Sonar monitoring shows largest Chinook run on Smith River since program began; seals and sturgeon observed

Project biologists reported to the Del Norte County Fish and Game Commission that sonar monitoring on the Smith River produced the largest single-season Chinook salmon estimate recorded by the project: 27,427 fish for the 2024–25 season. The presenter said the program recorded a single-day peak of 3,585 upstream-moving fish on Oct. 27 and a following day estimate of just over 1,800.

The presentation, delivered by the project’s sonar monitoring lead, described how two types of imaging sonar (DIDSON and ARIS units) produce digital imagery used to count and track fish movements. “The advantage of using these imaging sonars is that they can work in dirty water and in pitch darkness,” the presenter said, noting the equipment cannot reliably distinguish species by image alone and therefore uses Rowdy Creek Fish Hatchery trap data to apportion Chinook and steelhead in sonar counts.

Why it matters: the Smith River is monitored at a single downstream site that, the presenter said, captures runs from most spawning tributaries upstream. The commissioner meeting heard that the Chinook total of 27,427 was about 10,000 greater than the project’s average seasonal estimate (average cited as roughly 17,002), while steelhead counts remain below early-project levels and show year-to-year variability.

How the monitoring works and limits: the presenter explained DIDSON and ARIS technology and described trade-offs between range and image quality. He said the project samples 33 percent of sonar video (20 minutes per hour) for counts and applies hatchery trap proportions to estimate species totals. He added the program tracks “milling” behavior — repeated upstream/downstream movement — which complicates counts during low flows. “In low water salmon will swim back and forth waiting for a rain event,” he said, and that pattern can cause repeat counts of the same fish.

Pinnipeds and predators: the presentation included concerted pinniped (seal and sea lion) detection tracking begun under current project management. The presenter reported 673 pinniped detections noted last season and said most detections clustered in October–December, with a smaller uptick in March likely tied to downrunning steelhead. When asked whether increased pinniped numbers had been shown to reduce salmon runs, the presenter said the project cannot make that causal claim from these data: “We’re not able to make that conclusion. What we can say is that there appears to be a relationship between the number of seals and the size of the runs of fish that are coming.”

Other species and observations: the project documented sturgeon and river otters on sonar. The presenter described a roughly six-foot sturgeon recorded on multiple frequencies, noting sturgeon observations occur most years though counts vary; he said a handful of sturgeon were seen in spring during some years and far fewer in others. Cutthroat trout and juvenile coho were discussed as non-target species that complicate apportioning; the presenter said Rowdy Creek trap data and CDFW spawner survey totals are used to estimate coho, which the presenter said this season may number about 1,600–1,800 (the presenter described this coho figure as a working estimate drawn from spawner-survey adjustments to sonar totals).

Recording efficiency and data loss: the presenter reported seasonal recording efficiency near the project average but noted increasing data loss during high-flow events. Last season’s efficiency was reported as 92.4%, slightly below a multi-year average of 93.4%. Two principal causes of lost data were identified: equipment/network/power faults and proactive removal of sonars when river stages make the access road or equipment unsafe. The presenter warned that high flows (river stages in the mid-20-foot range and above) remove sonar coverage of portions of the channel and that estimating fish passage during those outages is difficult; he said estimates for outages due to network or power failures can be interpolated from adjacent days, but high-flow periods resist confident interpolation.

Funding and program context: the presenter said the monitoring began in 2010 under a California Department of Fish and Wildlife (CDFW) grant, paused when CDFW labeled the watershed a lower monitoring priority, and resumed in 2021 with tribal-nation involvement. He told the commission the project has been funded in part through the Rowdy Creek Fish Hatchery permitting and related hatchery genetic management planning; attempts to win broader federal funding (for example from the Pacific Coast Salmon Recovery Fund) were described as challenging because those funds prioritize listed endangered species.

Early season 2025–26 update: sonars were deployed Sept. 13 for the current season and, through the most recent sampling day shown in the presentation, the project was recording above-average Chinook movement for the elapsed period and had already produced one daily estimate above 2,000 fish. The presenter said ocean fishery closures likely allowed more salmon to survive to return as adults, contributing to stronger Chinook counts.

Questions from commissioners touched on species apportioning, whether sonar counts are used for fishery quotas (the presenter said the monitoring is not used to set harvest quotas), and how the project handles steelhead kelts (post-spawn fish traveling downstream) in the totals. The presenter said annual reports include separate treatments of downrunning kelt observations and that conservative reporting choices are used so annual published estimates can be interpreted with known caveats.

Votes at a glance: the commission approved a motion to accept the previous meeting minutes (voice vote; counts not specified in the transcript) and later approved a motion to adjourn (voice vote; counts not specified).