Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
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faunas have been under strong, long-term selection to deal with such conditions, and intolerant species would have been extirpated (C. L. Smith and Powell 1971). Examples of fish faunas inhabiting areas prone to disturbance include faunas of Great Plains streams. Fishes in these streams are subjected to periods of low flow and even dewatering as well as to intense periods of flooding. Such conditions likely occurred before widespread human-caused changes in the nineteenth and twentieth centuries, although the amount of silt has probably increased as a consequence of changes in land use (Matthews 1988).
In response to this environment, some fishes have evolved increased tolerances to low dissolved oxygen levels and high temperatures. Comparisons of physiological tolerances among minnow species from more benign upland streams in Arkansas with those inhabiting apparently harsher Great Plains streams in central and western Oklahoma generally showed that minnows from harsh environments were more resistant (Matthews 1987). As a group, Matthews showed that the minnows from the harsh streams had significantly greater tolerance to high water temperatures compared to fishes from the relatively benign streams, although one prairie minnow, the Emerald Shiner (Notropis atherinoides), had a critical thermal maximum (CTM) more in line with the upland fishes (Figure 6.1). However, Emerald Shiners, along with three other plains minnows, showed better survival at low oxygen levels compared to upland fishes or Blacktail Shiner (Figure 6.1).
FIGURE 6.1. Contrasts in physiological resistance of minnows from relatively harsh versus benign environments. Harsh environments include prairie streams from central and western Oklahoma; benign environments include streams from upland regions of Arkansas. Oxygen tolerance was measured as the percentage of fish surviving 8.5–10 h at low oxygen levels (0.2=0.9 ppm dissolved oxygen). Based on data from Matthews (1987).
Pupfishes (genus Cyprinodon) occur widely in fresh and brackish water habitats in Mexico (Miller 2005), in coastal brackish water areas along the Gulf of Mexico and Atlantic coasts (Johnson 1980; Nordlie 2003), and in desert regions of the southwestern United States (Naiman and Soltz 1981). In all regions, Cyprinodon species show high resistance to temperature extremes (Feldmeth 1981; Bennett and Beitinger 1997; Nordlie 2003). For instance, the Sheepshead Minnow (Cyprinodon variegatus) of the Atlantic and Gulf coasts can survive temperatures from a low of −1.8° C to a high of 43° C, the widest temperature range of any of the over 200 estuarine/salt marsh fishes reported by Nordlie (2003). Populations of cyprinodont fishes inhabiting the Death Valley region of Nevada and California also have wide temperature tolerances, being able to withstand temperatures of < 1° C to 40–44° C (Brown and Feldmeth 1971; Soltz and Naiman 1978; Feldmeth 1981).
Refuge-seeking behavior also allows fishes to resist adverse conditions in their environment and can have a role in surviving floods and droughts. Fishes in a variety of regions increase their resistance to downstream displacement during floods, especially during the winter when lowered water temperature limits their swimming ability, by actively selecting habitats with large structure such as woody debris, rocks, or other large and relatively immovable structures. Cutthroat Trout (Oncorhynchus clarkii) in a tributary of the Smith River in California showed twice the site fidelity in pools with large woody debris compared to those without (Harvey et al. 1999). During a winter flood event, trout in pools with large woody debris tended to remain in those pools in contrast to the greater movement shown by trout in less complex habitats. Fishes in an arid eastern Oregon stream also were more resistant to floods in structurally complex habitats compared to simple habitats (Pearsons et al. 1992). In the southeastern United States, Bayou Darters (Nothonotus rubrum), a species endemic to the Bayou Pierre system of Mississippi, responded to winter water temperatures (7.5–11° C) in an artificial stream by shifting their distribution to habitat patches with larger particle sizes of coarse gravel and pebbles (Figure 6.2A). In addition, under winter conditions, as flow increased from 14 to 35 cms-1, Bayou Darters selected habitat patches with large refuges to current (in this case bricks 14 × 7 × 7.5 cm) over habitats with just pebbles (Figure 6.2B).
FIGURE 6.2. Behavioral resistance of Bayou Darters (Nothonotus rubrum) to high winter stream flow in western Mississippi.
A. At cold temperatures (7.5=11.0° C) in a laboratory stream with a current speed averaging 31 cms-1, Bayou Darters shift their habitat selection to patches with larger particle sizes compared to habitat selection at warm temperatures (22° C).
B. Even in habitats with large substrata, Bayou Darters select patches with larger refuges (bricks) at moderate versus low current speeds. In both figures, bars are 95% confidence intervals. Based on data from Ross et al. (1992b).
FIGURE 6.3. Behavioral resistance of adult and one-day-old Sonoran Topminnow (Poeciliopsis occidentalis) to displacement by floods, compared to the nonnative Western Mosquitofish (Gambusia affinis). 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