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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