Feeding Behavior in Three Species of Sharksl

Feeding Behavior in Three Species of Sharks l EDMUND S. HOBSON 2 THIS REPORT concerns a study of the feeding environment-the aim of this investigation behavior in three species of sharks: Carchar has been almost completely neglected. hinus menisorrah Miiller and Henle, the grey In this study, a comparison is made of the shark (Fig. 1), Carcharhinus melanopterus feeding behavior of the three species. Behavior Quoyand Gaimard, the blacktip shark (Fig. 2), is further related to habitat and to distribution both of the family Carcharhinidae; and Triae within the lagoon. Experiments designed to il nodon obesus Riippell, the whitetip shark (Fig. lustrate the roles played by the major sensory 3), of the family Triakidae. The study was con modalities are presented. This last portion of ducted in the lagoon at Eniwetok Atoll, Mar the investigation, dealing primarily with grey shall Islands, during the summers of 1959 and sharks, includes a consideration of the specific 1960. It was a segment of a broad program of stimuli involved in releasing feeding behavior. investigation of shark behavior in which labora Consideration is also given to food preferences tory and field work were coordinated whenever and to factors which may possibly inhibit feed possible. The overall program, conducted at ing. both the Eniwetok Marine Biological Labora tory and the Hawaii Marine Laboratory, Coco GENERAL DESCRIPTION OF STUDY AREAS nut Island, Hawaii, was under the direction of Dr. Albert 1. Tester, with financial support The study was centered about two primary from the Office of Naval Research (Contract locations: (1) the lee of Engebi Island during Nom 2756(00), Project NR 104503). 1959, and (2) the lee of Bogen Island, adjacent Observations of sharks in their natural en to Deep Channel, during 1960. The Engebi vironment have been the basis for most of the Island site was in 15 ft of water over a relatively shark literature which is available today and shallow sand and coral rubble flat extending out yet comparatively little has been offered toward from shore for approximately 400 yd before a realistic understanding of shark behavior. dropping suddenly into the deeper regions of Most of this material has been written for popu the lagoon. Currents are weak in this area and lar consumption and is therefore oriented to the water is generally turbid, with underwater ward the sensational rather than the scientific. visibility commonly less than 20 ft. At the We do find scientifically oriented accounts in Bogen Island site strong tidal currents are pres the literature (as for example, Eibl-Eibesfeldt ent and underwater visibility often exceeds 100 and Hass, 1959; Limbaugh, 1958; and Wright, ft. The edge of Deep Channel at this point drops 1948) but these observations are limited largely abruptly from a depth of approximately 10 ft to incidental encounters. at the rim to 110 ft at the bottom. A critical study of shark behavior, undertaken At these locations, observations were made with planned experiments in the sharks' natural from the following vantage points: 1. An underwater chamber was fitted to a 1 Prepared as partial requirement for the Master of Science Degree, University of Hawaii. Contribution vessel moored in the lagoon. This chamber was No. 180, Hawaii Marine Laboratory, University of a metal cylinder, 14 ft long and 2Yi ft in di Hawaii, Honolulu, Hawaii. Manuscript received No ameter, open at the top and closed at the bot vember 23, 1961. tom, with viewing ports on three sides near the • Department of Zoology, University of Hawaii. Presently at the Department of Zoology, University of bottom. An observer in the chamber was situ California, Los Angeles 24, California. ated approximately 8 ft below the surface. 171 172 PACIFIC SCIENCE, Vol. XVII, April 1963 FIG. 1. The grey shark, Carcharhinus menisorrah. (Photo by E. Hobson.) 2. A wire cage was suspended from a raft GENERAL OBSERVATlONS OF BEHAvIOR anchored on the slope at the edge of Deep Channel. An observer in this cage was located Often when we were engaged in various immediately below the surface. types of activity in the lagoon the resulting 3. A canvas screen, anchored on the bottom, commotion, which commonly involved splash concealed an observer lying prone on the bottom ing on the surface or striking metal tools on wearing SCUBA equipment. hard objects underwater, was immediately fol 4. A 16-ft glass-bottom boat, having the lowed by the rapid approach of an obviously added advantage of mobility, provided for ob alerted blacktip or grey shark. servations of activity directly below. In spite of this initial attraction to many stim 5. In many cases, the observations involved uli, both species exhibited varying degrees of incidental encounters with sharks during the caution when encountering unfamiliar situa course of such routine underwater activity as tions. This was particularly apparent in