Mortality, Movements, and Migration Timing of Age-0 Cowichan Chinook Salmon Tagged in the Southern Gulf Islands in Fall 2017

Similar to other Chinook populations in the Salish Sea, the Cowichan River Chinook population has declined precipitously since the 1990s. Previous studies have tracked Salish Sea populations of Chinook during the winter and fall months and found that juvenile Chinook likely either remain resident in the Salish Sea beyond the time when they were tracked, or suffer very high mortality. In late summer 2017, we tested these hypotheses by acoustic-tagging 80 age-0 wild and hatchery Chinook salmon (>140 mm FL) resident near the outlet of the Cowichan River in Maple Bay and Sansum Narrows.

The fish were captured using microtrolling and then released near their capture locations after tagging. Mobile tracking was conducted for 10 days in September after tagging was completed, and sporadically throughout the fall. We will track their migration until spring 2018 with an array of special-purpose, short-term, receivers deployed to the north and south of the capture and release areas and at two harbour seal haul-outs.

The existing POST sub-arrays in the Strait of Juan de Fuca, Northern Strait of Georgia, and Discovery Islands will allow us to determine if fish remain the Salish Sea over winter. In addition to the acoustic tag, fish were implanted with a PIT tag which can ultimately be used to estimate smolt to adult survival back to the Cowichan River. Finally, we collected gill and scale samples for genetic stock dentification and to investigate infectious agents that might be present in tagged fish.i