The banning of DDT, the Clean Water Act, and increased conservation efforts of wetland and aquatic habitats formed the foundation for osprey recovery. (Photo courtesy Tom Koerner/USFWS)
March 07, 2024
By Glenn Zinkus
We had just run Whitehorse Rapids, a technical stretch of whitewater with a reputation for eating drift boats in its washing machine-like currents. We were now on a peaceful section along Oregon’s Deschutes River when our guide pointed to an osprey nesting platform, something installed in recent decades along the riverbanks. Looking up, we saw an osprey, now a common sight along this river, gliding downriver, eyes cast downward.
The Lower Deschutes had long been known as an outstanding dry-fly fishery, but the guide lamented, “You know, the dry-fly fishing here has changed over the years. It’s become something for the more advanced anglers.” Fish were known to hover in the enormous eddies of this big, western river located in Oregon’s high desert. He explained trout no longer feed out in the open, stacked up to sip on a watery conveyor belt of pale morning duns (PMDs) during the day.
The widespread use of the insecticide dichlorodiphenyltrichloroethane (DDT) from its advent in the 1940s until it was banned in the United States in 1972 had severe environmental consequences. DDT accumulated within the osprey, an apex predator, causing eggshell thinning. These fragile eggs were prone to breakage during incubation, and osprey populations plummeted. The banning of DDT, the Clean Water Act, and increased conservation efforts of wetland and aquatic habitats formed the foundation for osprey recovery. Osprey proved to be a resilient species, their population now surging in portions of their ever-expanding range.
Population increase estimates surpass 1,100 percent over the 40 years following the lows of the 60s and 70s. The Partners In Flight ornithological database estimates that there are approximately 400,000 ospreys in the US and Canada, although other population estimates are more modest. Some estimates peg the US population at 30,000 to 40,000 nesting pairs. Osprey populations, especially on the Deschutes, have boomed.
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The Lower Deschutes had long been known as an outstanding dry-fly fishery, some guides suspect the burgeoning osprey population has affected this behavior on its native trout. (Glenn Zinkus photo) My fishing buddies and I began a campsite discussion about ospreys and trout. We debated everyone’s observations, what’s true and what’s not, and concluded that we should expand this discussion to include other guides on other waters both near and far.
Starting first with other experienced Deschutes River guides who run this sweep of the Deschutes find many in agreement. Ethan Nickel, who runs his own Ethan Nickel Outfitters , explains when he started working on the Deschutes in the mid-90s, the osprey population was a fraction of what it is now.
“The change in the osprey population has absolutely influenced trout behavior,” Nickel said. “The fish are much less likely to rise out in the open, and when they do, they almost never suspend near the surface like they did in the old days, but rather ascend from depth to eat, then dive back down to safety, out of osprey range.”
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Dr. Alan Poole, a leading osprey expert and author of two books on ospreys estimates that osprey can dive to depths of approximately a meter.
Matt Ramsey, co-owner of Two Dudes Fly Fishing has more than 20 years of Deschutes River guiding experience. During the early 2000s, Ramsey and his clients could walk out to big eddies and cast flies to a plethora of trout targets.
“There was one eddy we called Viagra Eddy because the fish were always up,” Ramsey said.
That changed over the next ten years.
Many guides point out an example, a pole topped with a manmade platform intended for ospreys to create nests on. The Deschutes River platforms were installed at beginning of this century, several by Oregon Department of Fish and Wildlife, and more by the local electric utility company, Wasco Electric Cooperative. Because osprey preferentially build nests on these platforms, those on the Deschutes have a 100-percent occupancy rate.
Jon Belozer, a longtime guide and outfitter on this stretch of the Deschutes River recalls, “I’ve seen 10 to 12 ospreys in the air at once on this stretch.”
Dr. Jonathon Armstrong, an associate professor with Oregon State University’s (OSU) Department of Fisheries, Wildlife, and Conservation Sciences says, “It’s conceivable that there used to be a few stretches that were modified by osprey, but now there are many more.”
A professor at OSU’s Department of Fisheries, Wildlife, and Conservation Sciences says that there is a lot of evidence that the presence of avian and other terrestrial predators can cause fish to use less “risky” foraging behavior. (Photo courtesy George Gentry/USFWS) Oregon and Beyond So is this just a Deschutes phenomenon? Guides in other Oregon locations, and beyond, have seen more osprey recently but haven’t observed altered trout behavior on their home waters.
