Juvenile salmon smolt migrations through the Salish Sea: routes and survival of sockeye and steelhead

The Salish Sea has been labelled as a potentially critical lifestage for juvenile Pacific salmon smolts, and yet little is known regarding movement behaviors and survival in this ecosystem. Acoustic telemetry, or the insertion of electronic tags into fish and subsequently tracking movements in-situ, can provide great insight into the migrations of juvenile salmon. We have used acoustic telemetry to 1) estimate migration survival for wild sockeye smolts through the Salish Sea for the first time, 2) define migration routes of sockeye and steelhead within the Salish Sea, and 3) identify potential mortality hotspots for a hatchery steelhead population. Survival for a wild sockeye population (Chilko) was consistently poorest in small clear rivers and in the Salish Sea, with only 3-10% of smolts surviving the ~1000 km tracked. Migrations by sockeye smolts are rapid, traversing ~1000 km in ~45 days. Substantial contingents (20-50%) of both sockeye and steelhead across years and populations exhibit westward movements in the northern Strait of Georgia, indicative of potential milling behaviors. Migratory route through the Salish Sea influenced survival of steelhead, with fish taking the most eastern route the Malaspina Strait experiencing higher survival than fish travelling through the broader Strait of Georgia. Transport experiments of a hatchery steelhead population (Seymour) appears to identify Burrard Inlet as an area of elevated mortality, potentially due to predation. Smolts transported beyond Burrard Inlet had 3-times higher survival (39%) to Johnstone Strait than fish released in the Seymour River (13%). Our findings greatly contribute to understanding the migration ecology of smolts in the Salish Sea, and has identified specific regions where mortality is elevated.