Since 2008, a refuge population of Delta Smelt Hypomesus transpacificus has been cultured in a conservation hatchery to help protect the imperiled species from extinction. Culture techniques for this population have thus far focused on maximizing genetic diversity in conjunction with large-scale operations (40 larval tanks, 5,954-L larval system, 224,000 larvae capacity), which includes rearing multiple family groups within the same tanks. However, Delta Smelt production and associated research projects would benefit from also being able to rear fish on a smaller scale, such as in individual family groups. Recently, a new culture system and modified culture practices were developed for rearing small groups of larval Delta Smelt. We (1) determined the performance of the small-scale "experimental" rearing system from embryo to 80 d posthatch (dph) and (2) tested whether the performance of the experimental system met the standard set by the large-scale "refuge" rearing system at critical culturing life stages in terms of embryo hatching rate (10 d postfertilization) and larval survival, length, and weight (40 and 80 dph). From 1,000 embryos, the experimental system produced a mean AE SE of 376 AE 44 late-stage larvae. The hatching rate of embryos did not differ between systems. Early-stage larvae (0-40 dph) reared in the experimental system did not differ in survival rate but were longer and heavier than those reared in the refuge system. In contrast, late-stage larvae (41-80 dph) reared in the experimental system had poorer survival and were shorter and lighter than refuge-reared larvae. Thus, the experimental system met the refuge-system benchmark for embryo hatching and exceeded the benchmark in early-larval production, but it could be improved for late-larval rearing. Nevertheless, this small-scale rearing system will provide much-needed flexibility to enhance Delta Smelt culture practices and facilitate future research.

Ramshorn snail (Helisoma anceps) is a freshwater gastropod found all over North America and is also an essential component to the larval and juvenile culture of endangered Delta Smelt (Hypomesus transpacificus) at the UC Davis Fish Conservation and Culture Lab (FCCL). H. anceps has been proven effective in cleaning the excess algae while not harming the larvae. A challenge at the FCCL has been having a reliable source of these snails, since previously it has been dependent upon nature and never guaranteed there would be enough to meet the needs of the FCCL. Experiments were conducted to assess the effects of temperature, food types, rearing density and total ammonia nitrogen (TAN) concentrations. Procedures recommended to increase snail fecundity for the FCCL are to rear spawning parent snails and the resultant eggs at a high temperature (16-20°C) and with a low TAN concentration (0-5 mg/L). Newly hatched snails need to be cultured at a low density (about one snail/20 cm 2 ). After the snails grow to an acceptable size (1.3 cm diameter), they could be set aside and cultured in an environment with less optimal water quality parameters such as a high TAN level (as high as 20 mg/L) and low temperature (12°C) for quantity control prior to use. K E Y W O R D Senvironmental factor, freshwater snail, rearing, water quality wileyonlinelibrary.com/journal/are

A captive breeding programme was developed in 2008 for delta smelt Hypomesus transpacificus in reaction to dramatic population decline over several decades. We took 526 sub‐adult captive‐reared delta smelt and cultured them for 200 days without providing artificial food or water quality management to assess their performance once released in the wild. The results indicated captive‐reared sub‐adult delta smelt could survive in a semi‐natural environment with uncontrolled water quality and naturally produced wild prey through spawning and into their post spawning phase.

Background The application of otolith-based tools to inform the management and conservation of fishes first requires taxon- and stage-specific validation. The Delta Smelt (Hypomesus transpacificus), a critically endangered estuarine fish that is endemic to the upper San Francisco Estuary (SFE), California, United States, serves as a key indicator species in the SFE; thus, understanding this species’ vital rates and population dynamics is valuable for assessing the overall health of the estuary. Otolith-based tools have been developed and applied across multiple life stages of Delta Smelt to reconstruct age structure, growth, phenology, and migration. However, key methodological assumptions have yet to be validated, thus limiting confidence in otolith-derived metrics that are important for informing major water management decisions in the SFE. Methods Using known-age cultured Delta Smelt and multiple independent otolith analysts, we examined otolith formation, otolith-somatic proportionality, aging accuracy and precision, left-right symmetry, and the effects of image magnification for larval, juvenile, and adult Delta Smelt. Results Overall, otolith size varied linearly with fish size (from 10–60 mm), explaining 99% of the variation in fish length, despite a unique slope for larvae < 10 mm. Otolith-somatic proportionality was similar among wild and cultured specimens. Aging precision among independent analysts was 98% and aging accuracy relative to known ages was 96%, with age estimates exhibiting negligible differences among left and right otoliths. Though error generally increased with age, percent error decreased from 0–30 days-post-hatch, with precision remaining relatively high (≥ 95%) thereafter. Increased magnification (400×) further improved aging accuracy for the oldest, slowest-growing individuals. Together, these results indicate that otolith-based techniques provide reliable age and growth reconstructions for larval, juvenile, and adult Delta Smelt. Such experimental assessments across multiple developmental stages are key steps toward assessing confidence in otolith-derived metrics that are often used to assess the dynamics of wild fish populations.

Successful aquaculture operations strive to produce high-quality, healthy fish while minimizing costs. One way to do this is to wean fish off live feed as soon as possible to a formulated dry feed. In this study, Delta Smelt Hypomesus transpacificus were weaned onto a dry feed diet at three life stages: early larval (14 days posthatch [dph]), late larval (60 dph), and juvenile (105 dph), and compared to a control group of fish receiving live prey to 160 dph according to current practices. Delta Smelt did not have high survival or growth when dry feed was introduced at 14 dph, and showed significant increases in growth but low survival when the weaning process was started at 60 dph. Fish going through the experimental weaning process starting at 105 dph showed higher growth and survival rates than the control regime, indicating that this is a more optimal time to wean them off the live prey diet. Our findings suggest that Delta Smelt production methods could be improved by weaning fish earlier than the 160 dph currently implemented at the conservation hatchery.