San Francisco Estuary and Watershed Science

The release of hatchery-reared fish into the natural environment for conservation purposes may have unintended consequences for wild fish populations and the larger ecosystem. In situ enclosures are a tool that can be used to study cultured fish under natural conditions to help mitigate these risks or to acclimate fish to the surrounding environment during soft release. Despite widespread utility, few resources provide direction on material choice and enclosure design to conserve non-commercial and imperiled fish species that may have unique physiology and needs. Here, we designed, created, and tested an enclosure for the endangered Delta Smelt (Hypomesus transpacificus), an osmerid native to the San Francisco Estuary (estuary). We first performed hydraulic modeling and evaluated permeability to prey items by measuring energy dissipation and flow deflection of candidate screen materials in a flume. The final enclosure design was a cylinder made out of stainless-steel wire mesh, with 60% openness that measured 1.0 m in diameter and 1.3 m tall. We then investigated survival, growth, and feeding of cultured Delta Smelt in the enclosure in three 1-month-long deployments in an experimental pond. Fish survival was moderate to high (50% to 93%), and we observed that Delta Smelt in the enclosures spontaneously converted to live, natural food, which suggests that cultured fish will be able to forage when released into the estuary. We hope that the steps we have outlined and the final enclosure design will be instrumental in supplementing Delta Smelt and conserving other at-risk fish species in the future.