Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
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from static to mobile behavior. In other words, fishes are continually responding to their local environment relative to their physiological needs, and the variation in when and how much they move reflects this. Hence a “mover” one day might be a “stayer” the next.
Ontogeny and Movement
During their life cycle, freshwater fishes are faced with many challenges, including feeding, growth, predator avoidance, and reproduction. In some species and/or habitats, these activities may occur over a small spatial scale on the order of meters, whereas in others, tens or hundreds of kilometers may separate these and other activities (Figure 5.5). An important point that Figure 5.5 illustrates is that movement can occur at any life-history stage. For egg and larval stages, movement is generally passive and occurs via transport by water currents. The downstream drift of fish eggs and/or larvae is widespread, but by no means universal, among freshwater fish taxa (Gale and Mohr 1978; Brown and Armstrong 1985; Pavlov 1994). Entry into the drift by larval fishes can be due to turbulence that dislodges larvae (termed catastrophic drift) or because of active choice, as larvae swim up off the bottom. As described by Pavlov (1994), once in the water column, drift can be passive (most common for early larval stages), drifting downstream but not oriented to the direction of flow. Drift can also be active, where fish are actively swimming downstream, or active-passive, where fish show orientation to the current but only weak swimming ability. Several examples of species from different regions and habitat types in North America serve to illustrate these patterns.
MOVEMENT AT THE FERTILIZED EGG STAGE Various minnow species in Great Plains streams have adapted to life in large, turbid rivers with shifting sand substrata by having a semibuoyant egg stage. This is apparently an adaptation to the unpredictable summer flows, since eggs are released during high-discharge periods (Platania and Altenbach 1998; Dudley and Platania 2007), and perhaps to avoid the risk of suffocation by sediment accumulation that would threaten eggs deposited directly on a shifting substratum. In the Rio Grande drainage (including the Rio Grande and Pecos rivers), four native species in three genera form a reproductive guild of broadcast spawners with semibuoyant eggs. The species are the Rio Grande Silvery Minnow, Hybognathus amarus; Speckled Chub, Macrhybopsis aestivalis; Rio Grande Shiner, Notropis jemezanus; and Pecos Bluntnose Shiner, N. simus pecosensis (Platania and Altenbach 1998). Two additional taxa that were endemic to the Rio Grande (Phantom Shiner, Notropis orca; Rio Grande Bluntnose Shiner, Notropis simus simus) were also likely members of this guild; unfortunately these taxa are now extinct (Bestgen and Platania 1990; Platania and Altenbach 1998).
Embryonic and early larval development of fishes in this guild occurs as they drift downstream with river flow. The distances required for the egg and early larval development to occur are impressive—during the time of passive transport of the eggs, they could travel some 144 km. The newly hatched protolarvae, which also remain in the water column, could be carried an additional 216 km depending on water temperature (which controls the rate of development) and current speed (Platania and Altenbach 1998). Clearly, this guild of Rio Grande fishes shows that the eggs and early larval stages of certain species have the ability to move great distances. Upstream movements of juveniles and adults counter the downstream movements of the eggs and protolarvae. In relation to Figure 5.5, the distance (A) from adult habitat to spawning habitat can be quite short. In contrast, the distance (B) from the spawning habitat (open water) to the nursery habitat (downstream in shallow, slow water along the shoreline) can be a hundred or more kilometers. This occurs following the protolarval stage. Later larval stages and juveniles inhabit the shallow, warm, and productive river margins as they gradually move upstream. This upstream distance (C) is also on the same scale as the distance from the spawning habitat (B). This life-history pattern, while demonstrating the ability for long distance movement of early life-history stages and adults, is obviously very susceptible to man-made barriers in rivers and to flow modifications (Winston et al. 1991; Dodds et al. 2004; Dudley and Platania 2007). The semibuoyant eggs require at least some current speed to remain in suspension. Consequently, if they enter the slack water of a reservoir they tend to sink and die from suffocation. The high concentration of nonnative predators in reservoirs is also generally lethal to drifting eggs and larvae (Dudley and Platania 2007). Dams also preclude the upstream return movement of juveniles and adults. As a consequence, reproductive output of most of the breeding adults of silvery minnows seems to be lost as developing embryos and larvae drift into impoundments (Alò and Turner 2005); it is no surprise that most species of this guild are extirpated or have their ranges greatly reduced and have required federal listing as threatened or endangered.
