Author(s): Nathan A. Zorich, Michael R. Jonas, Patricia L. Madson
Author(s) Affiliations: Fish Field Unit, Portland District, USACE
Author Contact (Phone, e-mail): 541-374-8801, Nathan.A.Zorich@usace.army.mil
Avian predators continue to be a highly visible source of smolt mortality below hydropower dams. If sever, this mortality may impede FCRPS dams from meeting BiOp required survival goals for ESA listed salmonids. In 2011 the Fish Field Unit was tasked with determining the impact of avian predators on fish passing John Day and The Dalles dams. Our objectives were: 1) Determine species composition and numbers of piscivorous birds; 2) Estimate fish consumption and attack location of gulls; 3) Determine the effectiveness of intense boat hazing and avian deterrent line arrays at John Day and The Dalles dams.
To quantify avian consumption of fish, observers used binoculars to count gulls (Larus californicus), the number of attacks (i.e. dives), and to determine if an attack was successful (e.g. fish in bill) during the smolt outmigration between 10 April to 28 July 2010.
Gull collection for diet analysis was not permitted this year. Rather than reporting smolt consumption we report fish consumption estimates which include smolt, juvenile lamprey, and other fish. Additionally, counts of other fish eating birds were collected.
The daily abundance of gulls at John Day Dam ranged from zero on 15 April & 28 July to a high of 30 on 29 April with a seasonal mean of 5.7 gulls. At The Dalles Dam the daily abundance ranged from zero on 28 July to a high of 63 on 23 May with a seasonal mean of 14.7 gulls.
At John Day Dam preliminary estimates of fish consumption, which includes additive and compensatory sources of mortality, were 6,000 (95% CI 4,000 – 8,000). This is a reduction of 32,000 (84%) from 2010 when an estimated 38,000 fish were consumed. Gulls attacks were spatially uneven. We produced geospatial maps that show hot spots immediately downstream of the avian lines, primarily on the spillway side of the river.
At The Dalles Dam preliminary estimates of fish consumption, which includes additive and compensatory sources of mortality, were 16,000 (95% CI 13,000-20,000). This is a reduction of 70,000 (81%) from 2010 when an estimated 86,000 fish were consumed. Here also, gulls attacks were heterogeneous, focused immediately downstream of the avian lines, primarily on the spillway side of the river.
Hazing from boats and avian line arrays were as successful in protecting covered areas in 2011 and 2010. Because the deterrent effort was similar in both years we attribute the decreases in fish consumption to natural variation in the number of foraging gulls, not level of deterrent effort. Regardless, the management objective of reducing predation was achieved in both years.