By Peter B. Moyle & Robert A. Leidy

See Moyle and Leidy (2023) for much more detailed version of this essay. https://doi.org/10.1017/9781108758826

Few things give the authors of this essay more pleasure than swimming in a California stream on a hot summer day, wearing a mask and snorkel, and observing diverse native fishes behaving naturally. But being able to watch such fishes may be a passing phenomenon, not only here, but globally. Freshwater habitats are disappearing or being rapidly modified, a reflection of the ever-expanding demands of people, accelerated by global climate change. Not surprisingly, there is a global freshwater biodiversity crisis, of which fish are one of the most conspicuous indicators. There is widespread agreement that the crisis is real (e.g., Dudgeon et al. 2006; Strayer and Dudgeon 2010; Darwell et al. 2018, Tickner et al. 2020).

We started to document this crisis over 30 years ago (Moyle and Leidy 1992) when we concluded that, conservatively, 20% of the world’s freshwater fishes were already extinct or in severe decline towards extinction. Six years later, Leidy and Moyle (1997) confirmed this estimate using better information. In our most recent iteration, Moyle and Leidy (2023) concluded that “…without extraordinary measures, at least 40-50% of all freshwater fish species will be extinct in the wild or close to it by the end of the century, if not sooner (p. 201).” In the analysis, we relied on readily available data from the International Union for the Conservation of Nature (IUCN), which attempts to keep track of the status of all species of plants and animals. IUCN (2012) has nine categories for species status: extinct, extinct in the wild, critically endangered, endangered, vulnerable, near-threatened, least concern, data deficient, and not evaluated. Critically endangered, endangered, and vulnerable species are those ‘threatened with extinction’. Definitions of status categories are found in IUCN (2012) and Moyle and Leidy (2023).

The number of unique freshwater fish species on Earth is currently estimated to be over 18,000 (51% of all known fishes) and climbing (IUCN Red List 2022). According to IUCN, almost 3000 species (23% of the 13,276 species assessed) are threatened with extinction. These fishes are distributed among all major taxonomic orders, showing that they occur in freshwater habitats worldwide and are taxonomically diverse. Half of all assessed fishes are widespread or abundant, so they are not threatened with extinction and are classified by the IUCN as of “least concern”. Unfortunately, another 2556 fish taxa (19%) are ’data deficient’, lacking sufficient information for IUCN to assess extinction risk. The comparative lack of information on threatened fishes is reflected in the scientific literature on freshwater fishes, which is primarily about game fishes (Guy et al. 2021).

So how does California compare with the rest of the world in the proportion of its fishes on a pathway to extinction? The comparison of IUCN evaluations with California’s provides a test of both systems because the California freshwater fish fauna is comparatively well-documented, and the state is geographically well-defined. Of the 131 native taxa recognized, 81% are endemic to the state or share watersheds between just two adjacent states. We would not consider any of the native fishes to be ‘data deficient’, and the complete known fauna has been evaluated independently from the IUCN evaluations, using what we call the California Method for Status Evaluation of Fishes (Leidy and Moyle 2021). The method relies on combined scores of 1-5 for each of seven metrics, based on existing data and expert opinion (e.g., area occupied, estimated adult abundance). The method allows short and long-term trends to be evaluated rapidly (Moyle et al. 2011, 2015, Leidy and Moyle 2021, Moyle and Leidy 2023) and retrospective analyses are possible (Figure 1). To make the comparison, we first examined the status of California fishes as evaluated by IUCN, which mainly considers full species. Our list of California fishes includes 82 full species, 42 (51%) of which have been evaluated by IUCN. The IUCN found that 15 (36% of full species evaluated) are threatened with extinction (Vulnerable+Endangered+Critically Endangered); 3 of the 42 species were extinct in California, but still extant outside the state, and two endemic species were globally extinct (Clear Lake Splittail, Thicktail Chub).

For comparison, we examined California’s list of recognized freshwater fish taxa as of 2020 (131 taxa, 81% endemic, Leidy and Moyle 2021). This list includes 100 taxa with formal species or subspecies designations, seven undescribed subspecies, and 24 Distinct Population Segments (as defined by the federal Endangered Species Act of 1973). When we rated the status of all extant species in California using the California Method, 63 (49%) were scored in categories reasonably equivalent to the three IUCN categories lumped together as threatened. In short, our evaluation of the status of all native fish taxa resulted in about twice as many freshwater fishes regarded as threatened with extinction than the IUCN evaluations would indicate. However, when threatened percentages of fishes evaluated are compared (36% for IUCN vs. 49%), the numbers are more comparable. The IUCN percentage is close to the 32% of California fishes formally listed under state and federal Endangered Species Acts and the 30% of all freshwater fishes assessed by the IUCN.

Given that only about half of the 9800+ freshwater fishes have been assessed, 30% probably represents a conservative number of globally threatened fishes (ca. 3000 species). California, therefore, is a world leader in having its endemic freshwater fishes likely to be driven to extinction by the end of the century. This prediction becomes even more likely when the effects of global warming are considered, along with the increased demand for water and other resources by the planet’s expanding human population. Right now, we are facing the extinction of Delta Smelt, which exists today mainly as the result of a captive breeding program. The Winter Run Chinook Salmon also relies on captive breeding and is somewhat better off because a few fish still spawn naturally, albeit in the highly modified outflows of Shasta Dam.

Our conclusion: The best available evidence indicates that, if present trends continue, at least 40-50% of all freshwater fish species will be extinct in the wild or close to it by the end of the century. Aquatic habitats worldwide instead will support highly homogenized, if depauperate, fish faunas, part of novel ecosystems dominated by people and by non-native species such as common carp, tilapia, largemouth bass, a catfish or two, and mosquitofish. A few hardy native fishes might serve as a distinctive part of each local assemblage, reminders of past species richness (Moyle and Leidy 2023).

