The current study provides an examination of travel rate, residence time, and direction of travel upon ocean entry for acoustic-tagged yearling Chinook salmon, juvenile steelhead, and subyearling Chinook salmon at the mouth of the Columbia River and in the nearshore marine environment within the vicinity of the plume. Travel rate varied across species and age class, with steelhead smolts migrating out of the mouth of the Columbia River at a greater rate than yearling and subyearling Chinook salmon. Juvenile steelhead displayed shorter residence times in the transition area between the river mouth and the plume array than Chinook salmon smolts, although the majority of fish across species and age classes had residence times less than 3 days. Direction of travel upon exiting the river mouth also varied across species, age class, and season. We showed that smolt emigration from the river is highly influenced by outgoing tides and direction of tagged fish movement in the nearshore marine environment may be affected by ocean currents. This study also proved JSATS to be an effective tool for monitoring fish behavior in the marine environment.
Differences in travel rates between species at ocean entry and high variation in travel rates at the river mouth/plume array transition area may be associated with the level of smolt preparedness for ocean entry (for example, physiological, bioenergetics, size), tidal action, predation risk, channel morphology, and hydrologic characteristics [8, 14, 23, 39]. Depending upon timing of fish arrival at the mouth of the river, some fish may move into the ocean within a single tide change, whereas others may require multiple tide changes before moving out into the plume on a strong ebb tide. In trawl catches, Emmett et al.  observed higher subyearling Chinook salmon densities in the plume during ebb tide conditions and Clements et al.  observed tidal influences on juvenile salmonid migration in the Nehalem River estuary. This is consistent with our finding that most subyearling Chinook salmon were last detected in the river during ebb tide conditions. Cross-channel distribution of emigrating smolts at the river mouth may affect travel rate depending on the route taken through the lower reaches of the estuary, as fish in different areas of the channel may experience different water velocity and flow direction due to complexities in channel morphology and tidal influence . In freshwater environments, steelhead are typically larger and often migrate closer to the water surface, whereas Chinook salmon smolts tend to utilize deeper water [40, 41]. Thus, steelhead may use the higher velocity upper layer of freshwater to facilitate more rapid movement out of the river mouth with ebb tides, even though their larger size and orientation in the water column may present a greater risk of avian predation . Short residence times have also been observed for juvenile steelhead smolts migrating out of the Nehalem River and Alsea River estuaries towards the ocean [23, 39].
Residence time in the transition area between the river and the plume arrays
Subyearling Chinook salmon exhibited greater residence time in the transition area between the river and the plume arrays than yearling Chinook salmon or steelhead smolts. De Robertis et al.  suggested that juvenile salmonids use the low salinity plume waters as nursery habitat. However, it appears from the results of our study that most of the fish (steelhead smolts in particular) spent minimal time (that is, less than 3 days) within the confines of the plume array and migrated out to the ocean relatively quickly rather than using this habitat for rearing.
Most tagged fish were last detected at the river mouth arrays (rkm 2.8, 4.5, and 8.3) during ebb tides, which could have facilitated fish movement out of the river and into the plume. Of the subyearling Chinook salmon that moved back into the estuary, the majority did so during a subsequent flood tide. It is also possible that some juvenile salmonids in this study may have been eaten by predators, thus transferring the tag from smolt to a larger predatory fish, possibly explaining the movement in and out of the mouth of the river, which may have been characteristic of a predatory species, rather than a juvenile salmonid.
Migratory direction out of the Columbia River plume array
During early spring, prior to 18 May 2010, the majority of yearling Chinook salmon and steelhead smolts in our study were detected on the terminal and south sub-arrays in the plume. Conversely, previous studies have reported that yearling Chinook salmon and steelhead nearly always exit the Columbia River mouth by migrating northward [35–37]. Schreck et al.  placed two receiver lines off the north and south jetties and Rechisky et al.  placed a receiver array 131 km south of the Columbia River mouth (near Cascade Head, OR). The authors concluded in these former studies that very few smolts migrate south when exiting the Columbia River. It is clear, however, from the results of our study that many yearling Chinook salmon smolts (27%) and steelhead (47%) initially migrated south upon ocean entry. An array deployed at the river mouth off the south jetty by Schreck et al.  detected 12% of tagged Chinook salmon and 23% to 40% of tagged steelhead. The Cascade Head array deployed by Rechisky et al.  was located considerably farther south of the Columbia River mouth and only detected two fish (1%) tagged and released as part of a group (n = 196) transported by barge early in the season (April). Of the total number of fish tagged by Rechisky et al.  (n = 977), these two tagged smolts detected on the Cascade Head array were 0.2% of the tagged population in that study. In a separate study conducted by Pearcy and Fisher , a small percentage of coded wire-tagged yearling Chinook salmon were captured in purse seines south of the Columbia River mouth over a span of 5 years. It is uncertain how far south smolts move prior to reversing direction and migrating north. It appears, however, that direction of travel may be associated with the direction of ocean currents and potentially other ocean conditions (for example, current and wind direction, salinity, temperature) experienced by juvenile fish at the time they enter the marine environment.
