Meso-scale movement and mortality patterns of juvenile coho salmon and steelhead trout migrating through a coastal fjord

Early marine life is thought to be a critical period affecting recruitment of Pacific salmon populations, but movements and mortality patterns of juvenile salmon after ocean entry have been poorly documented. Transect surveys by boat with towed hydrophone and acoustic receiver, along with lines of stationary receivers, were used to quantify early ocean movement and mortality patterns of juvenile coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss) tagged with acoustic transmitters (> 700 tagged). Salmon smolts showed no behavioural preference with respect to distance from shorelines while migrating through Howe Sound, a coastal fjord. There was no evidence of spatial bias in mortality locations in terms of distance either to shorelines or from the river mouth, suggesting that mortality locations were scattered soon after ocean entry rather than concentrated right at the river mouth. Movement patterns of some tags (annual estimates of 5-20% of smolts that survived the downstream migration) were suggestive of estuarine predation, with detected tags likely inside predator stomachs. Using only detection data from mobile transects, a distance-based mortality rate was estimated for coho smolts while accounting for imperfect detection efficiency of transect surveys. The estimate of 2.4% per km during the 40 km migration through the fjord was comparable to average annual mortality rates estimated using detection data from stationary acoustic receivers, but required pooling multiple years of data. This suggests that mobile transect surveys of tagged migrating fish are likely insufficient for estimating annual mortality rates, but mobile detection data can complement those from stationary receiver arrays to further refine mortality estimates and provide information about fish movement patterns between lines of stationary receivers. This work provides an important methodological comparison between biotelemetry approaches for migrating fishes as well as the most comprehensive description to date of spatial marine mortality patterns of juvenile coho salmon and steelhead trout.