Search and detect
Predators can search for prey actively or passively. Water column searchers, such as herrings, anchovies, minnows, tunas, and billfishes rely heavily on vision, as do nocturnal plankton feeders. Olfaction, gustation, and hearing are also important for some water column searchers, particularly sharks. Low-frequency sounds of 20–300 Hz are especially attractive to sharks, whereas amino acids elicit feeding responses in many predatory fishes. Smell, taste, touch, or electrolocalization (passive or active) are employed extensively by benthic and nocturnal foragers such as eels, catfishes, gymnotid knifefishes, sea robins (triglids), goatfishes (mullids), and threadfins (polynemids), with polyodontid paddlefishes apparently using electrical cues to find plankton swarms. Chemoreception and touch are used by other groups that possess barbels, such as sturgeons, minnows, cods, and croakers. Some fishes search by speculation, much as chickens scratch where buried prey are likely to occur. Goatfishes move along the bottom probing into sediments with their muscular barbels that are equipped with abundant taste receptors; some goatfishes flush prey by inserting their mobile barbels into refuge holes where prey have sought shelter (Hobson 1974). Boxfishes (Ostraciidae) and triggerfishes (Balistidae) expel jets of water from their mouths to blast sand away from potential buried prey. Logperch (Percidae) roll stones with their snouts in search of hidden insect larvae. These speculating foragers frequently have attendant species that follow them and snap up prey disturbed by
the forager’s activity.
The energy expended in active search can be saved by camouflaged predators that lie in wait on the bottom or in other structure. Such camoufl age is often termed protective resemblance when hiding from predators, or aggressive resemblance when lying in wait (the latter usage is inaccurate behaviorally since “aggression” should be reserved for combat situations between animals, not for predatory activities). Benthic, camouflaged predators lie on rocks or soft bottoms or can be slightly (or greatly) buried by sediment. Their skin is colored to resemble algae-covered rocks, tunicates, sponges, and other bottom types. Wartlike and other fleshy outgrowths of skin and fins are common. These fish rush explosively from the bottom to capture prey or open their typically large mouths rapidly and inhale prey. Many scorpionfishes (Scorpaenidae), flatheads (Platycephalidae), seabasses (Serranidae), and hawkfishes (Cirrhitidae) rest exposed on the bottom, whereas lizardfishes (Synodontidae), stonefishes (synanceine scorpaenids), stargazers (Uranoscopidae), and flatfishes Pleuronectiformes) lie with only their eyes exposed above the sediment. For such liein- wait predators, vision is the primary sense mode by which prey are detected, except for the elasmobranchs which may also use electrical cues. Many benthic, immobile ambushers appear surprisingly conspicuous, at least to a human observer. They may rely on prey habituating to their presence and thus growing careless.
Some water column predators, including countershaded or silvery-sided fishes such as gars (Lepisosteidae), pikes (Esocidae), and barracuda (Sphyraenidae), also lie in wait, floating motionless near or below the surface and darting at prey that fail to recognize them. This group also includes substrate- and leaf-mimicking species such as trumpetfishes (Aulostomidae) and leaffishes (Nandidae). Many predators shift among search patterns. Trumpetfish lie in wait among gorgonian corals to ambush roving prey, hide behind swimming herbivores such as parrotfishes, or swim actively in the water column and attack relatively stationary schools of zooplanktivores. By day, torpedo rays erupt from the sand at prey that have wandered over them, whereas at night they swim actively above the bottom in search of swimming prey. Prey behavior and density often determine which search mode will be employed. For example, young lumpfish (Cyclopteridae) cling to rocks with their modified pelvic fins and make short excursions to feed on nearby zooplankton when prey densities are high. At low prey densities, the larvae swim through the water column searching for and feeding on plankters, thereby incurring the greater costs of active search but avoiding starvation (Brown 1986; Helfman 1990).
Considerable attention has been paid to the search tactics and detection capabilities of zooplanktivorous fishes. Fish swim through the water column scanning an area ahead of them that is shaped approximately like a hemisphere, the widest part being closest to the fish. The volume of this search space, the distance from objects at which fish react, and the size object that a fish is capable of detecting change with fish size, water clarity, illumination level, and current speed. Large juveniles can detect smaller objects than can small juveniles, and most fishes react further away in clearer water or after light levels exceed some threshold value (Hairston et al. 1982). Zooplanktivores that feed in currents employ searching tactics that vary as a function of current speed. Fish remain in place and wait for food objects to approach them; upon detection, fish then swim toward prey at low current velocities (10–14 cm/s) but fall back with the current at higher speeds (McFarland & Levin 2002).
Reaction distance is heavily dependent on prey size, to the extent that most zooplanktivores will react to and pursue the largest appearing prey in their visual field. This means that a small zooplankter near a fish may be taken preferentially to a larger plankter farther away because the smaller prey appears larger (the apparent size hypothesis). However, prey immobility and location also affect selection, smaller prey being preferred if they are mobile or are more directly in front of the forager (O’Brien et al. 1985; O’Brien 1987). The speeds at which fish search appear to approach the optimal in terms of maximizing intake relative to energy expense. For example, the actual sustained search speed of a 40 cm salmonid is 3 body lengths per second (BL/s), which is close to the calculated optimum sustained speed of 2.9 BL/s (Ware 1978; Hart 1993). Speeds vary as a function of fish size (=metabolic rate) and food concentration.
Although group formation is most commonly viewed as an antipredator response, grouped fishes may search more successfully than individuals. Foragers in groups may locate food sooner, ingest food faster, have more time available for foraging, and grow faster than solitary foragers. For example, in minnows (Phoxinus phoxinus, Cyprinidae), Goldfish (Carassius auratus, Cyprinidae), and Stone Loaches (Noemacheilus barbatulus, Cobitidae), shoal members spend less time before finding food than do solitary individuals, and the benefit increases with increasing shoal size (Pitcher & Parrish 1993). Accelerated rates arise because a fish in a shoal can search for food while simultaneously watching for signs of successful feeding in shoal mates, thus increasing the area over which it effectively searches. Also, the time each individual spends scanning for predators may decrease, leaving more time for feeding. These benefits are countered by intragroup competition for food, competition increasing as the group size increases.