MOVEMENT AT THE LARVAL STAGE In contrast to the previous examples of fishes with semibuoyant eggs, most freshwater fishes have eggs that are demersal and adhesive, remaining attached to bottom materials such as gravel, sand, wood, or other solid materials prior to hatching. However, larvae of many species do have a free-swimming stage and can enter the water column of streams. The diverse arrays of invertebrate and vertebrate organisms that are carried in the water column are collectively referred to as drift. The drift of larval fishes can occur at any larval stage but is most prevalent at the earliest (protolarvae) and the intermediate (mesolarvae) larval stages and can result in downstream movement on the scale of meters to hundreds of kilometers. Relative to Figure 5.5, distances of larval drift (B) and movement of juveniles to adult feeding areas (C) are generally roughly equivalent to distances moved by adults to the spawning ground (A). The duration of larval drift varies widely among taxa, from very long periods of drift in various species of minnows, as discussed previously, to very short time periods in the drift, as in certain species of darters (Slack et al. 1998). The density of drifting fish larvae can be impressive. In the Smith River, a coastal river of northern California, White and Harvey (2003) found that some 2.5 billion sculpin (Cottus) embryos and larvae move down the river to the estuary over a four-month period.
Movement via larval drift is common in many North American fish families. Of the 15 families that make up 90% of North American species (Chapter 1; Table 1.1), larval drift or drifting by early juvenile stages commonly occurs in 10 families (Table 5.1). Exceptions to this are livebearing fishes (families Poeciliidae and Goodeidae), fishes in the families Cyprinodontidae and Fundulidae where larvae tend to remain on or near the bottom (Foster 1967), and fishes where young are often guarded in a confined nesting cavity, such as madtom catfishes (Noturus), or closely guarded by parents (family Cichlidae).
The numerically dominant cyprinids are widely represented in drift throughout North American streams, including the western and central United States (Robinson et al. 1998), northern rivers in Canada and the United States (Muth and Schmulbach 1984), and southeastern streams (Gallagher and Conner 1980; Slack et al. 1998). As shown in the previous section for species with drifting eggs and larvae, distances traveled by larvae can exceed several hundred kilometers. In the Colorado River drainage, estimates of movement of native cyprinid larvae range from approximately 9 km for Humpback Chub (Gila cypha) and Speckled Dace in the Little Colorado River (Robinson et al. 1998), to over 200 km for Colorado Pikeminnow (Ptychocheilus lucius) in the Green River (Tyus and Haines 1991).
For percids, larvae of the commercially and recreationally important Walleye (Sander vitreus) and Yellow Perch (Perca flavescens) exhibit substantial drift with greatest abundances occurring at night (Gale and Mohr 1978; Corbett and Powles 1986; Johnston et al. 1995). The occurrence in drift samples of darter species in the genus Etheostoma has been documented in several studies, with peak abundances typically at night from 2100 to 0300 h (Gale and Mohr 1978; Lathrop 1982; Brown and Armstrong 1985; Paller 1987). Even Bayou Darters (Nothonotus rubrum), members of a genus that inhabits swift water and coarse substrata, show downstream drift of at least several hundred meters. This is at least far enough for them to travel between patches of riffle habitats that juveniles and adults of this species selectively occupy (Slack et al. 2004). Larval drift also occurs within the darter genus Percina. For instance, larval Snail Darters (Percina tanasi) show downstream transport of up to several kilometers, followed by return upstream movement of juveniles and adults (Kuehne and Barbour 1983). As shown earlier, this type of life cycle that includes downstream drift followed by upstream movement of juveniles and adults is greatly at risk from man-made barriers to movement. Indeed, the population of Snail Darters in the Little Tennessee River was extirpated by the infamous Tellico Dam (Ono et al. 1983). More recently, additional populations of Snail Darters have been