Juvenile summer/fall Chinook PIT tagging, Deschutes River, Oregon

Under this project the WST gained increased understanding of Deschutes River fall Chinook early life-history on timing of outmigration and growth rates in the Deschutes River upstream of Sherars Falls (Rkm 70), and on adult returns (e.g., SARs). Every spring since 2011, WST staff implanted between 20,000 to 25,000 juvenile fall Chinook salmon with PIT tags (PIT tags were paid for using BPA funds) just prior to their seaward migration (Table 1 in attached final progress report). Mortality rates from tagging were generally less than 1% of the total released. However, in 2013 the rate was 2.1%. The WST met their goals of being able to capture and tag between 20,000-30,000 juvenile fall Chinook per year with minimal mortality rates. Biomark developed specialized PIT detectors and modified a PIT tagging table. PIT tag data was entered into the Pacific States Marine Fisheries Commission database (PTAGIS) and made accessible to any agency personnel. Results were made available to ODFW co-managers for the Deschutes River and other interested parties.

Out of the 116,002 juvenile fall Chinook PIT tagged, there were 2,784 detected fall Chinook and 747 mortalities reported in PTAGIS (as of October 15, 2015). Of the 2,784 PIT tags detected (2.4% of all tagged fish), 2,454 first detections appeared to be outmigrants, as they were detected at the mouth of the Deschutes River (site DRM in PTAGIS) or in the Columbia River. Many of the detected fall Chinook had detections as juveniles going downstream and also as returning adults. The average first day of detection of outmigrants was 32.5 days from release and ranged from 3 to 337 days (Figure 1). Of these apparent outmigrants, 32 were first detected over 100 days after release. These may represent a life-history type of fall Chinook that rears in stream for a period before continuing downstream migration (Connor et al., 2005) and/or they may be spring Chinook, which display the stream-type life history type (Groot and Margolis, 1991). Outmigrants detected were between 1.6% and 2.9% of the total juvenile fall Chinook tagged each year (Table 1). Most were detected at Bonneville juvenile bypass or corner collector (2,148/2,454 = 87.5%). Additionally, 6.6% were detected at the array at the mouth of the Deschutes River and 5.3% were detected at the mouth of the estuary.

All of the benefits from this study have not yet been realized as only a proportion of the 116,002 PIT tagged fall Chinook have returned and many are not expected for three or four more years. However, this study has resulted increased understanding of juvenile fall Chinook outmigration. So far, we have data on timing and range of juvenile outmigration and are accumulating data to calculate survival between detection points. As the study continues, the WST will gain more data on adult returns. This data will enable the WST to fill in details of fall Chinook life history to advance understanding of population dynamics. One of the most important management implications with respect to Deschutes River fall Chinook are the control of predatory bird colonies on East Sand Island and Little Miller Island and the fishery at the mouth of the Deschutes River. The WST will be investigating harvest rates of Deschutes River fall Chinook with respect to adult return timing as indicated by detections at Bonneville and The Dalles dams. PIT tag detections are expected to help inform management decisions as well as advance ecological understanding of Deschutes fall Chinook.

Results from this study will advance ecological understanding of juvenile su/f Chinook in the Deschutes River. Currently there is limited information on juvenile growth rates. More accurate estimates of the number of juvenile outmigrants, survival to the mouth of the Deschutes River, and smolt-to-adult return ratios will also result from this project. This information may contribute to development of a life table to further understanding of fall Chinook population dynamics.

All returning adults are not yet represented in the number of detected PIT tagged fish. To date (October 2015), 175 returning adults from 2011, 77 from 2012, 56 from 2013, and 12 from 2014 were detected. Of the returning fall Chinook detected, most detections were made at Bonneville Dam, followed by The Dalles Dam and the lowest number were at the mouth of the Deschutes River (Table 2). Not every PIT tag detected at Bonneville Dam was detected at upstream sites and there were instances of tags not detected at lower arrays but they were recorded upstream. This is due in part to differences in detection efficiencies among arrays and also that fish may not have turned into the Deschutes River but continued upstream (i.e., strayed) or harvested at the mouth of the Deschutes River. Three tags from 2011 were detected at McNary Dam (and nowhere else). From release year 2011, PIT tagged fall Chinook were detected at Little Goose, Lower Granite, Ice Harbor, Lower Monumental dams (one tag per site), and in lower Tucannon River (one tag). Also from release year 2011, five fall Chinook were detected at Shitike Creek mouth in 2014 and one at Warm Springs mouth (also in 2014). From release year 2012, one PIT tagged Chinook was detected at the mouth of Shitike Creek (in 2014). These detections in Deschutes River tributaries suggest that spring Chinook were inadvertently tagged instead of fall Chinook or that fall Chinook spawn in and return to Shitike Creek and Warm Springs River (which has been suspected).