By Peter B. Moyle and William A. Bennett
Seven species of fish in the Delta are listed as threatened or endangered, including Delta smelt, salmon and steelhead. Although the ultimate cause of decline in these species is adverse water management throughout the Central Valley, there is a constant search for ways to increase their numbers without any water costs.
One such proposal is to kill as many striped bass as possible. Striped bass are an abundant non-native predator on fish and other aquatic organisms. So it is presumed that reducing striped bass numbers will increase populations of threatened fishes.
Over the past two years, this issue has been the focus of litigation, proposed legislation, and most recently a request by the National Marine Fisheries Service to the California Fish and Game Commission to remove all restrictions on striped bass fishing. Although on the surface this action seems logical (less predation = more fish) it remains an open question whether it would work. Here are some assumptions that would need to be true before native fishes might benefit from reductions in striped bass numbers:
1. Striped bass predation regulates populations of salmon, steelhead, and smelt, with other predators (other fish, birds, marine mammals, etc.) playing a minor role.
2. Other predators will not increase their predation on threatened fish if striped bass are removed.
3. Other species on which striped bass prey, such as Mississippi silverside, will not increase in abundance, causing harm by competing and preying on threatened species.
4. Reducing striped bass numbers would measurably compensate for the massive changes to the estuary and watershed caused by water diversions, loss of habitat, and other factors, which also reduce fish populations.
The above assumptions are unlikely to be true for a number of reasons:
1. Striped bass tend to forage on whatever prey are most abundant, from invertebrates to their own young to juvenile salmon and shad. Curiously, Delta smelt were a minor item in striped bass diets when smelt were highly abundant in the early 1960s, as well as in recent years at record low numbers.
2. Striped bass feed heavily on juvenile salmon and steelhead in the rivers. However, most salmon eaten are likely to be fish from hatcheries that are poorly adapted to the wild. High predation on them has little bearing on the degree of predation encountered by more wary juveniles from natural spawning. Predation on hatchery-reared juveniles may even buffer wild fish from such predation, given that wild fish are warier and less conspicuous than the more abundant hatchery fish.
3. All predation losses of salmon tend to be blamed on striped bass with the effects of other predators usually being regarded as insignificant. In fact, there are a multitude of other predators on juvenile salmon in the system, from birds (e.g., mergansers, cormorants, terns) to other fish, both native and non-native, including juvenile steelhead. The most abundant fish predator in the Delta today is probably largemouth bass—popular subjects of major catch-and-release fishing tournaments. If a control program for striped bass can be justified, then presumably one should be instituted for other predators as well.
4. Much of the predation on juvenile salmon (from multiple predator species) seems to take place in conjunction with artificial structures and poor release practices. These include hatchery releases and those trucked to the estuary from export facilities in the south Delta. Opportunistic predators such as striped bass are extremely quick to cue on these predictable events. Thus, apparent high predation rates may be the result of simple-minded management practices that create bass feeding stations.
5. If the striped bass is indeed the dominant predator on other fishes (the reason for a control program), then their decrease should have the most impact on species that are most frequently consumed. The ‘release’ from predation by striped bass is highly likely to benefit many other alien fish that are also known predators and competitors on endangered fishes. For example, Mississippi silversides are important in the diets of 1-3 year old striped bass, so bass predation may be regulating the silverside population. Fewer striped bass could result in greater silverside numbers, which may have negative effects on delta smelt through predation on eggs and larvae.
Reducing the striped bass population is quite likely to have a negative, rather than positive, effect on the species a control program is supposed to protect. By messing with a dominant predator (if indeed it is), the agencies are inadvertently playing roulette with basic ecosystem processes that can change in unexpected ways. Of course, if it is not a predator that is regulating native fish populations, this issue is moot.
The key to restoring populations of desirable species is to return the Delta to a more variable, estuarine environment. Reducing striped bass and other predator populations is unlikely to make a difference in saving endangered fishes, and will serve only to distract attention from the real problems. Any program to control striped bass should carefully consider the likely consequences. If initiated, it should involve an intensive study effort on the impacts of the program and an adaptive management plan (missing from all current proposals) to make sure the alleged cure is not worse than the supposed disease.
Peter B. Moyle is a fish biologist and William A. Bennett is a fish ecologist with the UC Davis Center for Watershed Sciences.
Bennett, W.A., and P. B. Moyle. 1996. Where have all the fishes gone: interactive factors producing fish declines in the Sacramento-San Joaquin estuary. Pages 519-542 in J. T. Hollibaugh, ed. San Francisco Bay: the Ecosystem. San Francisco: AAAS, Pacific Division.
Lindley, S.T. and M.S. Mohr. 2003. Modeling the effect of striped bass (Morone saxatilis) on the population viability of Sacramento River winter-run Chinook salmon (Oncorhynchus tshawytscha). Fishery Bulletin 101:321-331
Lund, J., E. Hanak., W. Fleenor, W., R. Howitt, J. Mount, and P. Moyle. 2007. Envisioning futures for the Sacramento-San Joaquin Delta. San Francisco: Public Policy Institute of California. 284 pp.
Lund, J., E. Hanak, W. Fleenor, W. Bennett, R. Howitt, J. Mount, and P. B. Moyle. 2010. Comparing futures for the Sacramento-San Joaquin Delta. Berkeley, University of California Press. 230 pp.
Moyle, P. B. 2002. Inland Fishes of California. Revised and expanded. Berkeley: University of California Press. 502 pp.
Moyle, P. B. and W. A. Bennett. 2008. The future of the Delta ecosystem and its fish. Technical Appendix D, Comparing Futures for the Sacramento-San Joaquin Delta. San Francisco: Public Policy Institute of California. 38 pp.
Nobriga, M.L., and F. Feyrer. 2008. Diet composition in San Francisco Estuary striped bass: Does trophic adaptability have its limits? Environmental Biology Fish 83: 495-503.
Stevens, D. E. 1966. Food habits of striped bass (Roccus saxatilis) in the Sacramento-San Joaquin Delta. Pages 68-96 in J.L. Turner and D.W. Kelley, eds. Ecological studies of the Sacramento-San Joaquin Estuary, part II: fishes of the Delta. CDFG Fish. Bull. 136.