Shoaling and search
The antipredation benefits of group formation apply to all phases of the predation cycle, including search. Fish in a shoal have a lower probability of being found by a predator than the same fish distributed solitarily (Brock & Riffenberg 1960). Shoals are undoubtedly more conspicuous than solitary fish, so providing no camoufl age value, although an inhibitory function may exist because, at the edge of visibility, a shoal may be mistaken for a large fish and therefore be avoided by an approaching predator (Pitcher & Parrish 1993). Shoal formation is probably common in prey fishes because of the necessity to move and find food, particularly among herbivorous and planktivorous fishes. Highly evolved protective resemblance is not an option for such fishes; hence group formation is an alternative.
Upon detection of a predator, fish in shoals typically shift to polarized, schooling tactics. Behaviors are emphasized that preserve the integrity of the threatened group (Pitcher & Parrish 1993). Subgroups stream toward the main group (but move as coordinated units, not as individuals), interindividual distances decrease, and movements become synchronized among school members. Heterospecific shoals (those containing more than one species) sort out by species, conspecifics associating with individuals of their own species and size. If few conspecifics exist, members of the minority species may seek shelter rather than wind up as the odd members of a school (e.g., parrotfish, Scaridae; Wolf 1985). In some situations, members of the prey group will actually move away from the shoal, approach the predator, and then return to the shoal. These predator inspection visits have been witnessed in Mosquitofish and Guppies (Poeciliidae), sticklebacks (Gasterosteidae), Bluegills (Centrarchidae), and gobies (Gobiidae). The behavior may: (i) allow prey fish to assess the identity, motivational state, or other traits of the predator; or (ii) inform the predator that it has lost the element of surprise and that an attack is unlikely to be successful (Magurran 1986a).
Prey can also discourage a searching predator by behaving aggressively. Several prey species actually attack potential predators and drive them from the area. This behavior, best known from bird studies and commonly called mobbing, has been documented for individuals or groups of squirrelfishes, snappers, grunts, goatfishes, butterfl yfishes, damselfishes, wrasses, and surgeonfishes interacting with predatory moray and snake eels, lizardfish, trumpetfish, scorpionfish, stonefish, flatheads, barracuda, and flatfish, and for Bluegill and Longear sunfish and Largemouth Bass interacting with turtles and water snakes. Mobbing fish may contact the head or tail of the predator, or may display in front of the predator by swimming in place and erecting dorsal spines and rolling the body. Mobbing reduces the predation rate in an area because mobbed predators take longer to return to an area than do predators that are ignored (Motta 1983; Ishihara 1987; Hein 1996). Predators may leave an area because the physical attacks of the mobbing fish are injurious or because the actions of the mobbers notify other prey individuals to the presence of the predator, which lowers the predator’s potential success in the area, analogous to the alarm calls of birds and small mammals (Helfman 1989).
Either inspection or mobbing might explain why some prey converge on or follow predators immediately after a successful attack on the group. This action has been observed in Yellow Perch attacked by Pike, in snappers attacked by jacks, in bluegill attacked by pickerel, in territorial damselfish attacked by several predators, and in planktivorous damselfish attacked by trumpetfish (Nursall 1973; Potts 1980; Dominey 1983; Ishihara 1987; G. S. Helfman, pers. obs.).
The focus of this discussion has been on avoiding detection by visual predators. However, many nocturnal predators and those that live in turbid habitats rely heavily on acoustic, bioelectrical, and chemical cues to find prey. Pacific Herring, Clupea pallasii, respond to sounds such as those emitted by echolocating dolphins by ceasing to feed, dropping in the water column, and schooling actively; fish already in schools drop in the water column and increase their swimming speed (Wilson & Dill 2002). Another clupeid, the American shad, Alosa sapidissima, first moves away from an echolocation sound and then swims erratically if the sound strengthens (Popper et al. 2004).
Little else appears to be known about mechanisms for confusing predators or avoiding detection via non-visual channels. In terrestrial environments, both predators and prey possess attributes that function to muffl e sounds, such as the serrated feathers on the leading edge of owls’ wings, or the pads on the feet of felids (or “quiet as a mouse”). In contrast, sound is difficult to localize underwater. Localization requires some difference in timing or amplitude upon arrival of a sound at members of a pair of receptors. Sound travels relatively rapidly in water (4.5 times faster than in air) and hence arrives on both sides of a fish at very nearly the same time. Predators often know that prey exist in the area but cannot tell in what direction or how far away. Sharks and a few teleosts (e.g., cod, Gadidae; squirrelfishes, Holocentridae; cutlassfishes, Trichiuridae) have been shown to localize sound and this ability might encourage selection for acoustic dampening structures or behaviors in prey.
Comparatively little is known about the behavioral ecology of electrolocalization (see Electrical communication), whether prey somehow insulate their electrical output or maximize their ionic similarity with their surroundings to avoid detection by passive and active electrolocators. Chemical detection of prey is well known. It has been suggested, although not demonstrated, that the mucous cocoons that many parrotfishes secrete from their gills while resting at night could seal off chemical cues used by predators such as moray eels (Winn & Bardach 1959), although tactile predators could also be deceived or deterred, especially given that parrotfish dash away when the cocoon is contacted (Videler et al. 1999; Light-induced activity patterns).
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