Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
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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, increased rapidly after one year; overall recovery of Cutthroat Trout, which required three years, initially began by immigration of juvenile fish (age-1+) into the disturbed area, followed in the second and third years by enhanced recruitment of fry.
Although responding to press disturbance, salmonids in a Canadian stream required at least a year to successfully recolonize a rewatered section of the Bridge River after a long period of no or greatly reduced water flow. The Bridge River, a British Columbia tributary of the Fraser River, was impounded in 1963 and most of the captured flow (annual mean discharge of 100 m3s−1) was redirected into another watershed for hydropower production (see also Chapter 14). As a consequence, there was no flow in a 4 km section below the dam, after which groundwater and small tributaries resulted in a small flow of 0.7 m3s−1 for the next 11 km before being substantially augmented by a large tributary (Decker et al. 2008; Bradford et al. 2011). Once the flow was restored in the 4 km reach, the flow was only at a level of 2–5 m3s−1, 2–5% of the original annual mean discharge (as the channel was regraded to accommodate the reduced flow), but did result in some positive responses. There was rapid recolonization of periphyton and aquatic insects, but colonization by fishes was much slower. Juvenile salmonids (primarily Steelhead and Rainbow Trout, Oncorhynchus mykiss; Coho Salmon, O. kisutch; and Chinook Salmon, O. tshawytscha) did not move upstream even though an invertebrate prey base was available within three months. Instead, colonization was primarily the result of upstream movement of adult anadromous fishes that successfully spawned in the restored habitat. Coho and Chinook salmon spawned in the fall of 2000 and Steelhead spawned the following year. By one year after the resumption of flow, populations of age-0 Rainbow Trout and juvenile Coho and Chinook salmon in the rewetted area were equivalent to downstream populations in the continuously wetted site.
Resilience is also shown by life-history responses of fishes, such as the timing and duration of reproductive cycles or the length of the reproductive life span. The Split-tail (Pogonichthys macrolepidotus), a cyprinid endemic to the Sacramento-San Joaquin Estuary in west-central California, requires inundated floodplains for successful reproduction (Sommer et al. 1997). Submerged terrestrial vegetation on inundated floodplains is used as a feeding area by prespawning adults, as a spawning substratum, and as a larval nursery area. Low-flow years result in substantial reductions in the production of age-0 fish, in contrast to large increases of age-0 fish during wet years. Because the adults have a reproductive life span of three or more years, as well as a high fecundity, the populations are moderately resilient to periodic drought years that limit successful reproduction (Sommer et al. 1997).
Fishes in a small Ontario, Canada, lake also demonstrate resilience to harsh conditions through survival of long-lived adults. The lake was acidified by the addition of sulfuric acid for eight years and then recovery studied for 13 years as part of a large investigation on the effects of acid precipitation (Mills et al. 1987). Three of the five species studied by Mills et al. (1987) (Lake Trout, Salvelinus namaycush; Pearl Dace, Margariscus margarita; White Sucker, Catostomus commersonii) survived the acidification but were not able to successfully reproduce as the pH level dropped. Once acidification stopped and the pH began to gradually rise, recruitment of all three species gradually resumed. Two other species, Fathead Minnow (Pimephales promelas) and Slimy Sculpin (Cottus cognatus), were extirpated from the lake during the acidification period, but only Fathead Minnow successfully recolonized from a nearby lake during the 13 years following acidification.