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Birds play role in sucker numbers

  • Sucker study< H&N file photo - Amari Dolan-Caret, a U.S. Geological Survey fish technician, measures an endangered sucker in May 2014 on the banks of the Williamson River. A new study indicates bird predation is playing a large role in sucker mortality rates.

    Results from a six-year study indicate bird predation could play a larger role than previously thought in regulating sucker populations at Clear Lake Reservoir.

    The reservoir is part of the Clear Lake National Wildlife Refuge in Northern California. Lost River and shortnose sucker populations were monitored for the study between 2009 and 2014.

    According to the study, results indicate predation rates vary by sucker species, age, bird colony location and year.

    According to Dave Hewitt, a fisheries biologist for the U.S. Geological Survey (USGS), birds preying on Lost River and shortnose suckers at Clear Lake could account for nearly 5 percent of adult sucker deaths.

    Hewitt said data from 2015 is similar to data from 2014, and it will be released in a separate report.

    The fish are being studied as part of a recovery effort after both species were given protections under the Endangered Species Act in 1988.

    Study methods

    For the study, biologists implanted passive integrated transponder (PIT) tags in Lost River and shortnose suckers in Upper Klamath Lake and Clear Lake. More than 7,000 Clear Lake suckers have the rice-size PIT tags inserted in their bellies.

    Hewitt said PIT tags are useful for monitoring sucker movements because the tag receiver sensors are underwater — all fish have to do is pass over, or nearby, the sensors to transmit a signal.

    Each year during the study biologists scoured American white pelican and double-crested cormorant nesting colonies for PIT tags that were ingested when birds ate a fish and then excreted on dry land.

    Eight islands or nesting colonies — three in Clear Lake and five in Upper Klamath Lake — were scanned for PIT tags following nesting seasons.

    Predation rates on suckers at Clear Lake were highest by birds nesting at the lake, according to the study. Hewitt said a half-moon-shaped rocky outcropping called Last Chance Island is the only land mass in the lake fully rimmed in water and that has sandy areas for nesting birds in drought years.

    “The predation on suckers in 2014 and 2015 pretty much came from whatever got to nest on Last Chance Island,” Hewitt said.

    Between lakes

    USGS Fish Biologist Eric Janney said evidence indicates that birds frequently fly from Clear Lake to Upper Klamath Lake to forage for fish, but Upper Klamath birds rarely fly south to Clear Lake.

    “As of now, we’ve never had an (Upper Klamath Lake) tag show up anywhere around Clear Lake,” Hewitt said.

    Janney said now that the USGS Klamath Falls office has a clearer picture of how much predation occurs, biologists hope to start unraveling other mysteries related to sucker survival, such as how bird predation changes with different lake levels. He said suckers may be more susceptible to predation in low water years, but conversely, birds may not nest in low water years because their nest islands become land bridged.

    “The birds know the coyotes will eat all their young so they just bug out and go somewhere else,” Janney said.

    Trying to attribute an exact amount of mortality due to bird predation is tricky. Janney pointed out that several things can happen to a tag after it’s been ingested with a fish, including being excreted somewhere inaccessible.

    “The hard part is trying to figure out what portion of the tags that are ingested end up on these colonies,” Janney said. “Typically the only places we can scan for the tag are in nesting and roosting areas.”

    A recovered tag indicated the biggest fish eaten by a pelican was a 29-inch-long female Lost River sucker, according to Hewitt.

    “I was shocked by that. I figured the only animals that would prey on an adult Lost River sucker would be river otters, maybe the occasional bald eagle might take one on,” Janney said.

    Funding for the sucker study is provided by the Bureau of Reclamation. According to Janney, the Bureau will continue funding the sucker studies in 2016.


    Far away from home

    According to Dave Hewitt, a fisheries biologist for the U.S. Geological Survey, 12 of the identifying tags recovered from a six year sucker study come from unidentified sources.

    Hewitt noted an external plastic tag that was attached to Lahontan cutthroat trout in Pyramid Lake, Nev., was found lying in the sand at the the Clear Lake Reservoir pelican colony.

    More than a dozen tags implanted in juvenile chinook salmon in the Columbia River Basin in Oregon, Washington and Idaho also made their way to Klamath Basin bird colonies.

    One tag was implanted in a fish at a rearing facility in Elk City, Idaho, nearly 400 miles from where it was found in the Klamath Basin.



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