By Liam Zarri, UCSC, Graduate Student, Ecology & Evolutionary Biology Department

How many invasive species do you think live in the San Lorenzo River? Many may be obvious, such as invasive plants on the shoreline. Others, however, lurk below the surface and disrupt local ecology in secrecy. In 2003, largemouth bass (Micropterus salmoides) were discovered in the San Lorenzo River. This species is originally from the Mississippi River watershed, but has been spread widely across North America, South America, Asia, Europe, and Africa. It is introduced as a game species, and is a popular sport fish in California.

Invasive species typically disrupt local ecology and displace native species. For example, the introduction of largemouth bass in Michigan decreased the numbers of plankton-eating fish who are prey for the bass (Carpenter et al 2001). This allowed zooplankton to increase in number, who then consumed a greater amount of photosynthesizing phytoplankton. The result? Water clarity increased greatly, fundamentally altering the ecosystem. This is called a trophic cascade, with largemouth bass being the keystone predator. High predation rates of this species can also negatively impact native species, such as tidewater goby (Swift et al 1997).

Long Lake

Figure 1 Water clarity change in Long Lake, Michigan, after a trophic cascade induced by largemouth bass (Estes et al 2011)

The largemouth bass has several characteristics which make it an excellent invasive species. They are resistant to poor water quality and are voracious predators. Larvae are guarded by the father and consume zooplankton, while the juveniles swim together and eat invertebrates or small fish. Largemouth bass are cannibalistic and a few fish in a juvenile school can be observed to grow quickly by eating naïve siblings. The adults are solitary hunters, using the cover of vegetation to ambush prey. In fact, the spread largemouth bass in the SF Bay-Delta is assisted by the spread of an invasive plant and common aquarium species, Brazilian waterweed (Egeria densa). Largemouth bass hunt by the light of day or moonlight, feeding most voraciously at dusk and dawn.

Although they have been sporadically detected, this species remains uncommon in the San Lorenzo River. How can that be, considering their environmental tolerance and parental care? They occur in Loch Lomond reservoir and ponds scattered throughout the watershed. Perhaps there is not enough food for one life stage of largemouth bass: not enough zooplankton for larvae, invertebrates for juveniles, or fish for adults. To understand a species, one must fully consider the requirements at each life stage. For example, threadfin shad have been introduced as prey for adult bass across the state, but they can outcompete larval bass for zooplankton and cause declines in bass populations (von Geldern & Mitchell 1975). Fisheries biologist Don Alley believes that the winter cold high flow events make this river inhospitable for largemouth bass. We can be grateful that the San Lorenzo River has not experienced an explosion of this invasive species like other areas in our state.

Questions? Email Liam at or visit Liam’s website to learn more about his work.



Nic Retford, City of Santa Cruz

Dr. Eric Palkovacs, University of California Santa Cruz

Don Alley, DW Alley & Associates



Carpenter, S.R., Cole, J.J., Hodgson, J.R., Kitchell, J.F., Pace, M.L., Bade, D., Cottingham, K.L., Essington, T.E., Houser, J.N. and Schindler, D.E., 2001. Trophic cascades, nutrients, and lake productivity: whole‐lake experiments. Ecological monographs, 71(2), pp.163-186.

Estes, J.A., Terborgh, J., Brashares, J.S., Power, M.E., Berger, J., Bond, W.J., Carpenter, S.R., Essington, T.E., Holt, R.D., Jackson, J.B. and Marquis, R.J., 2011. Trophic downgrading of planet Earth. science, 333(6040), pp.301-306.

Swift, C.C., Duangsitti, P., Clemente, C., Hasserd, K. and Valle, L., 1997. Biology and distribution of the tidewater goby (Eucyclogobius newberryi) on Vandenberg Air Force Base, Santa Barbara County, California. Final Report, USNBS Cooperative Agreement, pp.1445-007.

von Geldern Jr, C. and Mitchell, D.F., 1975. Largemouth bass and threadfin shad in California. Black bass biology and management. Sport Fishing Institute, Washington, District of Columbia, USA, pp.436-449.

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