I’ve read a couple fascinating studies applying acoustic telemetry to salmonid migration here in the Pacific Northwest, and thought those would be good to share for a Friday morning link dump:
First, and filed under geeky science stuff, ecologists from the Department of Energy’s Pacific Northwest National Laboratory have published a paper detailing their work on the out-migration patterns of juvenile salmonoids from the Columbia River. The paper is here (open access), and good press coverage on the article is available in the Columbian. In brief, the study used eraser-sized, battery-powered tags in more than 8,000 out-migrating salmonids (salmon and steelhead trout). Those fish were released in one of four points in the Columbia, and the tags emitted unique acoustic signatures that could be detected by underwater receivers deployed by the research team. Of the 8,000+ released fish, those receivers picked up signals from 1,700, remarkable when you consider that the fish had to swim within 250 meters of the receiver, and the receivers were placed 1.5 miles apart, 9 miles off the mouth of the Columbia. What the researchers saw was remarkable as well. It has been assumed that salmonids leaving the Columbia would hang a right and head north. Some fish did that, but others went straight out to sea, some went south, and some (young chinook) would cycle between fresh and salt water before ultimately heading out to sea. The study also found that steelhead trout would “bolt” to sea quickly, and that species was most susceptible to predation, perhaps because of shallow-water migration patterns. Besides being a really cool study design, this work has implications for a wide variety of management decisions related to salmon in the Columbia, including timing of release of smolts from hatcheries.
The second study I just read is a research on steelhead out-migration in Puget Sound (also open access), evaluating the impacts that the floating bridge at Hood Canal may have on mortality of out-migrating steelhead smolt. Like the above study, this work involved using acoustic tracking of steelhead smolt, but had the advantage of being able to place receivers at various choke points throughout Puget Sound–providing much better detection rates of out-migrating smolt. An analysis of the data generated over five years showed that there was extra mortality of steelhead smolts in the vicinity of the Hood Canal Bridge, a floating structure supported by pontoons that extend almost four meters into the water across 95% of the mouth of Hood Canal. Because steelhead smolt are shallow-water migrators, it is thought that the bridge pontoons may be inhibiting migration, through blocking routes, causing confusion, providing attractive shade, and inhibiting surface currents. The authors of the study documented longer time spent by smolt in the vicinity of the bridge, and, through a complex statistical analysis, calculated a mortality rate of about 5 to 36% of the smolt that were detected at the bridge in 2010. Amazingly, the study authors hypothesize that the steelhead smolt are being eaten by harbor seals, and that the documented behavior of the transmitters crossing back and forth under the Hood Canal Bridge (a behavior not seen elsewhere) is actually harbor seals carrying the transmitters in their gut across the receiving array. This crossing behavior went on for a few days before the harbor seal defecated the transmitter back into the water. Another interesting result of the study is that it documented no probable or possible mortalities in this area in 2009, which was when WSDOT replaced the eastern half of the bridge, resulting in half of the span not being in place during steelhead migration. This is certainly valuable research with respect to floating structures, especially the results from 2009, indicating that it may be a combination of depth and the extent to which a floating structure completely blocks a migration route that may be the determinative factors in whether that structure impacts salmonid populations.