We found the direction of fish movement in the nearshore ocean to be associated with southerly ocean currents prior to 18 May 2010, followed by a reversal in the direction of both fish movements and ocean currents thereafter. Surface currents are influenced by wind direction and our observations suggest that migration direction of tagged smolts may have also been affected by these conditions. Therefore, it is possible that differences in surface current dynamics (and smolt migration behavior) among years may explain some differences in migration direction observations between past and present studies. Although our box array design expanded on previous findings regarding the direction of movements in the nearshore ocean, a detailed study of juvenile fish behavior in relation to oceanic conditions is necessary to gain a better understanding of how the environment affects fish movement. Due to the dynamic nature of the plume, surface current direction is not static and conditions may change within a matter of days or hours [30, 31]. Therefore, pairing acoustic telemetry receivers with devices (for example, acoustic Doppler current profilers) deployed to collect environmental data would further expand our ability to associate fish movement with ocean conditions.
Distribution of the two species of salmonids (of varied age classes) tagged in our study varied within the plume array, as more yearling Chinook salmon were detected on the terminal sub-array than steelhead or subyearling Chinook salmon. Fisher et al.  reviewed data from juvenile salmonid trawls along the west coast and concluded that subyearling Chinook salmon were mostly distributed closer to shore in shallow water. This is consistent with our findings that most subyearling Chinook salmon were detected on the plume array north of the Columbia River and their distribution was skewed toward the shallower water off the beach. Given the large number of smolts in our study that were last detected on the terminal sub-array (located along the 100 m depth contour), it is possible that some fish move on to migrate over deeper water beyond the continental shelf, which is generally considered to be at the 200 m depth contour. Past studies have also found distributions of yearling Chinook salmon similar to our study [33, 45], while others report yearling Chinook salmon being distributed somewhat closer to shore . Many other studies, however, have shown inconsistencies in catch densities and distribution patterns of juvenile salmonids in the Columbia River plume across seasonal and annual sampling periods [24, 43, 44]. Burla et al.  suggested yearling Chinook salmon are likely to spend more time in the overall low-salinity plume habitat, staying closer to shore before moving north in May to July, whereas steelhead use the plume to quickly disperse from coastal habitats. However, De Robertis et al.  found that yearling Chinook salmon were abundant throughout the entire plume.
Due to attenuation of acoustic signals in marine environments, the detection range of JSATS was reduced in saltwater compared to freshwater (by approximately 50%; McMichael et al.
). We suspect the plume array detection efficiency was considerably lower compared to the arrays located at the river mouth because the spacing between receivers was larger in the plume array (approximately 3 km versus 150 m in the river) and the water within the plume array was more saline. Therefore, many tagged fish likely passed the box-shaped plume array undetected. In range testing conducted in the plume in 2009 with the receiver deployed in approximately 55 m of water, we determined that detection efficiency averaged about 20% at 100 m and between 5% and 10% at 150 m, while the greatest distance for valid detections of a JSATS transmitter occurred 250 m away from the receiver. These detection efficiencies are approximately half of those we have measured in freshwater and will be an important consideration in future array designs in plume and marine environments. Other studies have speculated that salmon smolts utilized waters nearer the surface in the plume [34, 43], and it is possible that fish closer to the surface (farther from the receivers near the sea floor) were not detected as efficiently as those travelling at greater depth. For example, a larger percentage of the subyearling Chinook salmon (26%) estimated to have left the river were detected on the plume array than either yearling Chinook salmon (16%) or steelhead (10%). From these data we might conclude that the deeper travelling subyearling Chinook salmon were closest to the relative position of the receivers in the water column and steelhead were farthest as they migrated closer to the surface, with the yearling Chinook salmon at intermediate depth. It is also possible that the tendency for subyearling Chinook salmon to migrate more slowly and closer to shore in areas where the total depth of the water was relatively shallow may have increased the detection efficiency of their transmitters.
Limitations and future directions
This study documents the successful use of the JSATS to track fish behavior in the nearshore marine environment. Further use of this technology in the future could improve array design by reducing the spacing between receivers, adding additional lines in a ‘net-like’ formation (that is, placing receivers outside of the current array intermittently between receivers in the existing formation at varied distances) outside of the box array and providing more extensive coverage up and down the continental shelf to better account for spatial variation of the oceanographic conditions in and around the plume, up and down the coast, and beyond the edge of the continental shelf. Expanding the array could help to elucidate direction of travel after juvenile salmonids exit the plume box array and allow for improved quantification of distance travelled by southward moving smolts. Adding additional receivers outside of the box array may also allow for estimation of smolt survival in the nearshore ocean environment using mark-recapture methodology. Finally, pairing receivers with devices deployed to collect environmental data may also help to identify conditions associated with fish behaviors, such as movement. Multiple years of telemetry data are necessary to observe the behavior of juvenile salmonids in the nearshore ocean under different environmental conditions.
Telemetry receiver array improvements will help provide more extensive and longer term smolt survival information and also help to capture fish behavioral responses to changing conditions. Because the transition between the river and ocean has been identified as a critical period that influences smolt-to-adult survival rates, these additional data sets will provide needed information to run forecasting models [29, 47]. In addition, the continued use of the JSATS will allow for smaller size classes (that is, <95 mm fork length (FL)) than ever before to be monitored, providing a more accurate representation of behavior characteristic to the population. Previously, most telemetry studies in the nearshore ocean have utilized lower frequency acoustic transmitters (for example, 69 kHz) that may potentially be within audible range of predatory marine mammals [48, 49]. JSATS transmitters have frequencies that are outside the audible range of marine mammals (416.7 kHz) and may reduce any potential study bias associated with predation.