Friday July 17, 2026
Maven’s Notebook —
Biologists studying the San Francisco Estuary’s endangered fishes are trying to unpack a puzzling but encouraging uptick of endangered longfin smelt in a small North Bay watershed. With the fish declining in most of the estuary, time is running out to stabilize the population. Now, scientists hope that understanding why longfin are aggregating in this small stream system could guide efforts to protect critical habitat and save the species from extinction.
In the last quarter century, the longfin smelt population has collapsed, following the path of most fishes native to the estuary. Loss of wetland habitat, depleted river flows, and climate change are the probable culprits.
However, in the past three years, the fish’s numbers have shown a modest bump—likely from the ecosystem benefits of three consecutive wet winters beginning in 2023. History has shown that winters of high precipitation prompt increases in longfin numbers; this pattern is accentuated when wet years occur consecutively.
“You get a compounding effect from these back-to-back wet water years,” says Colin Brennan, who supervises the California Department of Fish and Wildlife’s Interagency Ecological Program.
What has particularly caught scientists’ eyes, though, is the outsized concentration of longfin smelt in the marshy lower reaches of the Napa River.