The ideal invasive fish


There are hundreds of non-native fishes in the US. Some become harmful invaders, while others have established, but aren’t causing appreciable negative effects.

As human transport networks continue to facilitate new introductions, it makes sense to ask, ‘is there something special about certain fishes that make them invasive?’. This isn’t a new idea—plant ecologists have discussed ‘the ideal weed’ since the mid 1960s, and freshwater fish ecologists began trait-based risk assessment in the early 2000s.

The principle goes like this: think of a species as just a chunk of flesh defined by a unique combination of traits—things like body shape, mouth position, scale types, etc. Because of their unique evolutionary history, some fishes have evolved traits that make them better suited for establishing and spreading in new systems. Others, not so much.

So, what have we learned in the last twenty-odd years of researching traits of invaders? Below are a few (and by not even approaching exhaustive) traits/attributes that seem to be consistent among invasive freshwater fishes.

Diet. When introduced into a new habitat, a fish’s first order of business will be to find food. If you’re a highly specialized feeder that can’t easily adapt to a broad food source, you’re going to struggle to find your next meal. Spreading prolifically will be almost out of the question. Accordingly, several studies have found that invasive fishes have broader or more predatory diets than natives1,2,4, which can contribute to establishment success and/or spread3.

Effect sizes of key attributes of nonnative aquatic organisms, relative to natives.  From McKnight et al. 2016

Life history strategy. Here, we’re talking about a few key traits that determine pretty much everything about a fish’s biology—namely fecundity, juvenile survivorship, and generation time (or age at maturation). Compared to native species, successful invasive fishes generally grow larger, produce more eggs, mature later, and live longer3,6,7.

Native range size. Large native range size is a common feature among invasive fishes4,8,9. This attribute may cause an increased likelihood of environmental match between native and non-native habitats, or simply indicate success in the native range and thus potential for success in the nonnative range

Growth rate. A recent meta-analysis identified growth rate as the trait with the greatest difference between native and non-native aquatic organisms. Invasive fishes may grow faster than natives at early life stages1, which may allow them to compete with or escape predation from native species. However, there’s still more work to be done on understanding how growth rate determines invasion success in fishes, as the relationship is inconsistent among species5.

Human interest. We move around species we like to fish for and/or eat. Accordingly, some of the most prolific invaders are ones associated with human uses such as fishing, aquaculture, or the aquarium trade6,10,11.

Some of the most prolific nonnative fishes in the eastern US. All photos from Wikimedia Commons: Brown Trout, Bluegill, Largemouth Bass, and Rainbow Trout

Bringing it all together. Compared to natives, invasive fishes generally have broader diets, are more reproductively prolific, grow faster, have large native ranges, and have larger native ranges.

Does this sound like any fish you may know? Perhaps one you’ve pursued for fishing? Those traits make many fish ideal for fishing and aquaculture. Accordingly, some of the most prolific non-natives, at least in the US, are the ones that we have stocked heavily because of their traits. In the US, some of these include Brown Trout, Largemouth Bass, and Channel Catfish. So, while some traits do make fishes more adept at becoming established and/or invasive outside their native ranges, human introduction effort is an important contributor to that process.

Relationships between traits, fishing, and spread of nonnative freshwater fishes. From Peoples & Midway 2018

Is there an ‘ideal’ invasive fish, or can any fish become invasive if we simply introduce enough? At this point, it seems like the answer is, ‘yes…and yes’. Traits aren’t the only determinant of invasive fishes, and other factors like climate matching, introduction effort, and biotic interactions also play a role. We’ve learned a lot about nonnative and invasive fishes over the past two decades, but we’ve still got a long way to go. Understanding how traits interact with species interactions and environmental factors in the context of variable introduction effort is an important research goal for identifying and preventing new invasions.


  1. Kolar, C. S., & Lodge, D. M. (2002). Ecological predictions and risk assessment for alien fishes in North America. Science, 298(5596), 1233-1236.


  1. Mcknight, E., García‐Berthou, E., Srean, P., & Rius, M. (2017). Global meta‐analysis of native and nonindigenous trophic traits in aquatic ecosystems. Global change biology, 23(5), 1861-1870.


  1. Howeth, J. G., Gantz, C. A., Angermeier, P. L., Frimpong, E. A., Hoff, M. H., Keller, R. P., … & Lodge, D. M. (2016). Predicting invasiveness of species in trade: climate match, trophic guild and fecundity influence establishment and impact of non‐native freshwater fishes. Diversity and Distributions, 22(2), 148-160.


  1. Marchetti, M. P., Moyle, P. B., & Levine, R. (2004). Alien fishes in California watersheds: characteristics of successful and failed invaders. Ecological Applications, 14(2), 587-596.


  1. Rypel, A. L. (2014). Do invasive freshwater fish species grow better when they are invasive?. Oikos, 123(3), 279-289.


  1. Jeschke, J. M., & Strayer, D. L. (2006). Determinants of vertebrate invasion success in Europe and North America. Global Change Biology, 12(9), 1608-1619.


  1. Olden, J. D., Poff, N. L., & Bestgen, K. R. (2006). Life‐history strategies predict fish invasions and extirpations in the Colorado River Basin. Ecological Monographs, 76(1), 25-40.


  1. Ribeiro, F., Elvira, B., Collares-Pereira, M. J., & Moyle, P. B. (2008). Life-history traits of non-native fishes in Iberian watersheds across several invasion stages: a first approach. Biological Invasions, 10(1), 89-102.


  1. Sommerwerk, N., Wolter, C., Freyhof, J., & Tockner, K. (2017). Components and drivers of change in European freshwater fish faunas. Journal of Biogeography, 44(8), 1781-1790.


  1. García-Berthou, E., Alcaraz, C., Pou-Rovira, Q., Zamora, L., Coenders, G., & Feo, C. (2005). Introduction pathways and establishment rates of invasive aquatic species in Europe. Canadian Journal of Fisheries and Aquatic Sciences, 62(2), 453-463.


  1. Davis, A. J. S., & Darling, J. A. (2017). Recreational freshwater fishing drives non‐native aquatic species richness patterns at a continental scale. Diversity and Distributions, 23(6), 692-702.

One Comment Add yours

  1. Sol says:

    This was an interesting read! I’ve always wondered about certain fish that ‘rise to the top’ so to speak. There seems to be an element of Darwinism here – a fish that not only survives but thrives sounds good in theory, but I can imagine that some of these fish may be harmful as well! Would love to see a follow-up article on specific fish in this arena.



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