In situ measurement of coastal ocean movements and survival of juvenile Pacific salmon

Abstract

Many salmon populations in both the Pacific and Atlantic Oceans have experienced sharply decreasing returns and high ocean mortality in the past two decades, with some populations facing extirpation if current marine survival trends continue. Our inability to monitor the movements of marine fish or to directly measure their survival precludes experimental tests of theories concerning the factors regulating fish populations, and thus limits scientific advance in many aspects of fisheries management and conservation. Here we report a large-scale synthesis of survival and movement rates of free-ranging juvenile salmon across four species, 13 river watersheds, and 44 release groups of salmon smolts (>3,500 fish tagged in total) in rivers and coastal ocean waters, including an assessment of where mortality predominantly occurs during the juvenile migration. Of particular importance, our data indicate that, over the size range of smolts tagged, (i) smolt survival was not strongly related to size at release, (ii) tag burden did not appear to strongly reduce the survival of smaller animals, and (iii) for at least some populations, substantial mortality occurred much later in the migration and more distant from the river of origin than generally expected. Our findings thus have implications for determining where effort should be invested to improve the accuracy of salmon forecasting, to understand the mechanisms driving salmon declines, and to predict the impact of climate change on salmon stocks.

Tracking wild sockeye salmon smolts to the ocean reveals distinct regions of nocturnal movement and high mortality

“…As Chilko smolts passed northward through the Strait of Georgia, their migration rates were the slowest of all migratory segments in this study (10-20 km/d; 1-2 BL/s for a 12 cm smolt); these migration rates were very similar to those observed for hatchery-reared and largerbodied Cultus Lake sockeye acoustically tracked through this same area from 2004 to 2007 (Welch et al 2009(Welch et al , 2011, and residence time was similar to that estimated by Preikshot et al (2012) when scaled for body size. Chilko smolt migration rates increased in most years (up to 30 km/d; also observed for tagged Cultus Lake smolts [Welch et al 2009[Welch et al , 2011) as they passed through the final study segment (Discovery Passage; Array H-I), where a narrowing of the coastlines leads to surface tidal currents which can exceed 43 km/d (~50 cm/s) in the direction that smolts are migrating (Foreman et al 2012).…”

“…Chilko smolt migration rates increased in most years (up to 30 km/d; also observed for tagged Cultus Lake smolts [Welch et al 2009[Welch et al , 2011) as they passed through the final study segment (Discovery Passage; Array H-I), where a narrowing of the coastlines leads to surface tidal currents which can exceed 43 km/d (~50 cm/s) in the direction that smolts are migrating (Foreman et al 2012). In addition to the fact that smolts must actively swim through this area (McKinnell et al 2011), it is possible that smolts may modify their behavior to take advantage of the variation in tides to increase their net migration speed.…”

Survival of seaward-migrating PIT and acoustic-tagged juvenile Chinook salmon in the Snake and Columbia Rivers: an evaluation of length-specific tagging effects

“…One field study failed to detect a relationship between length and survival among acoustic-tagged Chinook salmon smolts emigrating through the Fraser River Basin [36]. However, this study utilized only larger Chinook salmon (that is, >130 mm) and evaluated the length-survival relationship by comparing the lengthfrequency distribution of all released fish to the distribution of fish known to survive to the mouth of the Fraser River.…”

Estimating behavior in a black box: how coastal oceanographic dynamics influence yearling Chinook salmon marine growth and migration behaviors

“…Most estimates of marine migration rate are based on average travel time between two points of detection (e.g., Thorstad et al 2007, Welch et al 2011. While these estimates have greatly increased our understanding of migration behavior, questions about the realized migration length or energetic expenditure of a migrating animal have remained unanswered.…”

“…To understand behaviors employed or stimuli used by migrating fish, movement dynamics must be estimated with fairly high precision (e.g., hourly or daily). However, many estimates of fish movement rely solely on net distance and speed between capture/release and recapture locations, assuming a straight-line trajectory (Thorstad et al 2007, Welch et al 2009, Welch et al 2011, Tomaro et al 2012. While such information is useful for characterizing spatial distributions, it cannot resolve the ecological and behavioral processes involved in a fish moving between two locations.…”