When Good Fish Make Bad Decisions

Carson Jeffres, Staff Research Associate, Center for Watershed Sciences

Canyon section of the Shasta River in the late fall when spawning conditions appear to be good. Photo: Carson Jeffres

Restoration of degraded habitat is generally considered to be a no-brainer.  But, what if by “restoring” the habitat, you inadvertently create a habitat that causes either the target species or other important non-target species to spiral towards extinction—that is, a place that looks good on the surface, but actually leads to poor outcomes for the population?  In riverine ecosystems, habitats are generally created through physical processes such as flooding and sediment transport.  Over time, fish have evolved the ability to adapt and thrive in the habitat that these processes create.  Human changes to river systems often disrupt underlying processes that created natural habitat, and can result in the elimination or degradation of such habitat.  When we try to mimic these habitats on the surface with restoration, but without the associated underlying processes, we can create an ecological trap that worsens the problem.

An ecological trap occurs when an animal seeks out habitat that ultimately reduces its survival or reproductive success (Robertson and Hutto, 2006).  Conditions which lead fish into poor quality habitats or away from high quality habitats can diminish populations.  Most fish have evolved mechanisms to recognize environmental cues that indicate favorable habitats.  Occasionally, however, cues that indicate high quality habitat can belie poor conditions and lead to errors in judgment that reduce fitness and survivorship.  If poor quality habitat is repeatedly chosen over time, population numbers will decrease and an increased risk of local extinction may occur.

At times, ecological traps also may be an unintended consequence of well-intentioned restoration or conservation activities (Robertson and Hutto, 2007; Hawlena et al., 2010).  Usually, such mal-restoration results from efforts to support a single life stage (e.g., spawning) of a single species.  Other species may be attracted to and utilize the restored habitat, but receive either no benefit from it, or are instead harmed by it.

Figure 1. Map of Shasta River watershed showing the two primary spawning locations for salmonids. The Canyon spawning complex is the location of historical spawning gravel placement.

An example of an ecological trap comes from the Shasta River, a Klamath River tributary in Northern California (Figure 1) (Jeffres and Moyle, 2012).  Coho salmon, once abundant, are nearing extinction in this tributary for many reasons.  Most notably, they are particularly susceptible to high spring and summer temperatures caused by land and water use activities, along with a geological quirk of the watershed.  Unlike juvenile Chinook salmon that migrate to sea before their first summer, juvenile coho spend a year and one-half in the watershed before leaving.  This exposes these sensitive fish to high spring and summer water temperatures (Null et al., 2010).  Historically, when temperatures warmed, juvenile coho would have survived by migrating upstream to cool water sources or moving out into the Klamath River for the summer.

The Shasta River passes through a steep, narrow canyon in its lowermost reaches, just above the confluence with the Klamath River.  When salmon return from the ocean in late fall/winter, multiple cues (such as clean gravels, well oxygenated cool water, and good flow velocities and depths) are present that signal that this is good spawning and rearing habitat.  But, juvenile coho that end up in this lower canyon reach inevitably die for three reasons.  First, once they emerge from the gravels, the stream is too steep for them to move upstream to cooler waters.  Second, if they stay, temperatures are almost always fatal by summer.  Finally, with poor water quality conditions in the Klamath River, fish that move into the mainstem are unlikely to survive.

In evolutionary terms, natural selection should weed out adult coho salmon that spawn in the lower Canyon, selecting for those that spawn near cold water sources in the headwaters.  This would be possible if the population were large enough to withstand the selection pressures, but selection pressures function poorly when the population is small.  Additionally, since coho are semelparous (i.e., they die after spawning), there is little opportunity for learning.  Finally, a significant number of these fish are strays from the Klamath River and the Iron Gate Hatchery, thus diluting the local gene pool where selection has already taken place.

Juvenile coho in the Shasta River. Photo: Carson Jeffres

Adding to the effectiveness of this trap are historical conservation efforts.  For many years gravels were added to the canyon reach to improve the quality of spawning habitat for Chinook salmon.  These “no regrets” efforts may have helped Chinook since they do not stick around after the water warms up.  But, coho salmon were drawn to these gravels as well, which left their offspring in a fatal trap.

A more holistic approach to restoration—addressing causes of habitat degradation rather than symptoms — addresses the underlying water quality limitations and spawning locations in the Shasta River while providing access to locations where oversummering habitat is available (Jeffres and Moyle, 2012).  One example comes from the efforts of The Nature Conservancy, aided by scientists from UC Davis, to improve habitat and water temperatures in areas accessible to coho salmon.  This successful approach focuses on all of the fresh water life stages of the fish and has helped to keep coho going in the watershed.

Ecological traps occur in many settings where efforts are underway to restore habitats in California.  Ecosystem-based approaches, rather than the more common species-specific or life stage-specific approaches, are more likely to be successful at avoiding the creation of these traps.

Further Reading:

Hawlena, D., Saltz, D., Abramsky, Z. & Bouskila, A. (2010). Ecological Trap for Desert Lizards Caused by Anthropogenic Changes in Habitat Structure that Favor Predator Activity. Conservation Biology 24, 803-809.

Jeffres, C. & Moyle, P. (2012). When Good Fish Make Bad Decisions: Coho Salmon in an Ecological Trap (PDF). North American Journal of Fisheries Management 32, 87-92.

Null, S. E., Deas, M. L. & Lund, J. R. (2010). Flow and water temperature simulation for habitat restoration in the Shatsa River, California. River Research and Applications 26, 663-681.

Robertson, B. A. & Hutto, R. L. (2007). Is selectively harvested forest an ecological trap for Olive-sided Flycatchers? Condor 109, 109-121.

Robertson, G. A. & Hutto, R. L. (2006). A framework for understanding ecological traps and an evaluation of existing evidence. Ecology (Washington D C) 87, 1075-1085.

About Elena M. Lopez

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