Data show mean number retained in a laboratory stream system for each of ten, 60 s trials with a 60 s rest period between each trial; six fish of each species were used in each trial. One-day-old Western Mosquitofish only survived for five replicates. Based on data from Meffe (1984). The shaded area of the map inset shows the approximate boundaries of the Sonoran Desert.
Sonoran Topminnow (Poeciliopsis occidentalis), a small poeciliid native to streams, marshes, and springs in the Sonoran Desert, has evolved behavioral responses to flash floods that commonly occur in the region, in contrast to the morphologically similar Western Mosquitofish (Gambusia affinis), which is not native to the region (Figure 6.3). In habitats that are not periodically disturbed, Western Mosquitofish can reduce or eliminate Sonoran Topminnow, probably through predation on the young (Meffe 1984). Using field observations in a tributary of the Santa Cruz River in southern Arizona, combined with laboratory experiments, Meffe (1984) showed that Sonoran Topminnow of all life-history stages responded to floods by rapidly moving to shoreline eddies and remaining there until high flows receded. In contrast, Western Mosquitofish responded more slowly and in a less organized manner to flooding and tended to move back out into high flows sooner, thus exposing themselves to downstream displacement. As a consequence, Sonoran Topminnow showed stronger resistance to downstream displacement by flooding in contrast to the nonnative Western Mosquitofish, although both species showed improvements in flood resistance through repeated exposure (Figure 6.3). By testing one-day-old fish, Meffe also demonstrated that the behavioral response to floods is innate in Sonoran Topminnow; one-day-old Western Mosquitofish were almost all displaced. Western Mosquitofish were also completely displaced from Sabino Creek, another southern Arizona stream, by a record winter flood, whereas a native minnow (Gila Chub, Gila intermedia) and a nonnative centrarchid (Green Sunfish, Lepomis cyanellus) were not (Dudley and Matter 1999).
Morphology, in concert with appropriate behavior, can also provide resistance to harsh environments. For example, most all fishes that obtain oxygen from the water will move closer to the water’s surface as oxygen levels are depleted—the response is termed aquatic surface respiration (ASR) (Kramer 1987). However, most fishes, because of jaw morphology and head shape, must spend additional energy through body or fin movements to maintain the extreme body angle required for ASR and cannot survive severe subsurface oxygen depletion for extended periods of time (Lewis 1970). Fishes such as livebearers and topminnows have flattened heads and superior mouths, morphologies that are particularly adapted to ASR, and are able to use the highly oxygenated surface film while remaining in a nearly horizontal (< 10°) position. By exploiting the surface film, these fishes can survive extended periods in water that is otherwise low in oxygen (Lewis 1970; Kramer 1987; Timmerman and Chapman 2004). More recent work indicates that ASR might be most important as an immediate response measure to low oxygen levels. For instance, Sailfin Mollies (Poecilia latipinna), as well as a variety of other fish groups, are able to gradually increase oxygen capacity of the blood when chronically subjected to an oxygenpoor environment (Timmerman and Chapman 2004). This occurs through an increase in red blood cells and through increased hemoglobin concentration.
Resilience
Fish populations are often faced with environmental perturbations that are too severe for one or all life-history stages to resist through morphological, physiological, or behavioral mechanisms. Such perturbations might include extreme floods, drying of essential habitats such as feeding or spawning areas, changes in water quality, or total drying of an aquatic habitat. Although the initial effect can be the total or partial loss of species making up an assemblage or the lack of successful reproduction, the ability to recover once environmental conditions become more favorable is described by resilience.
Resilience to major environmental perturbations is provided through the ability of fish populations and assemblages to repopulate an area once conditions improve. This may occur through the return of displaced individuals and through the often-accelerated production of new individuals (Ross et al. 1985; Matthews 1986b; Fausch and Bramblett 1991). The extent of resilience is influenced by the size of the affected habitat and by the size and proximity of refuges where fishes can survive. Watershed geometry (e.g., Chapter 4; Figure 4.3) plays an important role in resiliency (Grant et al. 2007).
In southern Oklahoma, fishes in Brier Creek (see Chapter 5; Figure 5.4) recolonized a dewatered section of stream within four months once flow resumed. Recolonization to this pulse disturbance was initially by movement of fish out of isolated pool refugia, followed by spawning. In a southeastern study, Albanese et al. (2009) removed adult and juvenile fishes from 416 m and 426 m reaches of two small streams, Middle Creek and Dicks Creek, located in the James River drainage of Virginia (Figure 6.4). They followed recolonization for approximately one year along a 130 m reach in Dicks Creek, and two years along a 126 m reach in Middle Creek. The reaches were located in the middle of each of the two removal sections (Figure 6.4). The larger Dicks Creek site involved 19 species, whereas the smaller Middle Creek site involved 6 species. The fish fauna in both streams was dominated by minnows, which make up 85% of the fish in Dicks Creek and 92% in Middle Creek.
The resilience of individual species studied by Albanese et al. (2009), measured by their rates of recovery to the simulated pulse disturbance, varied widely; rates of recovery also differed between the two streams. Mountain Redbelly Dace (Chrosomus oreas) rapidly recolonized, reaching over 60% of the original population size within one month in Dicks Creek but less than 20% in Middle Creek (Figure 6.4). At the end of one year, Mountain Redbelly Dace populations had fully recovered in Dicks Creek, but required an additional year for full recovery in Middle Creek. After one year, five of the eight censused species in Dicks Creek attained 80% or greater recovery; Shadow Bass (Ambloplites ariommus) and Blacknose Dace (Rhinichthys atratulus) showed much lower resilience. In Middle Creek, three of the five censused species reached 90% or greater recovery after two years, whereas Torrent Sucker (Thoburnia rhothoeca) did not recover and Rosyside Dace (Clinostomus funduloides) only reached 60% of the original population size. Even though species varied significantly in their resilience to the defaunation, the fish assemblages, as measured by pre- and postremoval similarity, appeared resilient. This occurred because abundant species remained abundant, and more abundant species have a greater effect on faunal similarity measures that include relative abundance (Matthews 1998). As stressed by Albanese et al. (2009), this has important conservation implications. Measures that focus only on the assemblage level could overlook the loss of rare species following natural or human-caused perturbations.
FIGURE 6.4. Varying levels of resilience of two southeastern fish assemblages as demonstrated by recolonization of experimentally defaunated areas. The map shows the study area, which was located in the James River drainage of Virginia; ovals are impoundments. Recovery was followed for approximately one year in Dicks Creek and two years in Middle Creek. Based on data from Albanese et al. (2009).
In western North America in the Willamette River drainage of western Oregon, Lambertiet al. (1991) followed recovery of Cutthroat Trout in Quartz Creek, a high-gradient stream that had suffered a pulse disturbance in the form of a catastrophic debris flow. The debris flow had severe impacts on the physical and biotic characteristics of a 500 m stream reach. Physical changes included loss of woody debris, loss of canopy cover, a reworking of channel sediments, and an overall simplification of the channel, resulting in reduced hydraulic retention. Chlorophyll α was low immediately after the debris flow, but the newly opened canopy and the reduction in grazing by macroinvertebrates later resulted in a doubling of chlorophyll α compared to a control reach. Macroinvertebrate density initially showed high variation, followed by recovery after one year to densities shown in the control reach, and recovery of species richness after about two years. Cutthroat Trout, the only fish species in Quartz Creek, were initially extirpated. Resilience, as measured by percent recovery to predebris flow conditions,
