For years at the Oregon Department of Fish and Wildlife’s Clackamas Fish Hatchery, birds have been feasting on defenseless juvenile chinook salmon, coho and steelhead while they were being reared in the three asphalt ponds and “raceways” until the fish were large enough to be released into the nearby Clackamas River to begin their long journey to the Pacific Ocean.
Hatchery managers estimate that birds were picking off 10 to 15 percent of the spring chinook salmon and to a lesser extent the steelhead stock each year.
“It has become increasingly clear the past couple of years that birds preying on hatchery smolts have had a noticeable impact on the number of adult fish that are returning to the Clackamas and Sandy rivers,” said Todd Alsbury, district fish biologist for ODFW’s North Willamette Watershed.
Rather than shoot or harass the birds, ODFW staff decided to close the fish buffet by placing nets over the three large ponds – no small task considering each pond is 300 feet long and 100 feet wide. One of the challenges of covering such large spans was keeping the nets from sagging into the water, which would have exposed the fish to the birds all over again. In addition, the nets had to be easy to remove so hatchery staff could regularly clean the ponds.
The solution was to string high tension cables across the ponds to suspend dozens of nylon mesh nets a foot or two above the water. The nets were attached to the cables using mountaineering carabiners, which allowed the nets to be easily moved along the cables to facilitate pond cleanings.
“This setup is pretty slick,” said Dan Straw, manager of the Clackamas hatchery. “It’s easy to manage and seems to be effective at keeping the birds out.” Straw estimated the cost of the system was approximately $20,000. The cost was reduced substantially, he added, by an in-kind donation from Portland General Electric, which sent in a crew to install the ground anchors to which the high tension cables were attached.
“We’ll recover the cost pretty quickly through reduced fish mortality,” he said.