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other·conservation biology, fisheries science, aquatic ecology, endangered species management·PMC10218733

Captive-reared Delta Smelt (Hypomesus transpacificus) exhibit high survival in natural conditions using in situ enclosures

PLOS ONE · 15 authors, 6 centres

AI SUMMARY

FIDELITY 97%

POPULATIONCaptive-reared adult Delta Smelt (Hypomesus transpacificus) from the UC Davis Fish Conservation and Culture Laboratory

INTERVENTIONHousing in three enclosure designs (41% open, 63% open, and 63% open with partial outer mesh wrap) deployed at two field sites in the San Francisco Estuary for four weeks

COMPARISONBetween enclosure types (A, B, C) and between two deployment sites (Sacramento River near Rio Vista and Sacramento River Deepwater Shipping Channel)

This summary was generated by AI from a single paper. It has not been reviewed by a clinician and is not clinical advice. Verify against the source before acting on it.

AGENT 01 · STRUCTURED SUMMARY

TRACE TO SOURCE

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TL;DR

Captive-reared Delta Smelt showed high survival (88–100%) over four weeks when housed in in situ enclosures in the San Francisco Estuary, with fish at one site (Rio Vista) gaining weight (+15.9%) while those at another site (Deepwater Shipping Channel) lost weight (-15.9%). Gut content analysis confirmed that fish successfully consumed wild zooplankton entering the enclosures, demonstrating that captive-reared individuals can forage and maintain condition under semi-natural conditions. These findings support the potential use of captive-reared Delta Smelt for supplementation and as surrogates to evaluate habitat management actions.

Full summary

4,054 CHARS

**Background:** The Delta Smelt (Hypomesus transpacificus) is an endangered osmerid fish endemic to the San Francisco Estuary, with wild populations at or below detection limits of monitoring programs. A captive breeding program has existed since 1996, but it was unknown whether captive-reared individuals could survive, feed, and maintain condition outside hatchery conditions. This study evaluated the use of in situ enclosures as a tool to test captive-reared Delta Smelt under semi-natural conditions and to inform future supplementation efforts.

**Methods:** In winter/spring 2019, captive-reared adult Delta Smelt (206–278 days post hatch) were deployed for four weeks at two sites: the Sacramento River near Rio Vista (RV) and the Sacramento River Deepwater Shipping Channel (DWSC). Three enclosure designs were tested: Design A (41% open perforated steel), Design B (63% open), and Design C (63% open with external woven mesh wrap on lower half). At RV, 64 fish were placed in each of 6 enclosures (2 replicates per design); at DWSC, 60 fish per enclosure. Fish were tagged with Visible Implant Alphanumeric tags, weighed, and measured before and after deployment. Survival was monitored daily. Zooplankton samples were collected weekly adjacent to enclosures. At study end, stomach contents from 10 fish per enclosure (60 per site) were analyzed. Statistical analyses included t-tests for pre/post comparisons, linear mixed-effects models for growth by enclosure type, and ANOVA for instantaneous ration and zooplankton biomass.

