Conservation of endangered fishes commonly includes captive breeding, applied research, and management. Since 1996, a captive breeding program has existed for the federally threatened and California endangered Delta Smelt Hypomesus transpacificus, an osmerid fish endemic to the upper San Francisco Estuary. Although this program serves as a captive refuge population, with experimental releases being initiated to supplement the wild population, it was uncertain how individuals would survive, feed, and maintain condition outside hatchery conditions. We evaluated this and the effects of three enclosure designs (41% open, 63% open, and 63% open with partial outer mesh wrap) on growth, survival, and feeding efficacy of cultured Delta Smelt at two locations (Sacramento River near Rio Vista, CA and in Sacramento River Deepwater Ship Channel) in the wild. Enclosures exposed fish to semi-natural conditions (ambient environmental fluctuations and wild food resources) but prevented escape and predation. After four weeks, survival was high for all enclosure types (94–100%) at both locations. The change in condition and weight was variable between sites, increasing at the first location but decreasing at the second location. Gut content analysis showed that fish consumed wild zooplankton that came into the enclosures. Cumulatively, results show that captive-reared Delta Smelt can survive and forage successfully when housed in enclosures under semi-natural conditions in the wild. When comparing enclosure types, we observed no significant difference in fish weight changes (p = 0.58–0.81 across sites). The success of housing captive-reared Delta Smelt in enclosures in the wild provides preliminary evidence that these fish may be suitable to supplement the wild population in the San Francisco Estuary. Furthermore, these enclosures are a new tool to test the efficacy of habitat management actions or to acclimate fish to wild conditions as a soft release strategy for recently initiated supplementation efforts.

Early one morning, a scientist collecting fish in the San Francisco Estuary was surprised to find a young lamprey. She knew lamprey were ancient fish without jaws, bones, or scales, and she wondered to herself: Where else are young lamprey in the estuary? She discovered that although lamprey existed long before dinosaurs, little is known about their lives in estuaries. She and her team decided to gather data from scientists who accidentally caught lamprey while studying other animals. This process of reusing data from unrelated studies, called data synthesis, is an important tool for answering unsolved mysteries. The team uncovered differences in where young lamprey live and that three species live in the San Francisco Estuary, including two species that live half of their lives in the ocean and the other half in freshwater. Knowing where young lamprey live can help scientists protect the habitats lamprey need to develop.

Conservation hatcheries are like luxury fish hotels that raise threatened and endangered fish that are nearing extinction in the wild. Raising fish in the controlled environment of the conservation hatchery usually takes away the issues that caused the population to dwindle in the first place. However, there is one problem: the fish get used to the conservation hatchery and become wimpy, meaning they become domesticated and do not do as well as wild fish in if they are returned to the natural environment. Managing the genes of hatchery fish is one way to block domestication and raise fish that are as close as possible to wild fish. In the San Francisco Estuary watershed, there are conservation hatcheries for the endangered delta smelt and winter-run Chinook salmon. Read on to learn about how these conservation hatcheries help hatchery fish be as tough as possible and survive in the wild.