the installing apparatus, etc. blacktip, a species which seemed especially sen Experimental procedures and apparatus will sitive to potential danger. When work was be be further described as they pertain to the re gun at Engebi, blacktips often circled at the port. limit of visibility in the baited area for as long as 2 or 3 hr before approaching the bait. Then, SPECIES STUDIED when an approach was made, it seemed to be Identification of the sharks is based on Schultz done reluctantly and was often cut short by et al (1953). All three species, Carcharhinus some stimulus, unnoted by the observer, which menisorrah, C. melanopterus, and Triaenodon startled the shark as it neared the bait and obesus, reportedly have a wide Indo-Pacific dis caused it to swim rapidly away. Usually the tribution. They are reported from the Red Sea shark repeated the approach, but in these early as well as the Maldive Islands by Klausewitz tests it was not until several sharks had become (1958, 19'59). Although positive identification active in the area that a blacktip finally took of the two carcharinids awaits a revision of the the bait. We subsequently found that we were family on a world-wide basis, all three species dealing with the same blacktips day after day appear to be prominent in the shark popula at Engebi and that as the work progressed, their tions of most Pacific atolls. Harry (1953) re initial caution steadily declined. This was pre ports them from the Tuamotus, as does Randall sumably due to a growing familiarity with the (1955) from the Gilbert group. The author situation. Nevertheless, a month of testing found them abundant at Palmyra and they were passed before the blacktips took the bait with the only species of sharks consistently seen in out hesitation. Once released, the attack of the the lagoon at Eniwetok during the present in blacktip was very fast and aggressive. vestigation. The grey was a notably bolder species. Al- Feeding Behavior of Sharks-HoBSON 173 though a blacktip often approached rapidly in took fish from the spears of divers before the response to splashing at the surface, it then gen fish could be removed from the water. Even in erally reversed direction and retreated just as these instances, the slow, deliberate actions of rapidly if the investigation of the splashing this shark did not give the impression of being brought it into an encounter with a human. In a threat to the diver. This impression may have the same situation, the grey ordinarily reacted been an illusion based on the sluggish behavior to the encounter with a human by continuing of the animal. On one such occasion a 6-ft the approach to approximately 5 yd from the whitetip bit the fish in half and then made a human, at which point it would veer aside and slow pass at the diver holding the other end of circle with an apparent cautious interest. If no the spear. This appeared to be a warning pass further stimuli were introduced, the grey moved at a potential competitor rather than an active on. arrack. Such warning passes were noted on sev Blacktips and greys showed a marked increase eral occasions directed at grouper or snappers: in excitement when feeding in numbers. This which approached a bait that was under attack phenomenon, generally referred to in extreme by a whitetip. Similar behavior was noted in cases as the feeding frenzy, has been observed greys and blacktips. Tester (personal communi in many species of sharks. In the blacktips and cation) observed a small blacktip pursue a greys, the presence of more than one shark ap grouper of comparable size from a bait which peared to lower the threshold for the release of had been placed on the bOttom in shallow water feeding behavior. near Aniyaani Island. In this case, the blacktip Whitetips did not show this group effect. showed considerably more than a warning pass Even when feeding in numbers,:members of as it actively pursued the grouper among a'con this species responded individually and without centration of small coral heads. We did not ob a notable increase in excitement. The whitetip serve these sharks exhibiting aggressive behav seemed to be relatively unresponsive to many ior toward members of their own species. One of the stimulus sltuations which elicited a sharp observation made on numerous occasions might reaction in the grey and blacktip. There was at first glance seem to oppose this view. When little overt response seen in this shark when en a bait which was too large to be immediately countering a human in the water for the first swallowed was presented to a group of feeding time. In this situation we did not see the curi greys or blacktips, the shark which succeeded osity frequently exhibited by the greys, nor in taking the bait would invariably swim rapidly the start and rapid flight usually exhibited by away from the area, shaking its head vigorously the blacktip.

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