MAJOR: PREDATOR PREY INTERACTIONS IN FISHES 



tions at different times of the year and under 

 different environmental conditions. 



BEHAVIOR OF 

 FREE-LIVING SILVERSIDES 



Between sunrise and sunset each day hundreds 

 to thousands of individual silversides could be 

 observed in large, often elongated, schools along 

 Lilipuna Pier and other structures over shallow 

 reefs, along the edge of reefs, and in quiet pro- 

 tected waters such as the HIMB Lagoon. At times 

 the silversides remained in the shadow of struc- 

 tures or overhanging vegetation, rarely venturing 

 into sunlit water. The schools were located just 

 under the surface of the water, with individuals 

 often forming single or multitiered layers. The 

 schools as a whole were largely stationary and in- 

 active except for the occasional individual that 

 darted out from and immediately returned to a 

 school. These individuals appeared to be feeding, 

 snapping at objects which I could not see when 

 they left the school. While in the large inactive 

 schools, individuals about one-half to two body 

 lengths apart were randomly oriented to one an- 

 other. However, upon the approach of a predator 

 or potential predator, or when attacked, the indi- 

 viduals rapidly became polarized, often less than a 

 body length apart as the school maneuvered about 

 the predator) s) in well coordinated patterns. 



When a predator slowly approached a school of 

 silversides it frequently penetrated into the 

 school. However, as the predator moved into and 

 through a school, the silversides split into two or 

 more smaller groups which passed around to the 

 sides of the predator to reunite behind and along 

 the path just traversed by the predator. This 

 maneuver resulted in the formation of a void or 

 halo of clear water around the entire predator as it 

 moved through the school. This halo was esti- 

 mated to average about one to two predator body 

 lengths in width in any direction from the 

 predator. Similar behavior has been reported and 

 illustrated by Breder (1959), Nursall (1973), and 

 Radakov (1973). When a predator actually at- 

 tacked, it usually dashed at high speed toward an 

 individual in or near a school or into a segment of a 

 school. When attacked, individuals in the imme- 

 diate area of the predator jumped out of the water 

 as they radiated out and away from the path of the 

 predator. In a larger school, silversides at increas- 



ingly greater distances from the attacking pred- 

 ator jumped less, the jump(s) grading into evasive 

 swimming; and in some instances, little or no 

 initial response was made by individuals some 

 distance from the predator. 



As jumping silversides reentered the water they 

 realigned with other silversides that had jumped 

 or evaded by swimming. At the same time there 

 was a general, though somewhat belated, move- 

 ment of individuals around into the wake of the 

 rapidly moving predator. When an attack was 

 prolonged, as when a predator chased an indi- 

 vidual or small group of silversides, a large school 

 often formed a number of smaller schools, which 

 occasionally coalesced later. Frequently, jumping 

 and/or evading individuals or segments of the 

 attacked school joined with one or more other 

 schools which were usually nearby but unaffected 

 by the predator(s). 



When a predator, such as a barracuda, attacked 

 from a horizontal direction, the silversides usu- 

 ally had a strong lateral component to their 

 jumps. Such jumps usually occurred at a shallow 

 angle just above the surface and less than 45° to 

 the surface. When attacked from directly below, 

 initial jumps tended to have a somewhat more 

 vertical than horizontal component, being greater 

 than 45° to the water's surface. Distances covered 

 during single horizontal jumps were not mea- 

 sured, but may have been as great as 5-10 times an 

 individual's body length; several meters were 

 spanned during a series of jumps. 



When more than one predator simultaneously 

 approached or attacked a school of silversides, 

 evasive maneuvering and jumping became con- 

 fused. The more rapidly increased numbers of 

 predators approached or attacked, the more 

 "disorganized" the silverside's evasive response 

 appeared to become. 



In Kaneohe Bay the most common diurnal 

 predators observed attacking and chasing silver- 

 sides were barracuda, Sphyraena barracuda; blue 

 jack, Caranx melampygus; leatherjacket, Scom- 

 beroides lysan; and lizardfish, Saurida gracilis. 

 Needlefish, Tylosurus sp., were also observed near 

 silverside schools, but attacks were not seen. 

 During the day, and particularly during the 

 evening twilight period, the jack, Caranx ig- 

 nobilis, may also have been a predator. This jack 

 readily attacked silversides in field and cement 

 enclosures. Recently ingested silversides were 

 occasionally found in the stomach contents of 



417 



