Canthidermis macitlalus also was a schooling spe- 

 cies and tended to be abundant in the collections in 

 which it occurred. This species did not show the 

 limited temporal and regional abundance described 

 for juvenile Chroynis, but the distribution of Can- 

 Ikklerrnis appeared to extend farther to sea than 

 other species. The collection farthest from shore 

 (200 nautical miles) contained 87 Canthidermis and 

 4 Batistes polyUpis. Only Canthidermis was ob- 

 served beneath other drifting material in the same 

 area. The four B. polylrpis were located inside the 

 cavit}^ of a large bamboo stem and probably did not 

 represent a usual component of high-seas aggrega- 

 tions. Had we taken more collections from flotsam 

 drifting 100 or more nautical miles from shore we 

 feel the frequencies for Canthidermis would have 

 increased proportionately. 



Decapterus sp. ranked fifth in abundance but only 

 once dominated a collection. This species nearly 

 always schooled with Selar crumenophthalmus but 

 was less abundant in the mixed schools. Decapterus 

 was captured wiUiout Selar in only 7 of 19 collec- 

 tions. Thus whenever a large number of Decapterus 

 was taken, the number of Selar was usually larger. 

 Hence, Decapterus rarely dominated a collection. 



The use of only the numbers of individuals for the 

 determination of dominance instead of numbers and 

 weights obscured some of the relations among 

 species. Had weights as well as numbers been used, 

 Canthidermis and Psenes probably would have 

 tlominated more collections and Chromis, Pscudu- 

 pcneus, and Ahudefduf fewer. Owing to their large 

 size range and abundance, little difference would be 

 e.xpected in the values for C. cahallus and Selar. 



OBJECT SIZE 



To determine if the length or the number of fishes 

 was related to the size of the object, we recorded for 

 each collection the volume of the object in cubic 

 centimeters, the total number of fishes captured, and 

 the mean length of all fishes in the collection. Of 

 the two variables only the number of individuals in 

 the collection was obviously related to the volume of 

 the object. Collections made beneath large objects 

 tended to be larger than those taken beneath smaller 

 objects (table 4). 



Field observations indicated that the frequency 

 of occurrence of larger fishes may be related to the 

 size of the object. The largest object studied, an 

 entire tree, was too large to be encompassed by the 

 purse seine. The tree was 1 m. in diameter at the 



root section, had a trunk diameter of 0.3 m. and was 

 over 10 m. long. Associated with the tree was the 

 largest aggregation of adult fishes seen during the 

 study. There were large schools of adult Sectator 

 ocyurus, Canthidermis maculatus, Coryphacna hip- 

 purus, and Euihynnus lineatus in addition to nu- 

 merous juvenile fishes. A portion of the school of 

 adult Sectator is shown in figure 7A, and in 7B some 

 of the adult Canthidermis are shown among the 

 branches of the tree. For comparison, two groups 

 of juvenile fishes that were associated with two 

 smaller objects are pictured in 7C and 7D. Yabe 



Table 4. — Number of collections made of fishes associated with 

 objicts of three size classes and Die number of fislies these 

 collections contained 



(Collections were made in the offshore waters of Central -America in 1963] 



• Two collections omitted owing to lack of volume measurements. 



and Mori (1950) captured, by hook and line, fishes 

 associated with a tree of similar dimensions (1 m. in 

 diameter at the butt and 15 m. long). The lengths 

 of the fish of the species they captured exceeded the 

 lengths of the fish in our purse seine collections by 

 about a factor of 10. In our study, the juvenile 

 fishes that were associated with the tree were of the 

 same species and about the same size as those col- 

 lected beneath smaller objects. Thus the size of 

 the object appeared to be related to the presence or 

 absence of large or adult fishes rather than differ- 

 ences among juveniles. 



ARTIFICIAL MOORED OBJECTS 



To study the rate of recruitment of fishes to float- 

 ing materials, eight objects of various types were 

 moored near the Costa Kican coa.st for periods of 15 

 hours to 4() days. Six objects were not visited from 

 the time they were moored until the day the collec- 

 tion was made. Two balsa logs were moored in the 

 same locality at the same time and were obser\ed 

 daily until collections were made on the fifth day. 



20 



U.S. FISH .\ND WILDLIFE SERVICE 



