by Peter B. Moyle
The Mississippi silverside (Menidia audens) is one of the most abundant fishes in the San Francisco Estuary and in the fresh waters of California in general. As the name indicates, it is not native to the state but was introduced into Clear Lake, Lake County, in 1967, from which it quickly spread widely, via the California aqueduct system and through angler introductions as a bait and forage fish (Moyle 2002). It is a small fish, 7-12 cm (3-4 inches) adult length but typically occurs in large schools. Its impact on native fishes is poorly understood but is most likely negative. This blog tells the story of how it came to be introduced, as a classic example of the Frankenstein Effect, where a well-intentioned, science-based introduction created an out-of-control monster. I then discuss the extent of its spread, including into marine environments, and attempt to explain why it has been so successful in California and why it is probably having negative impacts on native fishes.
The silverside was brought to California in 1967 from Lake Texoma, Oklahoma, as a joint project of the Lake County Mosquito Control Agency and the California Department of Fish and Game (as it was known then) (Cook and Moore 1970). The goal was permanent control of the Clear Lake gnat (Chaoborus astictopus). This non-biting gnat, once an important component of the food web of Clear Lake, was regarded as a nuisance species because in summer, adults emerged from the lake in enormous numbers at night and were attracted to electric lights. Gnat hatches restricted or interfered with barbeques and other important human outdoor activities.
As a result, starting in 1949, TDE (a relative of DDT) was applied to the entire 180 km2 (68 mi 2) lake. The application was deemed a success until 1957, when concern over the detrimental, cumulative effects of TDE on birds and people shut the program down. This problem was made famous by Rachel Carson (1957) in Silent Spring, who used Clear Lake as an example; she said: “Here the problem was resolved in favor of those annoyed by gnats, and at the expense of an unstated…risk to all who took food and water from the lake (p 59).” The response was, in 1962, to switch to lake-wide application of methyl-parathion (Prine et al. 1975). I watched the last application of this pesticide in 1973 and was awed by seeing large barges loaded with barrels of pesticide being towed back and forth across the lake, leaving a toxic wake. This event was the last application because parathion was not working well, apparently due to increased pesticide resistance on the part of the gnat. So, the gnat problem returned and the search was on for a ‘biological control’ which presumably would be less harmful than chemical control. The choice of an agent settled on the Mississippi silverside.
The silverside seems to have been chosen because it was a known planktivore/insectivore, had a one-year life cycle so could build up populations quickly, and could tolerate a wide range of temperatures and salinities. It was also readily available. It quickly became one of the most abundant fish species in the lake. This was demonstrated by our catches in the 30 foot long, quarter inch mesh, minnow seines that my colleague Hiram Li and I used, with students, to sample shallow water during a summer field course. In order to return captured fish to the water as quickly as possible, each student determined how many fish were in a handful and then reported counts in “standard handfuls,” which were converted to rough numbers. Hundreds of fish were captured in many of the hauls.
Unknown at the time of introduction was a feature that made silversides even more likely to be able to control the gnat population. While silversides were extremely abundant in shallow water during the day (littoral zone), at night they were scarce in our seine hauls. We found that silversides were moving off-shore at night, into the pelagic zone. This movement occurred at the same time as the nocturnal gnat larvae were emerging from the bottom of the lake to feed on zooplankton (Wurtsbaugh and Li 1985). As a result, gnats became scarce because of silverside predation. From a biological control perspective, the introduction thus was successful; the gnats at least have not been a problem since then. However, the effect that silversides have had on the Clear Lake ecosystem overall (Eagles-Smith et al. 2008) and to other systems they have subsequently invaded is not well understood but appears to be negative.
The rapidity with which silversides moved from Clear Lake and spread is impressive. I first collected them in lower Cache Creek, the outlet of Clear Lake, and Putah Creek in 1972. By 1975 they were well established in the SF Delta and by 1981 they were abundant in a number of coastal and southern California reservoirs (Moyle 2002). Subsequently, they have spread to most coastal streams in southern California, from the San Gabriel River on north. Swift et al. (2014) indicate that much of this recent dispersal has been through marine waters along the California coast, from the large populations now present in some estuaries. Their ability to survive in marine environments indicates that they should be expected in other estuaries and coastal environments, as well as in freshwater habitats. For example, UC Davis scientist Levi Lewis and his crew have found silversides to be one of the most abundant fishes in south San Francisco Bay, with occasional population increases (e.g., 2020) that have been characterized as fish “tsunamis” (L. Lewis, pers. comm. and https://www.ogfishlab.com/).
