We assessed thermal and salinity limits in several ontogenetic stages and acclimation states of Delta Smelt to evaluate sensitivity to climate change stressors. Thermal tolerance decreased among successive stages, and juvenile tolerance limits were closest to current environmental conditions. Salinity impacted juvenile and adult survival in exposures over acute timescales.

In response to Federal listing of the Delta Smelt Hypomesus transpacificus as a threatened species in 1993, intensive fish culture techniques were developed to provide a supply of fish for research activities. The Delta Smelt was listed as endangered by the state of California in 2009, and several agencies worked quickly to develop a captive refuge population under genetic management. Captive 2‐year‐old wild‐origin Delta Smelt served as the founding population in 2008. Each year, 250 genetically selected, single pair crosses are made in vitro, and the resultant full‐sibling families are combined to rear in multifamily groups. Typically, eight families are reared together from egg to adult stage, with 80% or more of the initial families represented at the adult stage. Multifamily rearing provides an efficient way of achieving a breeding population of 500 in a smaller facility. Juvenile survival increased from 18% in 2009 to 39% in 2010, as facilities and methodologies improved. Growth rate also increased significantly from 2009 to 2010 (from 0.19 to 0.25 mm/d). Subdermal alphanumeric tags identified individuals and allowed spawning of select individuals to preserve genetic diversity in the refuge population. Group marking, by adipose fin clip, provided efficiencies in time and space. Tagging and genetic analyses enabled in vitro spawning of recommended pair crosses each year. At present, we recommend completing the majority of spawning from February to mid‐May and continuing to augment the refuge population with wild fish each year. The refuge population provides one type of safeguard against species extinction and provides an example for endangered fish culture.

Genetic adaptation to captivity is a concern for threatened and endangered species held in conservation hatcheries. Here, we present evidence of genetic adaptation to captivity in a conservation hatchery for the endangered delta smelt (Fish Conservation and Culture Laboratory, University of California Davis; FCCL). The FCCL population is genetically managed with parentage analysis and the addition of wild fish each year. Molecular monitoring indicates little loss of genetic variation and low differentiation between the wild and conservation populations. Yet, we found an increase in offspring survival to reproductive maturity during the subsequent spawning season (recovery rate) in crosses that included one or both cultured parents. Crosses with higher levels of hatchery ancestry tend to produce a greater number of offspring that are recovered the following year. The recovery rate of a cross decreases when offspring are raised in a tank with fish of high levels of hatchery ancestry. We suggest changes in fish rearing practices at the FCCL to reduce genetic adaptation to captivity, as delta smelt numbers in the wild continue to decline and the use of FCCL fish for reintroduction becomes more likely.

Water exports have been implicated in the decline of fish populations in the upper San Francisco Estuary, California. We evaluated the relation between delta smelt salvage at the John E. Skinner Delta Fish Protective Facility (SFF) and underlying entrainment losses at the State Water Project (SWP, south Delta). We used cultured delta smelt in mark-recapture experiments in February and March 2009 (adults) and June 2009 (juveniles) to estimate: (1) the percent of fish recaptured at the SFF of the total released at the entrance of the SFF (fish facility efficiency), (2) the percent of fish recaptured at the SFF of the total released in Clifton Court Forebay (CCF), a reservoir for SWP exports, and (3) the fish losses in CCF and before the SFF (pre-screen loss). Mean fish facility efficiency was lower in successive releases: February (53.2%), March (44.0%) and June (24.0%). The mean percent recapture of fish released at the CCF entrance was also lower over time: February (3.01%); March (0.41%) and June (0.03%). Correspondingly higher mean pre-screen losses occurred over time: February (94.3%); March (99.1%) and June (99.9%). We concluded that: (1) entrainment losses of delta smelt could be higher at times, compared to other species previously studied at the SWP; (2) pre-screen loss was the largest source of mortality for delta smelt; (3) increased release distance from the SFF and residence time in CCF-and decreased exports-resulted in a lower percentage of recaptured fish at the SFF; and (4) salvage of delta smelt at the SWP does not seem to be a consistent index of entrainment.

Reciprocal diet switches were conducted with first-feeding white sturgeons Acipenser transmontanus to investigate possible mechanisms of food acceptance and food preferences. Survival and relative growth rates were significantly reduced following a switch from a more natural diet (chopped Tubilex sp. worms) to a prepared diet (Biodiet), whereas growth was improved, and survival was improved or unaffected, following the reciprocal diet switch. Feeding observations, followed by assessment of stomach fullness, indicate that strong or exclusive preferences for Tubifex can develop quickly. These data suggest that the animals are more "prepared" physiologically and anatomically to accept and assimilate the more natural diet than the prepared diet during the early life stages. Associative learning of pre-and postingestional stimuli may be important in the development of food preferences.