Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
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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. Population levels of fishes, especially the top predator, Lake Trout, had not reached preacidification levels by the end of the 13-year study of recovery—likely a reflection of the still recovering prey base.
Fishes also may show resilience to unfavorable spawning conditions by having extended reproductive seasons. The Longnose Shiner (Notropis longirostris), a cyprinid found in small, upland streams in the southeastern United States, has a short life span of only 1–2 years. However, resilience to poor spawning conditions is achieved by having a protracted spawning season that begins in February and can extend into October (Heins and Clemmer 1976; Ross 2001). Similar patterns of extended spawning seasons in association with short life spans are shown for numerous other southeastern minnows such as Red Shiner (Cyprinella lutrensis), Blacktail Shiner (C. venusta), and Weed Shiner (Notropis texanus) (Ross 2001).
In summary, resilience in fishes can be achieved by movement of adults or juveniles back into a previously disturbed area. Resilience to poor spawning conditions or unfavorable conditions for larval/juvenile survival occurs through elevated longevity of adults so that they can wait out poor years. On an annual basis, short-lived fishes show resilience to poor spawning conditions by having extended reproductive seasons. Overall, there is considerable variation in the resilience of fish species and fish assemblages to perturbations. Variation occurs across multiple levels including the nature, timing, and severity of the disturbance; the type and location of the aquatic system; species characteristics; and life-history stage (Schlosser 1985; Detenbeck et al. 1992; Albanese et al. 2009).
Levels of Persistence and Stability in Lotic Systems
Considering a wide range of studies, lotic systems tend to show moderate to high levels of persistence and low to moderate levels of stability, with the degree of stability influenced by the metric used to test it. In a survey of 49 primarily North American stream sites that had been subjected to various types of disturbance, Detenbeck et al. (1992) found full or nearcomplete recovery within two years. Analysis of 25 long-term studies (≥ 2 years; median = 11 years; range 2–45 years) designed or amenable to testing assemblage persistence and stability, and including from 3 to 95 species, showed 76% high persistence and 52% high stability in at least one type of measure (Table 6.1). Environmental harshness, especially if the harshness was related to anthropogenic impacts, had a strong effect on assemblage persistence and stability (Figure 6.5A). In systems judged to have low stress, 100% of the assemblages were persistent and 80% stable. In contrast, for systems judged to have moderate or high stress, only 22% were persistent and 11% stable. In systems with obvious human disturbance, only 14% of the assemblages were considered persistent or stable (Figure 6.5B). However, the sample size is too limited to separate the impacts of human versus natural disturbances; of the 10 studies having moderate to high disturbance, only three were disturbed by nonhuman impacts. The data in Table 6.1 are also biased by geographical region; most of the studies were at lower latitudes (mean latitude = 35.7°; range = 31°–42°) and 84% were done east of the continental divide. However, recall the challenges of assessing assemblage persistence and stability in western fish faunas with long-lived species. There is also a bias in stream size as only two studies dealt with large rivers.
FIGURE 6.5. Impacts of environmental stress (A) and the level of human disturbance (B) on the degree of persistence and stability of lotic fish assemblages, and (C) a comparison of assemblage persistence and stability in lotic and lentic systems. Numbers above bars show sample sizes. Based on data from Tables 6.1 and 6.2.
Examples of Persistence and Stability in Lotic Systems
Brier Creek, an Oklahoma tributary of the Red River (now inundated by Lake Texoma), is routinely subjected to extreme conditions and has been particularly well studied. In spite of extreme conditions, including total dewatering of some stream reaches, the fish fauna over an 18-year period showed strong persistence on a stream-wide basis, in that abundant species continued to remain abundant and rare species remained rare, with only a few exceptions. Stability of the Brier Creek fish fauna showed greater variation, as measured by indices of similarity of the sampled fish fauna among years. The fish fauna at individual collection sites (i.e., at the local assemblage level) showed less persistence and stability.
The timing of perturbations can have a major influence on the resultant impacts to aquatic organisms. If flooding in Brier Creek occurs when fish are spawning, there are severe impacts on larval survival. For instance, Harvey (1987) showed that larval cyprinids and centrarchids that were less than 10 mm TL were