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The Complex Task of Predicting how Temperature Changes will Affect Native and Non-native Fishes

Lindsey Bruckerhoff

10/05/2020

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Lindsey Bruckerhoff

John Wesley Powell (1987)1 described the Colorado River as running “from the land of snow to the land of sun,” and this description hints at the wide variety of habitats, flow conditions, and water temperatures once available to native Colorado River fishes. Of any large river basin in North America, the Colorado River has a high proportion of fish species that exist nowhere else on Earth. These “endemic” species evolved in the unique flow, sediment supply, and river temperature regimes of the Colorado River. But the river has been changed by development for water supply and hydroelectricity. The high dams, such as Hoover, Glen Canyon, and Flaming Gorge block those species that once migrated between Wyoming and Mexico and the remaining river segments between the dams have drastically altered flow and temperature regimes, and greatly reduced sediment loads that cause changes in habitat. During more than a century, many nonnative fish species have been intentionally and unintentionally introduced to the Colorado River system. These nonnatives sometimes prey on native species and compete with native species for habitat and food.

In the Colorado River Basin (CRB), dam operations (especially at hydroelectric dams) have a big impact on the hydrology of the river system, while reservoir storage decisions can have a disproportionate effect on the thermal regime of river segments downstream of dams. As Earth’s climate warms, runoff in CRB has declined. Population growth in cities and changing demands for water to support agriculture continue to stress the balance between the supply of water and consumptive use. Thus, decisions about where to store water, locations within the CRB where the most water is consumed, and daily and annual patterns of water release from reservoirs will have implications for the future of native fishes. Predicting the viability of native fish populations is dependent on our ability to predict how fishes will respond to future temperature regimes resulting from a warming climate combined with reservoir storage decisions.

The construction of high dams typically resulted in the release of cool water after the reservoirs fully filled. This cool water results from water being released from the cold, deep water of the reservoir (hypolimnetic releases). While this cold water is great for some introduced nonnative species, such as rainbow trout, cold water is not so great for native fishes. The shift from native species to more cold water-adapted species is evident in the tailwaters downstream from dams like Glen Canyon and Flaming Gorge. These tailwaters are now home to rainbow and brown trout, while native fish rarely occur in these cold river reaches.

Although one might assume that warming river temperatures downstream from dams might be good for native fishes, the reality is more complex. There are many nonnative species that do well in warm water (Figure 1) and their increases can have negative impacts on native fishes through increased competition and predation. Predation is particularly concerning, because nonnative warm water species often have larger mouths than native fishes, allowing them to prey on native species more effectively. To accurately predict how temperature will shape the future of Colorado River fishes, we must predict both how temperature influences individual species and the outcome of biological interactions between native and non-native fishes.

Temperature Effects on a Single Species

In general, species are best adapted to the temperature conditions in which those species evolved. Laboratory studies testing how different metrics such as swimming ability, hatch rates, or other physiological traits respond across a range of temperatures are the primary information used to predict how a species might respond to temperature changes in the river. While laboratory studies are a great way to determine the extreme temperature limits for any species’ survival, there is some uncertainty associated with the application of lab studies to field conditions. For example, laboratory studies often only consider constant temperatures, but we are learning that temperature variability is important as well. Further, it is difficult to predict how slow, long-term changes in temperature may influence species at both the individual and population levels. Individuals are able to acclimate to small changes in temperature, while, to some extent, populations can adapt over time to slow changes in temperature across generations. A warming climate that increases river temperature over time may be one example of the latter.

Even if changes in temperature are within the limits of what a species can physiologically handle, mismatches between resource availability and metabolic needs of species may be problematic. Since higher temperatures mean higher energy needs for all species, resources may become limiting in warmer environments. Predicting changes in resources is difficult and dependent on many factors. Warmer water can lead to increased food resources, but only if other factors, such as nutrients, are not limiting.  In the CRB, warmer water releases from Lake Powell are associated with low reservoir storage. While this warmer water would typically mean more algae available for insects and other consumers, the lower water levels are also associated with lower nutrient concentrations in the reservoir releases. Nutrient releases are low when water elevations are low, because the water being released is from the upper layer of Lake Powell. Because sunlight reaches this upper layer, nutrients in this layer are taken up by by phytoplankton, leaving reservoir release water depleted of nutrients. Low nutrient levels limit primary production and the amount of food available for higher trophic levels like insects and fish in downstream river reaches.

Temperature Effects on Species Interactions

After narrowing down which species have the potential to exist under future water temperatures, the next challenge is to predict the outcome of species interactions, particularly between native and non-native fishes. Understanding how species interact in natural systems, even under current conditions, is very difficult, because multiple aspects of the river system tend to change at the same time. For example, the construction of dams led to a decline in native fishes, but also an increase in non-native fish abundances and the introduction of more non-native species, making untangling the effects of the dams and non-natives difficult. Even without co-occurring factors, measuring effects of species interactions is challenging. Identifying competition between species is dependent on showing that resources are limiting (we typically have very limited data on resource availability) and that a species is actually having a negative effect on another species. When measuring predation, it is easy to observe that it is happening, but it is much more difficult to understand how much predation is occurring, and if it contributes significantly to population persistence over time.

When we do have information on how species interact (typically from laboratory experiments), the next challenge is to predict how temperature might change those interactions. We might predict, for example, that rising water temperatures could increase both growth of prey species and consumption rates of predators. If the increased growth of prey species outpaces the increased consumption rates of predators, it is possible survival of prey may actually increase if faster growing, larger prey escape predation. Conversely, predators may also experience faster growth and survival, meaning more, and potentially hungrier, predators. To make things more complicated, other factors like turbidity (water clarity) and discharge may also influence how native and non-native fishes interact in response to temperature changes. For example, Smallmouth Bass can tolerate colder water temperatures than many native fishes (Figure 1), but because bass rely on vision to feed, their predation effects on native fishes may be dampened in turbid water.

Looking Ahead

The effects of changes in water temperature will likely have variable and complex responses on fish communities. Such effects will not occur in a vacuum. There are three endangered species programs focused on invasive species management and experimental flow actions intended to help advantage native fishes. Despite this complexity and the various levels of uncertainty around predictions about future fish communities, scientists have predictive tools available to help us prepare for the future. Predictive models based on the best available information allow us to explore these different sources of uncertainty. Developing future scenarios and comparing multiple hypothesized mechanisms allows us to explore the range of possible ecological outcomes—for example, we can compare outcomes of warmer water temperatures where predator consumption increases more than prey growth and vice versa. Embracing uncertainty allows us to explore a wide range of future possibilities and work to inform management to avoid the worst possible outcomes for native fishes.

1: Powell, J.W. (1875) Exploration of the Colorado River of the West and its Tributaries, 1869–1872. Washington, DC, US Government  Printing Office, Smithsonian Institute Publication, 291.

figure 1

Figure 1. The optimal (points) and range (lines) of temperatures suitable for several species in the CRB to complete important life history stages, including larval growth, egg incubation, and ovulation. Many non-native fishes (grey) share similar temperature tolerance ranges to native fishes of the CRB (black), but nonnative fishes have a wider span of potentially suitable temperatures, from ~8-32 degrees. This indicates some nonnative species may be able to use habitats that are not suitable for native species, such as the tailwaters below dams.