The question then becomes, Can California be the leader in developing conservation strategies that reverse the decline of freshwater ecosystems and their diversity, including fishes? We think the answer to the question is ‘yes,’ but whether California citizens and their leaders have the will to do so is questionable, as our inadequate response to climate change shows (even though it is better than most of the rest of the USA). Widely recognized among aquatic ecologists and biologists (and other professionals) is that climate change is accelerating the need for large-scale actions to slow or even reverse the rapid decline of fish abundance and diversity. However, the severity of the global situation is underestimated by most people. The ever-increasing trend in use and abuse of our fresh waters means that extinction of fish species on a large scale is a near-certainty in the foreseeable future. Given the scope of the problems and conflicts that fish conservation engenders, large-scale solutions are unlikely to arise, such as those discussed by Moyle and Leidy (2023) and references therein. Meanwhile, more local actions might save a few species from extinction.

We prefer optimism to the gloomy vision projected above because healthy waterways for fish are also good for people, as the Clean Water Act has long been recognized. An optimistic vision includes systematically protecting and managing diverse rivers, streams, lakes, and wetlands in each global region to ensure that some of our aquatic diversity survives into the distant future. This would include managing whole watersheds for their biota, with native fishes as indicators of success. Such Freshwater Protected Areas cannot just exist as incidental parts of terrestrial protected areas but must be a central focus of a system of aquatic reserves. Successful Freshwater Protected Areas will need to be linked in a broad scheme of stream and lake management that integrates the needs of humanity with those of fishes and their ecosystems.

We need the fishes and the fishes need us.

Peter Moyle is an emeritus professor in the Center for Watershed Sciences, University of California, Davis. Robert Leidy is a research scientist affiliated with the Department of Environmental Science, Policy and Management at the University of California, Berkeley.

Further reading

Closs, G.P., P. L. Angermeier, W. R. T. Darwell, and S. T. Balcombe. 2015. Why are freshwater fishes so threatened? Pp. 37-75 In G. P. Closs, M. Krkosek, and J.D. Olden eds. Conservation of Freshwater Fishes. Cambridge University Press

Darwell, W. R.T. and J. Freyhof. 2015. Lost fishes: who is counting? The extent of the threat to freshwater fish biodiversity. Pp.1-36 In G. P. Closs, M. Krkosek, M. and J. D. Olden. eds., Conservation of Freshwater Fishes. Cambridge University Press

Darwell, W. R. T., and 22 co-authors. 2018. The Alliance for Freshwater Life: A global call to unite efforts for freshwater biodiversity science and conservation. Aquatic Conservation: Marine and Freshwater Ecosystems: 2018: 1-8. https://doi.org/10.1002/aqc.2958

Dudgeon, D. et al. 2006. Freshwater biodiversity: importance, threats, status and conservation challenges. Biological Reviews 81:163-182. DOI: https://doi.org/10.1017/S1464793105006950

Guy, C.S., Cox, T.L., Williams, J.R. et al. 2021. A paradoxical knowledge gap in science for critically endangered fishes and game fishes during the sixth mass extinction. Scientific Reports 11, 844. https://doi.org/10.1038/s41598-021-87871-y

Harrison, I. et al. 2018 The freshwater biodiversity crisis. Science 362: 1369. https://doi.org/10.1126/science.aav9242

International Union for the Conservation of Nature (IUCN). 2012. Red List Categories and Criteria: Version 3.1. Second Edition. Gland, Switzerland and Cambridge, UK: IUCN. iv+32pp.

Leidy, R. A., and P. B. Moyle. 1998. Conservation status of the world’s fish fauna: an overview. Pp.187-227 In P. L. Fiedler and P. M. Karieva, eds. Conservation Biology for the Coming Decade. N. Y.: Chapman and Hall

Leidy, R. A. and P. B. Moyle. 2021. Keeping up with the status of freshwater fishes: a California (USA) perspective. Conservation Science and Practice 3(8), e474. https://doi.org/10.1111/csp2.474. 10 pages.

Moyle, P.B., J. V. E. Katz and R. M. Quiñones. 2011. Rapid decline of California’s native inland fishes: a status assessment. Biological Conservation 144: 2414-2423. https://doi.org/10.1016/j.biocon.2011.06.002.iardi

Moyle, P. B., and R. A. Leidy. 1992. Loss of biodiversity in aquatic ecosystems: evidence from fish faunas. Pp. 128-169. In P. L. Fiedler and S. A. Jain (Editors), Conservation Biology: The Theory and Practice of Nature Conservation, Preservation, and Management. New York: Chapman and Hall.

Moyle, P.B., J. D. Kiernan, P. K. Crain, and R. M. Quiñones. 2013.Climate change vulnerability of native and alien freshwater fishes of California: a systematic assessment approach. PLoS One. http://dx.plos.org/10.1371/journal.pone.0063883.

Moyle, P.B. and A.L. Rypel. 2023. Monster fish: lessons for sturgeon management in California. https://californiawaterblog.com/2023/05/14/monster-fish-lessons-for-sturgeon-management-in-california/

Rypel, A.L., 2023. Facing the dragon: California’s nasty ecological debts. California WaterBlog, June 11, 2023. https://californiawaterblog.com/2023/06/11/facing-the-dragon-californias-nasty-ecological-debts/

Strayer, D.L. and D. Dudgeon. 2010. Freshwater biodiversity conservation: recent progress and future challenges. Journal of the North American Benthological Society 29: 344-358. DOI: 10.1899/08-171.

Tickner, D.,et al. 2020. Bending the curve of global freshwater biodiversity loss: an emergency recovery plan. BioScience 70: 330–342. https://doi.org/10.1126/science.aav9242