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Birds of a Feather Excrete PIT Tags Together: Twenty Years of Salmonid Mortality from an Eastern Oregon Great Blue Heron Rookery

James Harbeck, Neal Espinosa, Ryan Rumelhart

Nez Perce Tribe, 500 North Main Street, Joseph, OR 97846

Salmon and steelhead serve as powerful cultural and social symbols for tribal and non-tribal people of the Pacific Northwest. Yet despite the significance of these icons, there have been widespread and dramatic declines in their populations over the last century. These declines were also witnessed in populations of northeast Oregon including in the Imnaha and Wallowa subbasins.

We investigated a great blue heron (Ardea herodias) rookery located near numerous nursery tributaries and hatchery acclimation release sites within the Imnaha and Wallowa subbasins. Our study was motivated in part by the increasing attention directed toward avian predation and its impact on juvenile salmonid survival during emigration through the Snake and Columbia river corridors. Furthermore, the Independent Scientific Review Panel (ISRP) strongly recommends managers and researches to not only estimate mortality (1 – Survival), but also identify the source of the mortality. 

We used PIT tag data from tags located within the heron rookery to examine the magnitude of juvenile salmonid mortality from this specific source. In 2020 test tags were randomly sown under and around the rookery site to calculate a detection probability. A six acre grid with flagged columns was laid out to help systematically scan for potential PIT tags. Over a two day period 2,402 tags were detected within the grid by scanners using Biomark BP Plus portable antennas with HPR Plus readers. Detections were from fish tagged and released in the years 2000 through 2019. Fish species consumed by the herons include hatchery and natural steelhead (Oncorhynchus mykiss), hatchery and natural Chinook Salmon (Oncorhynchus tshawwytscha), hatchery Coho Salmon (Oncorhynchus nerka), and natural Bull Trout (Salvelinus confluentus).These Snake River steelhead, Chinook Salmon and Bull Trout populations are all listed as “Threatened” under the Endangered Species Act (ESA).

Over the course of 20 years adult herons foraged over a three state area covering 18,118 km² in northeast Oregon, southeast Washington and central Idaho. Adults primarily sought and returned prey within 30 km of the rookery. But distances from prey sources to the rookery ranged from 4.1 to 142 km. Based upon fish-per-tag figures and detection probabilities the estimated number of salmonids consumed and regurgitated at the rookery averaged 8,207 fish per year, but were highly variable (1,679 – 16,444). Annual fish numbers consumed were consistently highest for hatchery steelhead followed by hatchery Coho Salmon (when available) and natural steelhead. Minimum predations rates were likewise higher for hatchery steelhead than other groups (0.46% - 2.34%).

Our results suggest great blue herons from the Wallowa rookery are a significant source of mortality for hatchery steelhead during the birds’ nesting season. To a lesser degree the herons also preyed against other ESA-listed and non-listed salmonids throughout a wide ranging forage area. Therefore, proper due diligence would support continued monitoring of this source of mortality and whether it serves as a limiting factor impacting recovery.