Jeff Perin, the owner of The Flyfisher’s Place in Sisters, Oregon and a longtime guide on the Cascade Lakes tells of his experience with trout behavior on the high country stillwaters.
“Fish are cruising the along the edge of East Lake eating callibaetis,” Perin said. “I’m thinking oh my god, the fish are out there exposing themselves to the osprey, but then the osprey don’t even work that zone.”
When I asked Poole about this behavior and whether osprey may avoid trout and other fish in water that is too shallow to dive into, he commented that “they’re good at targeting fish in shallow water…..they end up surface snatching instead of diving.”
Dee Chatani, a fly-fishing guide on the Bow River in Canada since 1988, hasn’t seen changing trout behavior because of ospreys.
“We have a lot of ospreys,” he said, but there are not apparent changes to trout holding and feeding behavior from the ospreys. Pelican populations are increasing, he told me, and could be similarly changing the fishery.
Josh Almond at Blue Ribbon Flies in West Yellowstone has not seen changed feeding or other behavior in trout. The guides have no complaints. And in the east, Pennsylvania guide, Rick Nyles, owner of Sky Blue Outfitters has also not encountered changes due to osprey. Much of the eastern population is centered around Chesapeake Bay.
Ecosystem Complexity Ecosystems are complex with many variables. Whether trout behavior changes due to a large presence of osprey has no definitive answer, and not every guide tells the same story.
Other variables play into the Deschutes River story. Anadromous fish restoration was a requirement for dam relicensing in the early 2000s. To achieve this restoration, the dam owners designed and built a Selective Water Withdrawal (SWW) tower to create surface currents over the feed waters of the Lower Deschutes in a reservoir named Lake Billy Chinook (LBC). SWW draws more water from the top of the dam, creating surface currents. These currents guide juvenile salmon and steelhead into collection facilities at the Round Butte Dam for transport downstream. However, this top layer of water is warmer and richer in nutrients from agricultural runoff.
The top-water draw warms the river and creates a slick snot over the rock and gravel bottom. This algal coating chokes off the once-abundant mayflies, caddis, and stoneflies. The bugs are still there but in lower numbers. But the taxa are also shifting from aquatic flies to swelling populations of snails and worms, so the trout diet changes.
Brian Silvey, a long-time well-known guide on the Deschutes tells me about the ospreys.
Osprey population increase estimates surpass 1,100 percent over the 40 years following the lows of the 60s and 70s. (Photo courtesy George Gentry/USFWS) “There are a ton of them in there,” Silvey said. “But I think the hatch timing and the tower are the problems.” Brian explains he still sees massive hatches, but they all come off in May with many of the major hatches overlapping each other.
John Hazel, owner of The Deschutes Angler fly shop in Maupin, and a long-tenured guide on the Lower D goes further to opine “why the trout are not in the back eddies is not because of a high osprey population. There’s no food in the back eddies.” Macroinvertebrate studies before and after SWW show numbers of mayflies, caddisflies, and stoneflies are down from years past. Despite this, other guides observe trout consistently rising, but under the protection of an overhead tree and very rarely out in the open.
Nate Turner, owner of Skookum Outfitters , says, “There’s a lot more bow and arrow casting under low-hanging alder trees” to get a dry fly to a rising fish.
Dr. Seth White, a professor at OSU’s Department of Fisheries, Wildlife, and Conservation Sciences told me that there is a lot of evidence from other systems that the presence of avian and other terrestrial predators can cause fish to use less “risky” foraging behavior .
It very well may be that trout behavior on the Deschutes is influenced by both the hatches and the osprey. A deeper dive into available macroinvertebrate studies and the statistical analysis reveals that PMDs, a regular component of the Deschutes River trout diet, declined by 8 percent during that time since the SWWs were implemented. Other mayfly, stonefly, and caddisfly populations decreased as well, some substantially more. While we await the results of the most recent sampling events to understand the latest insect population trends, the guides all advise on their change in tactics with their clients, especially those who eschew anything but dry flies.
This raises the question, what do you see on your home waters?
Glenn Zinkus is an outdoor writer and photographer from Oregon. More of Glenn’s work is at www.glennzinkusoutdoors.com .
