Eastward migration or marshward dispersal: understanding seasonal movements by delta smelt

Sometimes, the result of the same people talking to the same people in the same institutions leads to commonly held conclusions that are not supported by available data and analyses. Such was the case with disinformation on dispersion of delta smelt prior to their spawning each year.  Numerous presentations at numerous conferences, as well as conceptual diagram repeated in agency documents, depicted delta smelt adults migrating eastward migration taking them from the center of their range into the central and south delta where the population would be vulnerable to entrainment at state and federal water-export facilities. That description of inter-seasonal movements of delta smelt stands in contrast to findings drawn from previous studies, which describe movements by pre-spawning delta smelt from open waters in bays and channels to adjacent marshlands and freshwater inlets. In an effort to resolve disagreement regarding delta smelt movement, publicly available data on the distribution of the fish was used to generate maps and draw inference on its seasonal patterns of dispersal. Inter-seasonal dispersal of delta smelt was found to be more circumscribed than previously reported. The characterization of a spawning migration is not appropriate. Rather than migrating eastward en masse, the predominant movement of pre-spawning delta smelt adults is northward to Suisun Marsh and the north Delta, with only a small fraction of adults finding their way to the south Delta. This conclusion has real-world implications for efforts to conserve delta smelt. These findings indicate that a conservation strategy for delta smelt should focus on habitat restoration and management efforts for tidal marsh and other wetlands in north Delta shoreline areas directly adjacent to open waters that have been documented to support higher concentrations of the fish. Importantly, the hypothesis that entrainment at water project pumps represents a substantial threat to the delta smelt population is not supported by the available data.

Murphy DD, Hamilton SA. 2013. Eastward migration or marsh-ward dispersal: exercising survey data to elicit an understanding of seasonal movement in delta smelt. San Francisco Estuary and Watershed Science 11(3):1-20.