimensions of the resource application/pdf Language A language of the resource English