**Key Results:** Survival was high across all enclosure types and sites: at RV, mean survival was 96.4% ± 0.8% (range 87.5–100%); at DWSC, 98.6% ± 0.3% (range 96.7–100%). At RV, fish gained weight (mean change +0.197 ± 0.006 g, +15.9%; t = -34.791, df = 597.72, p < 2.2e-16), increased fork length (+0.164 ± 0.011 cm, +2.9%; t = -4.5123, df = 684.31, p = 7.547e-06), and increased condition factor K (+0.040 ± 0.004 g; t = -6.8555, df = 687.21, p = 1.6e-11). At DWSC, fish lost weight (mean change -0.248 ± 0.009 g, -15.9%; t = -14.814, df = 493.77, p < 2.2e-16) and decreased in K (-0.111 ± 0.004; t = 11.802, df = 366.76, p < 2.2e-16), with no change in fork length. There was no significant effect of enclosure type on weight change at either site (RV: ANOVA F = 0.2232, df = 2, p = 0.8122; DWSC: ANOVA F = 0.5411, df = 2, p = 0.583). Instantaneous ration differed across enclosure types at RV (ANOVA, F₂,₅₇ = 4.25, p = 0.019) but not at DWSC (ANOVA, F₂,₅₇ = 2.53, p = 0.089). Diet analysis showed fish consumed wild zooplankton: at RV, diet was dominated by cladocerans (79.1%, primarily Daphnia) and cyclopoids (16.6%); at DWSC, fish consumed cladocerans (47.8%, primarily Chydorus sp.), cyclopoids (39.1%, primarily Acanthocyclops sp.), and calanoids (10.0%). Zooplankton biomass was significantly higher at RV than DWSC (ANOVA, F₁,₆ = 14.67, p = 0.009). Water velocity reduction by enclosures averaged 70.68% for Design A, 43.98% for Design B, and 50.47% for Design C.

**Clinical Implications:** This study provides the first evidence that captive-reared Delta Smelt can survive and forage successfully under semi-natural conditions in the wild, with survival rates far exceeding the authors' initial expectations of low survival. The enclosures represent a new tool for testing habitat management actions and for acclimating fish prior to release (soft release strategy) for supplementation programs. The contrasting results between sites (weight gain at RV vs. weight loss at DWSC) highlight the importance of site selection, with floodplain-influenced locations like Rio Vista (downstream of Yolo Bypass) providing better food resources. The lack of significant differences among enclosure designs suggests that a range of mesh sizes can support Delta Smelt, though the larger mesh (Design B) showed modest foraging benefits at one site. These findings support ongoing supplementation efforts for Delta Smelt and provide a methodological framework applicable to other sensitive endangered fish species.

PICO

PPOPULATION

Captive-reared adult Delta Smelt (Hypomesus transpacificus) from the UC Davis Fish Conservation and Culture Laboratory

IINTERVENTION

Housing in three enclosure designs (41% open, 63% open, and 63% open with partial outer mesh wrap) deployed at two field sites in the San Francisco Estuary for four weeks

OOUTCOME

Survival, growth (weight, fork length, condition factor K), and feeding efficacy (instantaneous ration, diet composition)

STUDY TYPE

other

SPECIALTY

conservation biology

SUMMARISED BY

AI pipeline

FIDELITY CHECK

97% · A

ORIGINAL PAPER

PMC10218733

Captive-reared Delta Smelt (Hypomesus transpacificus) exhibit high survival in natural conditions using in situ enclosures

PLOS ONE

READ THE FULL PAPER

ABSTRACT

Abstract 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.

CITATION EXPORT

Melinda R Baerwald, Nicole Kwan, Catarina Pien et al.. Captive-reared Delta Smelt (Hypomesus transpacificus) exhibit high survival in natural conditions using in situ enclosures. PLOS ONE. (2023). https://doi.org/10.1371/journal.pone.0286027

AUTHORS

15 · 6 CENTRES

Melinda R Baerwald, Nicole Kwan, Catarina Pien, Grace Auringer, Evan W Carson, Dennis E Cocherell, Luke Ellison, Nann A Fangue, Amanda J Finger, Daphne A Gille, Haley Hudson, Tien-Chieh Hung, Ted Sommer, Troy Stevenson, Brian M Schreier

1California Department of Water Resources, West Sacramento, California, United States of America
2Department of Animal Science, University of California – Davis, Davis, California, United States of America
3San Francisco Bay-Delta Fish and Wildlife Office, U.S. Fish and Wildlife Service, Sacramento, California, United States of America
4Department of Wildlife, Fish and Conservation Biology, University of California – Davis, Davis, California, United States of America
5Department of Biological and Agricultural Engineering, University of California – Davis, Davis, California, United States of America
6CIFRI: Central Inland Fisheries Research Institute, INDIA