Steelhead (Oncorhynchus Mykiss) are the same species as Rainbow Trout and life history strategies of this species can be highly variable. Juvenile O. mykiss may spend their entire lives in fresh water (Rainbow Trout) or may outmigrate to the ocean (steelhead). To further confound the issue, some juveniles from two resident Rainbow Trout parents may outmigrate to the ocean; conversely, some juveniles from two anadromous steelhead parents may never outmigrate. The timing of outmigration can also vary widely and interpreting migrational movements can be challenging. In order to monitor instream abundance and outmigration of juvenile O. mykiss within the Okanogan subbasin, OBMEP has used a number of different approaches.
Snorkel surveys of juvenile fish are conducted at base flow periods in late summer and early fall at all habitat monitoring sites. Surveys are conducted on the mainstem Okanogan and Similkameen Rivers by using 5 snorkelers floating in a downstream direction over 500 meter long survey sites. Observers count all fish species as they float and record observations on a wrist-slate. On tributaries, one or two observers crawl in an upstream direction counting fish over 150 meter long sites. Snorkel surveys are conducted each year at 50 sites, across both the Washington State and British Columbia portions of the Okanogan subbasin. Long term data from snorkel surveys can show changes in observed fish abundance over time. In general, results from snorkel surveys conducted since 2005 show very low numbers of juvenile steelhead in the mainstem and considerably higher densities in tributaries.
In addition to snorkel surveys, the program has used additional methods to determine abundance of fish in streams. One of the most common approaches is the use of electrofishing and mark-recapture studies. Fish are sampled with a backpack electrofisher in late fall, marked with a PIT tag upon capture, and released back into the stream. Each uniquely numbered tag can then be detected at a later date if the fish is recaptured, or can be detected at various locations as the fish migrates to the ocean and back as an adult (e.g. screw traps, instream PIT tag antennas, or at mainstem dams). The primary study goals are to: (1) estimate abundance of juvenile O. mykiss in small streams, (2) calculate precision of estimates, and (3) calculate a stream-based outmigration estimate from PIT tagged fish. These methods allow the program to more accurately monitor annual abundance of juvenile steelhead in the Okanogan, estimate precision and bias associated with methods, and to examine trends in juvenile abundance, spatial distribution, and diversity through time.