Living with non-native fishes in California requires using the right words

Redeye bass in the Cosumnes River: a non-native species that deserves the title of Alien Invader because it has eliminated native fishes from most of the river. It was introduced assuming it would have a mutualistic relationship with people (Moyle et al. 2003).

by Peter Moyle

Everywhere you go in California, people live in landscapes where non-native species are conspicuous:  European grasses turning the hills golden, earthworms tilling our garden soil, exotic trees providing shade, bullfrogs jumping into backyard ponds, starlings making tight maneuvers overhead. In this blog, I want to describe the language of our relationships with non-natives and the nature of those relationships as biological phenomena, using fishes and other aquatic organisms as examples.  Reconciling our relationships with non-native species requires a vocabulary that reflects our attitudes towards them and their management.


In a recent WaterBlog, Stompe et al. ( argued that non-native fishes thrive in California because we have created habitats much like the ones to which they were native. These habitats, alas, are mostly quite different from the ones to which our native fishes are adapted.  To some extent, native and non-native fishes can form coalitions (novel ecosystems) where the resources available are divided up among the species, much as it supposedly is in undisturbed habitats (e.g. Aguilar-Medrano et al. 2019).  But, in general, non-native fishes are replacing natives as habitats change, mostly as we have changed them. How do we live with this change but still save native fishes?  Let’s start with language.

  • I use non-native as the general, seemingly neutral, term for species from elsewhere that have extended their range into California, thanks to being brought here by people. Most are introduced species, another neutral term.
  • The word exotic is a somewhat positive term, bringing to mind exotic people and places, as well as nature specials on television.  Today, it is most often applied to fish and plants in the aquarium trade.
  • Non-indigenous is the term often used by government agencies because it is neutral, hard to pronounce, and can be bureaucratically abbreviated to NID species.
  • Invasive is the term for non-native species that are harmful and/or spreading.  Unfortunately, it is widely applied to all non-native species, even though many are not demonstrably harmful.
  • Naturalized can be used to describe non-native species that were introduced so long ago they have adapted to California’s distinctive environment and are integrated into the biotic communities. Fish examples include striped bass, American shad, and common carp.
  •  Alien is a negative term for non-native species, enhanced by science fiction movies.  The worst thing you can call a non-native species is alien invader.  This term fits realistically the northern pike, highly predatory species, on which California was willing to spend millions of dollars to eradicate before it spread widely, devouring native fishes.
Examples of non-native fish species in California. Upper left: Shimofuri goby, USBR (60 mm, commensal).   Upper right, American shad USFWS (adult, naturalized, mutualist); Lower left: Mississippi silverside NANFA (75 mm, mutualist turned alien invader); northern pike UC ANR (adult, alien invader, eradicated).


The diversity of words we use to describe non-native species reflects the ambiguity in our attitudes towards them.  This partly reflects the diverse symbiotic relationships that we humans have with other species, especially non-native species.  Symbiosis in its simplest sense means ‘living together’ and usually includes mutualism, commensalism, parasitism, competition, and predation.

The symbiotic relationship looked upon most favorably is mutualism, where both species benefit from a relationship.  Most sport fishes brought into California by agencies could be considered as mutualists, when people benefit from the fishery and the fish benefit from having their range expanded, important in the game of long-term evolution and survival. Of course, as human attitudes change and protection of native species from non-native species becomes a priority, the relationship between humans and non-native fish becomes more complicated.

For some species, the relationship with people is commensalism, where one species benefits but the other species neither benefits nor is harmed.  An example is the shimofuri goby, a small fish with a life cycle that permits it to be carried across oceans in the ballast water of ships. It became established in the Delta in the 1980s and is now a common species, enjoying life amongst the rip-rap and trash, feeding primarily on non-native invertebrates that are not eaten by other fishes.  This benign relationship with people may change if it manages to invade (which it probably will) estuaries in southern California where it can compete with/prey on the endangered tidewater goby.

A poorly understood (for California fish) symbiotic relationship is parasitism, including disease, where the parasite benefits from the relationship but the host is harmed.  For example, whirling disease, scourge of both hatchery and wild populations of rainbow trout, is native to Europe and was brought to North America in trout from Europe.  It has forced some California trout hatcheries to shut down with considerable impact on recreational fisheries.

Competition and predation are often not listed as forms of symbiosis but they are interspecies interactions of great consequence. Humans, for example, compete with freshwater fish for water (a resource in short supply), with the fish usually losing, regardless of native vs non-native status, unless people choose to back away from the relationship.  Likewise, people are the biggest predators on fishes, via fisheries, management of which can favor non-native species (e.g. largemouth bass) over native fishes (e.g. Sacramento pikeminnow).

Regardless of the initial reason people and non-native species wind up living together, the relationship often becomes complicated. The Mississippi silverside was introduced into Clear Lake for an assumed mutualistic relationship, to control pestiferous gnat populations, which it did. It also became abundant in the process, an expected result of the relationship.  But what was not expected was that silversides were carried from the lake, down Cache Creek and into the Delta, where they likely prey on the eggs and larvae of native fishes, such as delta smelt. They also serve as prey themselves for native birds like herons and egrets. They are now abundant in reservoirs in southern California, with unknown effects.

