Using DIDSON to Provide Adult Steelhead Escapement Estimates in a Small Coastal Drainage of Central California

Steelhead (Oncorhynchus mykiss) in central and southern California are listed as Threatened and Endangered under the Endangered Species Act.  Obtaining escapement estimates is critical for measuring progress of recovery planning.  However, monitoring these fish presents difficult challenges since their abundances are so low that a complete census of these populations is needed.  Dual-frequency identification sonar (DIDSON) is a promising new technology that could potentially be used to monitor steelhead runs in California streams and rivers under highly variable environmental conditions.  We tested the feasibility of using DIDSON to estimate steelhead escapement in a low abundance setting in Scott Creek, a small coastal watershed in central California.  DIDSON uses sonar to produce high-quality images in turbid water, which allows for detection and enumeration of fish, as well as estimation of fish size and swimming direction.  We deployed the DIDSON in Scott Creek over three steelhead run seasons from 2008-2010 and compared the results to those from a floating resistance-style weir located 200 m downstream.  The DIDSON estimates (153, 57, and 84) were approximately two to three times those from the weir (50, 23, and 40).  The differences may have been due to differences in operation time, as the DIDSON remained operational during periods of high flow when fish were migrating and the weir was over-topped, allowing fish to pass through undetected.  Our DIDSON sampling has also shown that steelhead behavior in the stream is fairly complex.  Issues with fish behavior (i.e., milling) made image analysis difficult, especially later in the spawning season as small numbers of spawners were still heading upstream and large numbers of kelts were traveling downstream.  Species differentiation can also be problematic when using DIDSON for this type of monitoring if species of similar body morphology or size are present over the same time period as the target species.  The main advantages to using DIDSON for monitoring listed fish are that fish passage is not impeded and fish do not need to be handled to be enumerated.  Our results indicate that DIDSON can be a useful tool in providing escapement estimates for steelhead in low abundance situations.