Delta smelt (Hypomesus transpacificus) is an endangered fish species endemic to the Sacramento/San Joaquin Delta. A refuge population is maintained in culture by the UC Davis Fish Conservation and Culture Lab (FCCL) in Byron, CA. The FCCL also produces adult delta smelt for supplementation to prevent extinction of the species in the wild. The potential of supplementation at earlier life stages may be feasible, but its potential is understudied. Supplementation at the egg stage via egg frame boxes (EFB, Figure 1), for example, is used to support recreational fishing of the closely related wakasagi in its native habitats in Japan. Adaptation of wakasagi EFB for use in supplementation of delta smelt, therefore, may be a viable method to facilitate conservation of delta smelt in the wild via egg-stage supplementation.
We took a stepwise approach to assess the feasibility of EFB for supplementation of delta smelt eggs. First, we identified Cache Slough as a candidate site for EFB deployment based on annual monitoring survey data, which show that a majority of ripe female delta smelt are captured in this region during the spawning season. We then conducted computational fluid dynamics (CFD) simulations to evaluate if flow conditions in the slough are suitable for EFB deployment during this period. These simulations incorporated USGS flow data obtained from two Cache Slough stations in February. Next, we investigated potential effects of transportation on egg survival. We ran ‘transportation’ trials at FCCL by soaking EFB in water and then exposing the box to direct sunlight for the approximate travel time from FCCL to the deployment site. Our next step is to investigate delta smelt embryo development and hatching rates based on the range of humidity obtained from the first trial (Figure 2). These studies aim to inform EFB design and deployment conditions that would facilitate egg-stage supplementation of delta smelt.