species composition, dominant species, and tlie time 

 at which each species was fu'st observed. Xo ordered 

 recruitment of species was evident (table 5). 



The number of fish collected beneath inflated 

 truck inner tubes varied greatly (table 6). All the 

 tubes were identical in size and shape \vith the except- 

 tion of the tube that had 10 manila lines attached. 

 After periods longer than 5 days the number of fish 

 collected beneath the tubes did not increase sub- 

 stantially with time through 20 or more days. The 

 number of fish appears to increase rapidly during the 

 first few days and thereafter to remain at about the 

 same level. 



Because a drifting object passes through inshore 

 spawning areas, juveniles of species that spawn 

 inshore would be expected to be more abundant 

 beneath a drifting object than beneath an object 

 moored offshore. Abndefduf sa.ratilis and perhaps 

 Seriola sp. spawned inshore. Both species showed a 



Table 5. — Sj>ecies recruited to two balsa logs, A (mlumc, 0.021 

 m.' and B {volume, 0.065 /«.') moored 100 m. apart 7 nautical 

 miles from the Costa Rican coast in 196S 



' No underwater observations were made on tlie dav the log was moored 

 (Day 1). 



T.\Bi,E 6. — \umber of fish and species recruited to various ob- 

 jects moored near the Costa Rican coast in 1963 



' Attaelied to this inner tube were ten 9^-incij (19 mm.) manila lines 10 m. 

 long. AH tubes had a volume of 0.286 m.' 

 • Established at same time at same locality. 



high freciucncy in the fall collections, but neither was 

 found under the two balsa logs moored in the fall. 

 With these two exceptions, no difference existed in 

 the species composition or in the size of the individu- 

 als between populations of fishes associated with 

 moored objects and those associated with drifting 

 objects. 



BEHAVIOR 

 DISTRIBUTION AND FRIGHT BEHAVIOR 



When disturbed, nearly all species swam toward 

 the drifting object and maintained a position much 

 closer to it than when undisturbed. The fishes 

 showed tliis behavior when a school of porpoise, 

 Sknelln qnif/nani, passed a log, when four ()orpoi.se, 

 Tiu:<fiop.s sp., passed nearby, when a school of black 

 skipjack approached a log, and when a shark, Car- 

 charhinuf! azureus, swam beneath a log. The move- 

 ments of a diver, skiff, and the research vessel also 

 induced this response. The fishes rapidly habitu- 

 ated to the movement of the outljoard skiff; after 

 several passes of the skiff near the same log the fear 

 reaction no longer could be evoked. 



Most species showed a marked change in behavior 

 when disturbed. Abudcfdiif m.ratilis frequently en- 

 tered holes or crevasses on the surface of the log. 

 The kyphosids moved in and out of holes and swam 

 back and forth rapidl.v over the log so close to it that 

 their fins almost touched it. Schools of all species 

 became more compact; sometimes indi\-iduals that 

 were a part of a diffuse aggregation of several species 

 separated into monotypic schools. For example, 

 when only a few Chromi.'t ntrilohala were jiresent 

 under undisturbed conditions they remained with 

 the carangids in a loose aggregation, but when fright- 

 ened they formed a compact monotypic school. 



The fear response produced a marked vertical 

 stratification of species beneath the object. Species 

 distributed at various distances from the log, or 

 members of a common loose aggregation, separated 

 into discrete compact groups. If large numbers of 

 fishes were grouped in this manner the distribution 

 usuall.v resembled a cone, the apex of which was at 

 the underside of the object. The base was usually 

 foiTued by a large group of juvenile Sidar crumenoph- 

 Oialmiis and Dccuplcrua sp. The fish were always 

 in this ])()sition wheti the object was approached by 

 the research vessel or the skiff. It was only after 

 the skiff had been near the log for a half hour or 

 longer that the fish lost these more rigid groupings 



22 



U.S. FISH AND WILDLIFE SERVICE 



