Simulated smelt distribution (green dots) representing responses to salinity and turbidity. The area outlined in red is the region of simulated release (black outlined areas are different study regions). Image: Gross, et al.
New insights into Delta smelt swimming behavior could help locate the increasingly elusive fish and prevent losses at the pumps.
Scientists know that smelt use tidal ebbs and flows to migrate landward to spawn, but the degree to which external cues influence behavior remains unclear. In a new study published in the March 2021 issue of San Francisco Estuary and Watershed Science, researchers used computer modeling to predict smelt distribution
based on hypothesized swimming behaviors. Six increasingly complex behaviors were tested. For example, the “passive” category assumes that smelt do not swim at all, simply drifting with currents and tides. At the other end of the spectrum, the smelt respond to both turbidity and salinity cues. After assigning a behavior to simulated smelt distributed across the Delta, researchers ran a 133-day simulation of water year 2002, then compared the modeled movements with observations from the 2002 Spring Kodiak Trawl Survey. The movement models that most closely matched distributions indicated by the survey data were those that incorporated some form of salinity response triggering migration (either overall salinity in the surrounding water or the rate of salinity increase). The study acknowledges that further refinements are needed to assess the influence of turbidity on smelt movement, due to the challenge of accurately predicting what is essentially a localized, constantly changing effect. In addition to providing insight into how Delta smelt respond to environmental cues, the study’s findings could also be used to anticipate the risk of fish being caught in water project facilities. According to Ed Gross, an author of the study, “Delta smelt behavior is likely an intermittent tidal migration, because passive behavior would not accomplish landward migration and persistent tidal migration also results in unrealistic distribution.” If turbidity and modeling refinements are made, and one or more behavior models match survey observations from multiple water years, the modeling approach developed by the study could provide real-time entrainment risk analysis based on water conditions in the Delta.