One of the questions the Coastal Watershed Council hears most about the San Lorenzo River is, “How are the steelhead doing?” To monitor population changes of steelhead trout, or Oncorhynchus mykiss, the City of Santa Cruz Water Department and contracted fish scientists have long conducted summer surveys for these iconic fish in the San Lorenzo lagoon, and these surveys have been annual since 2008. The warm water of the lagoon creates an important habitat for young steelhead to eat and grow. If you’ve come across a survey at the river in action, you may have seen large seine nets like the one shown in the photo below, which are used to catch fish.
The City of Santa Cruz samples for steelhead in the San Lorenzo River using a seine net.
The fish monitoring surveys typically take place monthly from June through September. Scientists count and measure the steelhead caught in the seine, and tag them with rice-grain-sized devices called PIT tags as part a of a mark-recapture study. The number of tagged fish that are recaptured during a later survey are used to calculate two population estimates for the total number of O. mykiss in the lagoon each year: one for the spring at the start of the surveys, and one for the fall when surveys end. The fish caught in the surveys are mostly juvenile O. mykiss, typically 5-7 inches long; however, adult steelhead like the one shown below occasionally turn up.
Adult steelhead found in the San Lorenzo River lagoon in 2023. Photo credit: City of Santa Cruz
So what have more than 15 years of monitoring revealed? In general, more water means more fish.
Steelhead numbers are in the lagoon are typically higher in wet years with high rainfall, or in the year following a wet year. Michelle Tarian from the City of Santa Cruz Water Department shared this observation in a presentation about the steelhead monitoring program at the 2025 Salmonid Restoration Conference in Santa Cruz, noting recent highs in abundance between 2016 and 2020 corresponded with wet years in 2017 and 2019. The spring estimate for these years ranged between 2,378 fish and 26,815 fish for the spring, and between 704 and 18,720 fish for the fall (Figure 1).
Figure 1. Steelhead abundance estimates in the San Lorenzo River lagoon by year and water year type. CD = Critically Dry, D = Dry, N = Normal, W = Wet, VW = Very Wet. Source: City of Santa Cruz
Conversely, steelhead numbers were low in surveys from 2011-2015, years that coincided with a historic drought. The population in those years was estimated to be in the hundreds, with the spring estimate ranging from 60 to 559 fish, and the fall estimate ranging from 138 to 714 fish.
The most recent surveys in 2022 and 2023 again revealed lower numbers of steelhead, with the spring estimate at 53 and 444 for the two years, respectively. However, the fall estimate for 2023 was back in the thousands, at 2,046 fish. While the survey’s abundance estimates do not account for steelhead in the river outside the lagoon, they are still helpful for observing trends in the fish population. The 2024 steelhead monitoring report will be published soon, and will reveal whether the fish are on an upward trend following a very wet 2023 and a normal water year in 2024.
Monitoring surveys also provide a snapshot of other fishes that use the San Lorenzo River estuary. These typically include fish adapted to both saltwater and freshwater, including topsmelt, barred and shiner surfperch, Pacific herring, starry flounder, staghorn sculpin, and threespine stickleback. A few notable fishes are the endangered tidewater goby, and the non-native yellowfin goby, which turned up in the 2023 surveys.
Surveying for steelhead using a seine net in the San Lorenzo River lagoon. Photo credit: Chris Berry, City of Santa Cruz
Steelhead numbers are not as plentiful as they were a century ago, when production from an active fish hatchery attracted scores of avid anglers and helped earn the San Lorenzo the title of “The Most Fished River in California.” However, the City’s consistent monitoring indicates these resilient fish have the potential to rebound when conditions are right. As the third graders in CWC’s Watershed Rangers program learn, we can all take action to conserve water and support steelhead in our watershed. Other important actions include protecting habitat along rivers, properly managing septic tanks and sewer laterals to prevent leaks, and preventing littering and waste dumping in the river. Working together toward a healthier San Lorenzo River ecosystem is an important step in creating a home for our local trout to thrive.
You can find the past steelhead monitoring reports on the City of Santa Cruz web page for Water Resources Management here.