In the San Francisco Estuary, they occur in water ranging from fresh to near-marine. In the Delta they are most abundant in the warm (25-30 degrees C), turbid water that occurs in summer when inflows are low (Mahardja et al. 2016). In South Bay, they live under warm, near-marine conditions and coexist with brackish and marine species. If their behavior in the estuary is the same as that in Clear Lake, then they are interacting with other fishes in both the littoral (inshore) and pelagic (offshore, open water) zones. It may be significant that, in the Delta, silversides have increased, while abundance of other plankton feeding fishes have decreased (Mahardja et al. 2016). In particular, the decline of Delta smelt and longfin smelt may be related to silversides foraging in shallow water habitats where the smelt spawn. Their eggs and larvae would be easy prey. Moyle and Bennett (1996) note that under experimental conditions silversides can be effective predators on fish larvae of other species. Likewise, even when smelt are scarce, delta smelt DNA can be detected in guts of some silversides (Baerwald et al. 2012). Silversides themselves are frequent prey for non-native predators, such as largemouth bass, white and black crappie and other inshore fishes. It remains to be seen whether or not they have reciprocal interactions with these species, such as embryo and larval predation.
In short, Mississippi silversides are a hyper-invasive fish species. They can colonize new, diverse habitats rapidly and quickly become abundant. They have demonstrated extreme adaptability to highly altered habitats in California, including polluted or otherwise altered lakes, streams, and estuaries, under salinities ranging from fresh to salt. They co-occur with a diverse array of fishes including both potential predators and prey. It therefore is important to discover what regulates silverside populations and how to reduce negative effects where possible. Understanding their biology and interactions with other species (more than fishes!) is especially important for the San Francisco Estuary where millions of dollars are being spent for native fish conservation. Their story is also a good lesson in why bringing new species of any sort into California should be avoided if at all possible. It is predictable that new introductions will have unexpected consequences.
 The Mississippi silverside has been considered by many as part of the widespread Inland silverside (Menidia beryllina) complex (e.g., Moyle 2002), but recent studies indicate it is a distinct freshwater species. See discussion in Fluker et al. (2016).
Peter Moyle is an emeritus professor in the Center for Watershed Sciences and Department of Wildlife, Fish and Conservation Biology, University of California, Davis.
Baerwald, M.R., Schreier, B.M., Schumer, G. and May, B., 2012. Detection of threatened Delta Smelt in the gut contents of the invasive Mississippi Silverside in the San Francisco Estuary using TaqMan assays. Transactions of the American Fisheries Society, 141(6): 1600-1607.
Cook Jr, S.F. and Moore, R.L., 1970. Mississippi silversides, Menidia audens (Atherinidae), established in California. Transactions of the American Fisheries Society 99(1):70-73.
Eagles-Smith, C.A., Suchanek, T.H., Colwell, A.E., Anderson, N.L. and Moyle, P.B., 2008. Changes in fish diets and food web mercury bioaccumulation induced by an invasive planktivorous fish. Ecological Applications 18(sp8): A213-A226.
Fluker, B.L., Pezold, F. and Minton, R.L., 2011. Molecular and morphological divergence in the inland silverside (Menidia beryllina) along a freshwater-estuarine interface. Environmental Biology of Fishes, 91(3): 311-325.
Mahardja, B., Conrad, J.L., Lusher, L. and Schreier, B., 2016. Abundance trends, distribution, and habitat associations of the invasive Mississippi Silverside (Menidia audens) in the Sacramento–San Joaquin Delta, California, USA. San Francisco Estuary and Watershed Science, 14(1).
Moyle, P. B. 2002. Inland Fishes of California: Revised and Expanded. Berkeley, University of California Press.
Moyle, P.B., and Bennett W.A. 2008. The future of the Delta ecosystem and its fish. In: Lund J, Hanak E, Fleenor W, Bennett W, Howitt R, Mount J, Moyle P. 2008. Comparing futures for the Sacramento–San Joaquin Delta. San Francisco (CA): Public Policy Institute of California. Technical Appendix D. Available from: http:// http://www.ppic.org/content/pubs/other/708EHR_appendixD. pdf
Prine, J.E., G. G. Lawley, and P.B. Moyle (1975). A multidisciplinary approach to vector ecology at Clear Lake, California. Bulletin of the Society of Vector Ecologists 2: 21-31.
Swift, C.C. Howard, S., Mulder, J., Pondella, D.J., and T.P. Keegan. 2014. Expansion of the non-native Mississippi Silverside, Menidia audens (Pisces, Atherinopsidae), into fresh and marine waters of coastal Southern California. Bulletin of the Southern California Academy of Sciences 113(3):153-164,
Wurtsbaugh, W. and Li, H., 1985. Diel migrations of a zooplanktivorous fish (Menidia beryllina) in relation to the distribution of its prey in a large eutrophic lake 1. Limnology and Oceanography 30(3): 565-576.