Zach Sherker

Supervisors: Dr. Andrew Trites

Education: 2016 BSc Aquatic Resources & Biology from St. Francis Xavier University, Antigonish, NS

Research Description: 

An array of opportunistic foragers (brown trout, sculpins, common mergansers, North American river otters, American mink, and Pacific harbour seals) are suspected of preying on juvenile salmon in rivers and estuaries—and may account for critical low numbers of Chinook salmon in British Columbia. However, there is another piscivore predator that has been left off the list of usual suspects—the Pacific great blue heron. My thesis project investigated the role that herons might be playing in the decline of salmon by estimating rates of mortality caused by herons on juvenile Chinook salmon tagged with PIT tags in 2014, 2015, 2016 and 2017 (~10,000 tags per year) in the Cowichan River on Vancouver Island. I scanned three heron rookeries located ~10, 20, and 30 km from the mouth of the river using a mobile detector, and found 410 tags in fecal remains under nests. Most of the tags (406) came from the closest rookery with ~100 nests, and the remaining few were from the two smaller more distant rookeries (7 nests, ~30 nests). Predation occurred primarily in the lower river and was higher during years of low water flow. Recovering so many tags at heron rookeries was unexpected, and indicates that blue herons are a major predator of juvenile Chinook.  The location of heron nests relative to the distance to salmon bearing rivers is likely a good predictor of the impact on salmon runs, and a potential means to assess coast-wide impacts of great blue herons on salmon recovery.