turbulent flow in a boundary layer. Re<1 indicates a totally viscous environment and Re >1,000 indicates a totally inertial environment; at intermediate values both forces are represented (Lauder and Tytell 2006), but for values of Re below 300–450, viscous forces predominate over inertial forces (Webb and Weihs 1986; Fuiman 2002). It is difficult for us to really imagine life at low Reynolds numbers. Purcell (1977), in discussing swimming in microorganisms and the impact of the primacy of viscous over inertial forces, said: “If you are at [sic] very low Reynolds number, what you are doing at the moment is entirely determined by the forces that are exerted on you at that moment, and by nothing in the past.”
Once the yolk sac is absorbed, larval fishes generally swim at 1–3 body lengths per second (Fuiman 2002). Thus Re of a 5-mm larval fish would be 25–75, at the lower end of the intermediate range, and subject to viscosity effects. In a viscosity-dominated environment, pushing against the water by an elongate body is more effective than using caudal fin propulsion (Webb and Weihs 1986), but because of the unimportance of inertial forces, larvae must swim continuously to move. (Recall that the law of inertia, or Newton’s first law, states that a particle at rest or moving in a straight line with a constant velocity will continue to do so, provided the particle is not subject to an unbalanced force.) As soon as the larvae stop actively swimming, they come to a halt (Purcell 1977; Blake 1983a). As fishes increase in size, the importance of inertial forces increase relative to viscous forces so that once Re reaches 300–450, they can employ an energy-saving burst-and-glide approach to locomotion (Fuiman 2002).
NON-ANGUILLIFORM BCF LOCOMOTION Increased posterior localization of body undulation and power and the development of distinct caudal fins characterize the traditional modes of subcarangiform and carangiform locomotion (Table 7.1). Subcarangiform swimming occurs in the majority of nonlarval North American freshwater fishes, including salmonids, cyprinids, catostomids, centrarchids, and percids; however, specific studies on swimming are limited to relatively few species of salmonids, cyprinids, and centrarchids (Blake 1983a; Lauder and Tytell 2006). Subcarangiform fishes typically have fairly flexible but low-aspect-ratio caudal fins, such as the fins of many minnows, suckers, catfishes, sunfishes, and darters (Figure 7.4). Aspect ratio expresses the amount of lift generated by a hydrofoil, with lift increasing with aspect ratio, and is determined by the square of fin span divided by fin area. Examples of carangiform swimmers within North American freshwater fishes are less common but potentially include herring and shad (family Clupeidae). Carangiform swimming also is likely approached by two large cyprinid fishes endemic to the Colorado River system, Humpback Chub (Gila cypha) and Bonytail Chub (G. elegans); both have high-aspect-ratio caudal fins and narrow caudal peduncles, although there are no supporting biomechanical or hydrodynamic studies on these species. Consequently, in terms of BCF swimming, the vast majority of North American freshwater fishes occupy the anguilliform-subcarangiform range of swimming modes.
FIGURE 7.6. A. Bluegill (Lepomis macrochirus), with the dotted line showing the outline of the pectoral fin used in labriform locomotion.
B. Gait change and relative metabolic power and cost as a function of swimming speed in Bluegill (mean length 19.5 cm). The gait transition occurs at approximately 1.3 body lengths per second. Based on data from Kendall et al. (2007).
MPF LOCOMOTION Similarly to categorization of BCF swimming modes, Breder (1926) recognized six undulatory modes of MPF locomotion and one oscillatory mode, based principally on the median or paired fins involved, the general appearance of the waveforms, and the length of the fin relative to body length, but not on functional aspects of fin kinematics (Blake 1980, 1983a). In a simplified system, Blake (1983a; 2004) recognized the distinction between undulatory and oscillatory modes and divided undulatory modes into two groups based on fin kinematics. Type I includes fishes using fins with high amplitude, low frequency, and long wavelengths, such as dorsal fin locomotion in the Bowfin (Amia calva). Amiiform locomotion is also advantageous in allowing backward as well as forward locomotion (Webb 2006). Type II includes fishes using fins with low amplitude, high frequency, and short wavelengths, such as pipefishes (Syngnathidae), a primarily marine group but with some species, such as the Gulf Pipefish (Syngnathus scovelli), entering into fresh water. Because thrust is achieved most efficiently by accelerating a large mass of water to a low velocity rather than the reverse, type I locomotion is more efficient than type II (Blake 1983a).
Oscillatory fin locomotion, also referred to as labriform swimming, is shown by fishes using pectoral fins for locomotion, such as mudminnows (Umbra spp.), sticklebacks (Gasterosteidae), and certain centrarchids (Lepomis and Pomoxis) (Figure 7.6; Drucker and Lauder 2000; Walker 2004; Jones et al. 2007). Fishes using MPF locomotion are common in complex habitats such as weedy ponds, lake margins, or streams with abundant submerged or emergent vegetation or woody debris and are adept at backward as well as forward locomotion (Webb 2006).
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