by Peter Moyle
With a big collective sigh of relief, Californians rejoiced that we have largely recovered from 2012-2016 drought. Streams are flowing. Reservoirs are full. Crops are watered. Native fishes are reproducing But this not a time for complacency; if the 2012-2016 drought, the hottest and driest on record, had lasted another year or longer, California ‘s farms, cities, and ecosystems would have been in a dire state, with water in short supply everywhere (Mount et al., 2017, 2018). This should thus be a time to develop new and better strategies for reducing impacts of severe drought on both natural and developed systems, following the California Water Model (Pinter et al. 2019) of “resilience through failure.” California has a history of learning from past failures of water management, especially floods, and of avoiding repeating mistakes as a consequence.
Drought, of course, is not confined to California, but is a global problem, which is increasing as the planet warms. Fish and other aquatic organisms bear the brunt of drought because severe competition with people for water destroys their habitats. Worldwide, they are facing extinction at increasing rates. This is one lesson from a review of drought effects on freshwater fishes from Canada, California, and Australia (Lennox et al. 2019). This review shows how fish responded to drought in natural and highly altered waterways worldwide to gain insights into how to keep native fish from going extinct as droughts become more severe. California’s fishes provide especially good insights because most are highly adapted for persisting through long natural droughts; however, many are now threatened with extinction because of actions that exacerbate drought impacts.
To make our review more accessible, we condensed our findings into a list of maxims or aphorisms. These maxims contain general truths to help managers deal with the enormity of fish conservation problems in an increasingly drought-stricken world. Here are the maxims, somewhat modified from Lennox et al. (2019).
- Future droughts will be longer, more frequent, and more severe, creating more stressful conditions for freshwater fishes.
- Climate change and human alteration of aquatic ecosystems combine to increase the negative effects of drought.
- Fishes survive natural droughts in their native waters through physiological and behavioral adaptations, but all have limits.
- Different species respond to drought in different ways, so post-drought fish assemblages are hard to predict, especially in highly altered habitats.
- When natural environmental variability is suppressed by human activity, aquatic ecosystems become homogenized, dominated by a few drought-tolerant fish species, often non-native.
- Fish survive droughts by dispersal or migration to other habitats or by finding refuges in remnant habitats where conditions are physiologically suitable.
- The most abundant stream fishes in regions with frequent natural droughts are those with the ability to rapidly disperse and recolonize as both adults and juveniles.
- The best drought refuges are usually large rivers, lakes, spring-fed streams, and deep permanent stream pools with ground water inflow.
- The bigger and more diverse structurally the drought refuge, the more fishes it can shelter.
- Connectivity among habitats is essential for recovery of fish faunas in streams and lakes stricken by drought.
- Ground water is essential for maintaining stream refuges through drought.
- Human activity often produces perpetual drought conditions in streams through surface and groundwater removal.
- Poor water quality, especially low dissolved oxygen and high temperatures, followed by predation, are primary causes of fish mortality in drought refuges.
- Changes in severity or frequency of drought alter fish assemblages, with the effects of the changes depending on the history and physical characteristics of the system.
- Fisheries will decrease if human-mediated drought results in habitats unable to support large populations of harvestable fish.
- Translocation of species to new waters and artificial propagation are desperation measures unlikely to ameliorate the ecosystem effects of human-expanded drought.
These maxims point to the need to have some aquatic ecosystems in California that are managed largely for native fishes, in ways that make them drought-resistant. California’s native fishes are actually adapted for surviving severe droughts, but they evolved in large, inter-connected river systems where there were always refuges somewhere. We cannot bring back those large river systems as habitat but we can create refuges that are closely monitored and managed, with native fishes and the aquatic ecosystems they represent, getting a fair share of the water (Mount et al. 2017).
Lennox R.J., D.A. Crook, P. B. Moyle, D. P. Struthers, and S. J. Cooke 2019. Toward a better understanding of freshwater fish responses to an increasingly drought-stricken world. Reviews in Fish Biology and Fisheries 29:71-92 https://doi.org/10.1007/s11160-018-09545-9.\ Open Access.
Mount, J., B. Gray, C. Chappelle, G. Gartrell, T. Grantham, P. Moyle, N. Seavey, L. Szeptycki, and B.Thompson. 2017. Managing California’s Freshwater Ecosystems: Lessons from the 2012-16 Drought. Public Policy Institute of California 52 pp.
Mount, J. B. Gray, and 28 others. 2018. Managing Drought in a Changing Climate: Four Essential Reforms. San Francisco: Public Policy Institute of California. 30 pp.
Pinter, N., J. Lund, and P. Moyle. 2019. The California water model: resilience through failure. Hydrological Processes 2019: 1–5. https://doi.org/10.1002/hyp.13447
Peter Moyle is Distinguished Professor Emeritus at the University of California, Davis and is Associate Director of the Center for Watershed Sciences.