All these types of interactions between people (a non-native species) and fish show that understanding the biology of each species and our relationships with them is important. Like it or not, we humans need to have this understanding for living symbiotically with non-native species so we don’t have to consider ourselves at war with them.

Living with non-native species.

The widespread colonization of highly altered freshwater environments by non-native species is resulting in increased biotic homogenization. This is especially true in California and the American west because of intense development of water for people has resulted in massive changes to our waterways. A few natives can survive in new habitats such as reservoirs, but most cannot.  Our inability to stop the juggernaut of habitat change and non-native species demonstrates “…we are losing the battle against extinction…and against the seizure of aquatic ecosystems by alien invaders (Moyle 2021, p 70)”.  To take this militaristic analogy further, one of the best ways to save native fishes seems to be through creation of a system of fortresses, aquatic preserves that are bulwarks against species invasions.  For non-native fishes, a triage system for management can be recognized: eradication, control, acceptance. 

Eradication should be the main option for new arrivals that have a limited enough distribution that eradication is possible (e.g., Northern pike in Lake Davis). It is also a good option for populations of non-natives confined to treatable areas, such as brook trout in Sierra Nevada lakes where eradication via gill nets is possible, one lake at a time.  This strategy is important to restore native amphibians and invertebrates to the lakes.

Control can be an alternative where complete eradication is not possible. This approach has been used to create stream refuges for golden trout in California: a barrier is first built and then non-native trout eliminated above the barrier using a degradable fish poison. Golden trout are then re-introduced.  Such control of non-natives is rarely permanent, however.  In regulated streams, some control over non-native species can result from a tightly managed flow regime that favors native fishes.

Acceptance of non-native fishes is hard for those of us engaged in native fish conservation, but reality dictates that there is often little choice.  This does not mean they should not be managed at all, but instead managed in ways that minimize negative impacts on native species.  It is not worth spending time and energy on ‘saving’ native fishes where environments are so severely altered that native species will not persist whether or not non-native species are present (e.g.  most reservoirs).  On the other hand, where major positive environmental change is likely, as through dam removal, its effects on both native and non-native fishes will need to be considered.  Sometimes environmental restoration to favor native species will instead favor non-natives (e.g., Williamshen et al. 2021).

An approach that I think is helpful in resolving the native vs non-native fish dilemma is reconciliation ecology.  Reconciliation ecology is defined as “the science of inventing, establishing, and maintaining new habitats to conserve species diversity in places where people live work and play” (Rosenzweig 2003, p 7). This concept acknowledges that people dominate most ecosystems today, which means we determine what species they will contain as time goes on.  Many, if not most, ecosystems can support both native and non-native species. If we understand the role of non-native species, we are more likely to also keep native species as significant parts of California’s unique ecosystems.

This blog is based, in part, on Moyle (2020).

Further reading

Aguilar-Medrano, R., J. R. Durand, V.H. Cruz-Escalona and P.B. Moyle.  2019. Fish functional groups in the San Francisco Estuary: understanding new fish assemblages in a highly altered estuarine ecosystem. Estuarine, Coastal and Shelf Science 227106331

Moyle, P.B., 2020.  Living with aliens: nonnative fishes in the American Southwest. Pages 69-78 In D.L. Propst, J.E. Williams, K.R. Bestgen, and C.W. Hoagstrom, eds., Standing Between Life and Extinction: Ethics and Ecology of Conserving Aquatic Species in North American Deserts. Chicago: University of Chicago Press.

Moyle, P. B., P. K. Crain, K. Whitener, and J. F. Mount. 2003. Alien fishes in natural streams: fish distribution, assemblage structure, and conservation in the Cosumnes River, California, USA.  Environmental Biology of Fishes 68: 143-162.

Rosenzweig, M.L., 2003. Win-win Ecology: How the Earth’s Species Can Survive in the Midst of Human Enterprise. Oxford University Press on Demand.

Williamshen, B.O., T.A. O’Rear, M. K. Riley, P.B. Moyle, J. R. Durand. 2021. Tidal restoration of a managed wetland in California favors non-native fishes.  Restoration

Ecology. Society for Ecological Restoration. doi: 10.1111/rec.13392 12 pages

Peter Moyle is Distinguished Professor Emeritus at the Center for Watershed Sciences, UC Davis

About jaylund

Professor of Civil and Environmental Engineering Director, Center for Watershed Sciences University of California - Davis
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2 Responses to Living with non-native fishes in California requires using the right words

  1. bruceherbold says:

    I am puzzled by the statement: “… people are the biggest predators on fishes, via fisheries, management of which can favor non-native species (e.g. largemouth bass) over native fishes (e.g. Sacramento pikeminnow).”

    This strikes me as incorrect on two counts —
    1. most fish die as larvae and young often via predation, so the impact of fishes on adults has to be smaller, however great it might be.
    2. I am puzzled because there’s pretty much no fishery on pikeminnow, but a huge one on largemouth bass. And yet, LMB are thriving spectacularly, which seems to contradict the idea that fishery management is favoring LMB.

    Am I misreading?

  2. Brian Healy says:

    Control can work, including in Grand Canyon – I was just reading your Moyle 2020 book chapter and noted the reference to our Bright Angel Creek invasive trout control work (and yes, they are invasive in our system). Here’s a link to an update. Cheers!

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