12 Feeding Ecology of Piscivorous Fishes FRANCIS JUANES, JEFFREY A . BUCKEL AND FREDERICK S . SCHARF 12 .1 INTRODUCTION group, however, are difficult to categorize and describe, which perhaps explains why no reviews Fish exhibit tremendous diversity in feeding of their ecology exist . habits and the morphologies associated with feeding. A recent book (Gerking 1994) and various other overviews of fish feeding exist (Wootton 12 .2 ADAPTATIONS FOR 1990; Hobson 1991 ; Hart 1993) . However, most of PISCIVORY these tend to be general reviews of theory or focus 12.2.1 What is a piscivore? on smaller non-piscivorous fishes . Our intent here is to review the feeding ecology of piscivorous fish, We define piscivorous fish as carnivorous fish that a subject which to our knowledge has never previ- consume primarily fish prey. Most fish species are ously been reviewed . We focus on teleost fish, and opportunistic and flexible in their feeding habits on species and sizes that consume juvenile and (Dill 1983) and no species consumes only fish prey; adult prey and do not consider those that feed pri- however many do ingest fish as the main prey marily on larvae or fish eggs . item. Fish that eat other fish are second in propor- tion to those feeding on benthic invertebrates and are present in a variety of freshwater, estuarine 12.1.1 Whypiscivorous fish? and marine systems . They are equally common in Piscivorous fish are broadly distributed phyloge- tropical and temperate ecosystems . Keast (1985) netically and geographically, occur in most habi- examined the piscivore feeding guild of small lakes tats and generally occupy the top of most aquatic and streams and categorized piscivorous fish into trophic webs. Piscivorous fish also generally primary and secondary piscivores . Primary or achieve the largest body size within fish com- 'specialized' piscivores shift to piscivory within munities, are represented by some of the largest the first few months of life, whereas secondary pis- species (many elasmobranchs, tunas, billfishes) civores only become fish-eaters later in life . Keast and have potentially large impacts on their com- further suggests that secondary piscivores switch munities through predation . Finally, many pisci- to fish as a way to maintain energetic efficiency vorous fishes, because of their ubiquity and large as they grow. This can only be achieved by eating body size are among the most valuable harvestable progressively larger prey and, at a certain point, species in many of the world's fisheries . In some fish are the only prey available . Furthermore, sec- cases, they compete with humans for commer- ondary piscivores are not structurally adapted for cially important resource species (V . Christensen piscivory other than acquiring a large mouth as 1996; Buckel et al . 1999a) . Piscivorous fish as a they age . 268 Chapter 12 12.2.2 What are the adaptations strikes occur at high speed and missed prey are for piscivory? rarelypursued, or they can be pursuers such as trout (Salmo trutta ), where attacks start from a C-shaped Feeding behaviours and position, strikes occur at low speeds from a short morphological adaptations distanceand missed prey are chased. When observed in detail it becomes clear that pis- civore feeding behaviour is complex and flexible in Ambush This strategy is used by species that dealing with different prey types . Behaviours can attack from seclusion, although an element of generally be grouped into the following categories, chasing may also occur. Examples are morays which are also associated with particular morpho- (Muraenidae), pike (Esocidae) and summer floun- logical adaptations . der (Pleuronectidae) . Few morphological similar- ities exist in species that use this behaviour other Luring Luring is a sit-and-wait behaviour where than the ability to use camouflage and high-speed prey are attracted by a 'lure', which consists of a attacks. However, distinct patterns of kinematic, stalk topped by a device that resembles a source of pressure, electromyographic and behavioural pro- food; the lure is a modification of the first dorsal files of prey capture are exhibited by ambushers spine (Gerking 1994) . Luring is typical of the an- andpursuers . glerfish (order Lophiiformes) . In addition, among the frogfish various morphological features allow Other A variety of other rarer feeding habits each the predator to blend into the rocky environments with their own specialized morphologies, particu- in which they live as a form of camouflage. Angler- larly in the dentition, include forms of parasitism fish have a considerable gape and ingest prey sizes such as blood-sucking in lampreys (Petromyzonti- that are large compared to other piscivores . dae) and catfishes (Trichomycteridae and