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US Fish & Wildlife Service FieldNotes

STOCKTON FWO: Qualitative Modeling for the Delta Smelt Community: Stability and Response to Perturbations

Region 8, October 13, 2012
Satellite view of the Delta in the upper San Francisco Estuary. Indicated are the positions of X2 at 74 and 85 km, denoting high and low Delta outflows, respectively.
Satellite view of the Delta in the upper San Francisco Estuary. Indicated are the positions of X2 at 74 and 85 km, denoting high and low Delta outflows, respectively. - Photo Credit: n/a
Delta smelt, a listed endemic species of high management importance in the upper San Francisco Estuary.
Delta smelt, a listed endemic species of high management importance in the upper San Francisco Estuary. - Photo Credit: n/a

By Gonzalo Castillo, Fish Biologist

Estuarine ecosystems are particularly complex and such complexity is amplified by anthropogenic effects. In Summer 2012, the U.S. Fish and Wildlife Service began to explore the use of qualitative models to address community-level questions in the highly developed upper San Francisco Estuary (Delta). Modeling will be conducted by the Service in collaboration with scientists from the Interagency Ecological Program (IEP) of the Sacramento-San Joaquin Estuary and other partners. The focus will be on species interactions and abiotic influences for the delta smelt (Hypomesus transpacificus), a species listed under both the state and federal endangered species acts.

The goal of this research is to evaluate the delta smelt community over several temporal and spatial scales and for different life stages of delta smelt. The modeling approach is based on qualitative analysis of community interactions and addresses important system-level knowledge gaps, namely, the potential implications of past and current perturbations, natural or otherwise, on community structure and function and the observed or anticipated population responses to such perturbations.

The four objectives of this research are: First, to evaluate the response of the delta smelt population to the fall X2 conditions in 2011, where X2 is the location in the estuary with two percent salinity as measured at the bottom of the channel. Second, to analyze the stability of the estuarine community and the response of the delta smelt population to prevailing species interactions and abiotic factors and stressors following the Pelagic Organism Decline (POD). Third, to compare the stability patterns for the delta smelt community over different historical periods prior to the POD, and fourth, to investigate the likely mechanisms affecting the stability of the delta smelt community over different historical periods.

Initial work has focused on the first objective, an area of major research and management interest by the IEP’s Fall Low Salinity Habitat Program (FLaSH) ,which addresses the importance of the low salinity habitat (LSH) to the ecology of the Delta and its likely response to higher outflows and increased delta smelt habitat, growth and survival. During September-October 2011, favorable fall flow conditions were observed to evaluate FlaSH hypotheses (X2~74km; the distance in kilometers from the Golden Gate Bridge to the upstream location where the bottom salinity is two percent). As part of the first objective, qualitative community models were used by the Service to evaluate potential ecological interactions and the underlying mechanisms that may affect the delta smelt population.

The observed changes in relative abundances for different trophic groups in the fall 2011 were compared to FLaSH hypotheses and qualitative models predictions to investigate underlying ecological mechanisms. The overall consistency of model predictions and field observations for delta smelt and other trophic groups tends to support the FLaSH hypothesis that fall flows have positive effect on the delta smelt population. Future field and modeling work will help to refine our understanding on the mechanisms controlling delta smelt response to high fall outflow.

Contact Info: Gonzalo Castillo, 209-334-2968x323, gonzalo_castillo@fws.gov