by Dylan Stompe, Teejay O’Rear, John Durand, and Peter Moyle

            The San Francisco Estuary (estuary) is sometimes called the most invaded estuary in the world, and for good reason. Through many avenues, hundreds, if not thousands, of species have been introduced to San Francisco Bay, the Delta, and their rivers. Some introductions were byproducts of human activity and include organisms that “hitchhiked” on the bottom of boats or as stowaways in ballast water carried by international shipping vessels. Others were deliberate and undertaken either legally by the government or illicitly by individuals for biocontrol, fisheries, or disposal of unwanted pets.

            The U.S. Fish and Wildlife Service (USFWS) defines aquatic invasive species as “aquatic organisms that invade ecosystems beyond their natural, historic range.” Under this definition, any species brought into the estuary and establishes a self-sustaining population would be considered an aquatic invasive. However, we challenge that assertion given the current state of much of the estuary. If we focus on the historic range of an organism strictly as a function of geography, then the organisms introduced by people to the estuary are invasive aquatic organisms under the USFWS definition. If, however, if we interpret the natural range to encompass the habitats to which species are native, then many non-native species would be considered right at home in the estuary.

            For example, much of the Delta is made up of waterways that resemble southeastern lakes (such as Lake Okeechobee) much more so than they resemble the historic Delta habitat of sloughs and marshes. These new habitats are largely constrained by levees, eliminating the vast marshes and floodplains that once existed. Compounding these landscape changes are highly altered flow regimes. Upstream reservoirs capture water and control its release, dampening winter floods and increasing summer flows. The flatter hydrograph and modified landscape have made the Delta much more suitable habitat for many introduced species. These species are well-adapted to the low flows, increased water clarity, higher temperatures, large beds of aquatic weeds, and other features of the modern Delta. 

             Sadly, some native species, such as Delta smelt, are actually strangers to these altered habitats. While they are geographically native, the traits that once made them so abundant in the Delta are maladaptive in these new habitat conditions. Much like we would not expect Delta smelt to succeed if introduced to Lake Okeechobee, it should not be surprising that they are no longer successful in the warm, clear, and highly vegetated waters of the Delta.

            Take a breath! Contrary to what you may be thinking right now, we are not proposing to give up on species such as the Delta smelt. We are suggesting that trying to reestablish this species in poorly suitable habitats is extremely difficult. While there is a legal and moral obligation to help sustain Delta smelt and other critically endangered species, the Delta will require radical restoration for Delta smelt to persist in their native geographic range. State and federal agencies, as well as some private groups, have begun implementing a number of measures, including habitat restoration and hatchery supplementation, but these are slow to implement and unlikely to reverse the immediate extinction spiral (Börk et al. 2020). Rather than wait for better Delta solutions to emerge, creative solutions to sustain wild Delta smelt outside of the hatchery setting should be explored.

            One potential solution is to establish self-sustaining, non-hatchery-supplemented Delta smelt refuge populations in reservoirs with suitable conditions. A proof-of-concept has already been established by the Delta smelt’s cousin wakasagi (Hypomesus nipponensis), which was planted and is currently thriving in several Sierra reservoirs, such as Oroville, Rollins, and Almanor. Habitat requirements for these species are similar, and many reservoirs are cool, dark at depth, and have abundant zooplankton – conditions Delta smelt need. Wakasagi and Delta smelt will hybridize, so Delta smelt cannot be stocked into reservoirs that already contain wakasagi. But many reservoirs similar to Oroville and Almanor exist where wakasagi are absent: Mountain Meadows, Union Valley, Davis, Frenchman, and Britton, to name a few.

This form of “assisted migration” has worked for another Delta fish that recently became extinct in its “natural, historic range”: Sacramento perch. Sacramento perch was once so abundant in the Delta that it supported a commercial fishery, but it is no longer found in its historic waters. It has avoided extinction, however, by its successful introduction to farm ponds and reservoirs across the American West. While these populations of perch are not strictly “native,” they have saved the species from extinction and preserve some of their cultural, ecological, and recreational value.

            Sometimes we daydream about what our estuary once looked like and the epic amounts of salmon, smelt, and other native species that once swam in its waters. Unfortunately, it’s wishful thinking to believe that the daydream will be anything more than that given the magnitude of change in the estuary. The Delta smelt inhabits a novel ecosystem that contains very little habitat that could be considered natural for it. Additionally, some invasive species, such as the overbite clam (Potamocorbula amurensis) and Brazilian waterweed (Egeria densa), have caused habitat shifts that are nearly irreversible and these species are going nowhere.  Further, they’re no doubt going to be joined by new species that will further alter the Delta from its historic state, such as the recently introduced alligatorweed (Alternanthera philoxeroides; Walden et al. 2019). Therefore, novel management strategies must be used to keep species such as Delta smelt from going extinct. So, this begs the question – if we have thousands of Delta smelt in a hatchery, why not take a page out of the Sacramento perch playbook and plant them in some reservoirs while we still have smelt to plant?

            Do the best you can with what you’ve got – the Delta is currently not the best we’ve got for Delta smelt. The best we got – it’s the reservoirs.

Dylan Stompe is a fisheries researcher and graduate student at the Center for Watershed Sciences. Teejay O’Rear is a fish ecologist at the Center for Watershed Sciences and lab supervisor for Dr. John Durand. Peter Moyle is Distinguished Professor Emeritus at the University of California, Davis and an Associate Director of the Center for Watershed Sciences. John Durand is a researcher specializing in estuarine ecology and restoration at the Center for Watershed Sciences.

Further Reading

Börk, K., Moyle, P., Durand, J., Hung, T., Rypel, A. L. 2020. Small populations in jeopardy: delta smelt case study. Environmental Law Reporter, 50(9), 10714-10722

Cohen, A.N., Carlton, J.T. 1998. Accelerating invasion rate in a highly invaded estuary. Science, 279(5350), 555-558.

Crain, P.K., Moyle, P.B. 2011. Biology, history, status and conservation of Sacramento perch, Archoplites interruptus. San Francisco Estuary and Watershed Science, 9(1).

Moyle, P.B. 2021. Can Japanese Smelt Replace Delta Smelt? California Water Blog. https://californiawaterblog.com/2021/02/07/can-japanese-smelt-replace-delta-smelt/

O’Rear, T., Moyle, P.B., Durand, J.R. 2018. Delta Smelt and Salmon Habitats Beyond the Estuary. Presentation. https://watershed.ucdavis.edu/library/delta-smeltand-salmon-habitats-beyond-estuary.

Walden, G. K., G. M. S. Darin, B. Grewell, D. Kratville, J. Mauldin, J. O’Brien, T. O’Rear, A. Ougzin, J. V. Susteren, P. W. Woods.  2019.  Noteworthy collections, California (Alternanthera philoxeroides). Madrono 66(1): 4-7.