Cetopsi- dae), scale-eating in many cichlids and characoids, Stalking Stalking is the unobtrusive pursuit of and fin and eye-biting (Gerking 1994 . prey before the attack occurs . This strategy is com- A novel feeding behaviour common to plankti- mon to trumpetfish (Aulostomidae), longnose gar vores has recently been observed for a few species (Lepisosteidae) and needlefish (Belonidae) . These of piscivores (Sazima 1998) . Ram suspension (or species have similar morphologies, long slender filter) feeding is defined as swimming through the bodies with a long snout and sharp teeth . water with the mouth wide open and opercles flared as a way of filtering small prey items out of Chasing Large piscivores are able to chase and the water column without directing attacks to- 'swim-down' prey. This strategy is best represented wards individual prey. This may be a mechanism by the billfish (Xiphiidae and Istiophoridae) and used by piscivores feeding on relatively small prey some tuna (Scombridae) andyellowtail orpompano in high concentrations (B . Hanrahan and F. Juanes, (Carangidae) . These fish must be able to attain personal observation( . high cruising and accelerating speeds . Their bodies can be described as thunniform or carangiform, Most piscivores ingest their prey whole . A few where thrust is maximized by a lunate tail with a species are able to tear off and ingest pieces and high aspect ratio, a narrow caudal peduncle that good examples are piranhas (Serrasalmus spp .) minimizes sideways thrust and a large anterior and African tiger fish (Hydrocyon vittatus) . The body depth that minimizes recoil of the head end . most detailed analysis of partial prey eating has These same features, along with a relatively rigid been performed for bluefish (Pomatomus and streamlined body, also minimize drag (see saltatrix) (Juanes and Conover 1994a ; Scharf et al . Brix, Chapter4, this volume . Chasers canbe either 1997). Juvenile bluefish switch from taking prey Jungers, such as the pike (Esox spp . ), where attacks whole to partial prey consumption when the prey are startedat close range from an S-shaped position, to predator size ratio is about 0.35 independent of Feeding EcologyofPiscivorous Fishes 269 prey type (Scharf et al . 1997( . This ability allows length, has a fineness ratio between 3 .5 and 5 .0 bluefish to attack much larger prey sizes than can (Juanes et al . 1994) . Interestingly, offshore inverte- predators of similar size and is likely to be a func- brate-feeding juveniles (<40 mm) are already mor- tion of specialized musculature and dentition. phologically specialized for piscivory, suggesting Bluefish are also unique in that they ingest prey a trade-off between feeding efficiency and future tail-first, whereas most other piscivores that have diet. A result of this trade-off may be to accelerate been examined eat prey head-fast . Tail-first inges- the onset of piscivory. tion may be a result of pursuing prey rather than ambushing it, or a way to reduce prey mobility Life history thereby increasing prey vulnerability, as has been observed in piranhas . Onset of piscivory Most piscivorous fish under- go ontogenetic shifts in diet (Werner and Gilliam 1984; Keast 1985 ; Winemiller 1989) . These shifts Schoolingin predators generally progress from consumption of zooplank- Schooling by prey is generally thought of as an ton to consumption of benthic macrofauna or prey adaptation for evading, confusing and reducing the fish, with a concomitant increase in mean prey efficiency of predators (Pitcher and Parrish 1993 . size as predators grow . There is much variation in Schooling also has hydrodynamic and foraging the timing of the shift to piscivory among primary functions . Little empirical work has been done on and secondary piscivores (Mittelbach and Persson whether schooling by predators enhances preda- 1998; Mittelbach, Chapter 11, this volume) . A tion efficiency in piscivores . Field studies have few species of scombrids apparently forgo the shown that, in general, group attacks tend to result zooplanktivorous stage and start eating fishes at in higher capture success rates (Pitcher and Parrish first feeding, whereas others shift early in the 1993). Eklov (1992) has shown that the foraging larval period (Tanaka et al . 1996) . The shift to efficiency, measured as growth rate, varies be- piscivory invariably results in an increase in pre- tween a group-foraging, actively searching pisci- dator growth rate (Buijse and Houthuijzen 1992 ; vore (Eurasian perch, Perca fluviatilis) and a Juanes and Conover 1994b, Olson 1996) . Among solitary-foraging, stalking piscivore (pike, Esox the scombrids studiedby Tanaka et al . (1996, there lucius) . Under similar conditions, grouped perch was a direct correlation between the