birds.) A school may intermittently sound to greater depths after feeding actively for a while. 

 This is generally shown by the birds, which stop "working" whenever the school sounds. Occa- 

 sionally, when a school is completely lost, the birds rest on the surface of the water. Such 

 resting birds are believed to signify fish at greater depths. 



Table 2. --Composition of bird flocks on two CHARLES H. GILBERT scouting cruises 



— None in southern part of area. 



In addition to the terns and shearwaters, the frigate bird ( Fregata minor ), which usually 

 does not flock, is also a species important to the Hawaiian fishery. One to five of these birds 

 frequently hover high over a flock of terns and shearwaters, and the fishermen believe that their 

 presence indicates a good-sized skipjack school which can be successfully fished. In any event 

 these large (wing spread 5 to 7 feet), high-flying birds can be seen great distances, and on several 

 occasions we have discovered low-flying flocks after noticing a high-flying frigate bird. 



In addition to the flocks of oceanic birds which were obviously feeding or "working", some 

 migrating flocks were seen. At the two westernmost stations on the November cruise (SMITH 

 cruise 24) a total of 10 tightly clustered flocks were sighted flying near the sea surface. They 

 were all headed about southwest at carefully estimated speeds of 40 to 60 knots. None were cap- 

 tured or seen closely enough to establish positive identification, but they were tentatively identified 

 as a species of shearwater other than the common wedge-tailed one. These flocks, of course, 

 were not included in our counts of "working" flocks. 



5/ 

 EVALUATION OF VESSEL SCOUTING- 



Distance and Bearing of Flocks from Vessel 



Our fishermen, who are accustomed to looking for bird flocks, can spot them as much as 

 4 miles from the vessel, and even without the assistance of binoculars they can see birds 2 to 3 

 miles away. However, on GILBERT cruises 11 and 13 the vast majority of the flocks were first 

 seen within 2 miles of the vessel and many were not seen until a half nnile or less from the vessel 

 (table 3), 



The relation among the nunnbers sighted at the various distances when first seen provides 

 an estinnate of the efficiency of sighting. We have in table 3 the actual number seen in each suc- 

 cessive, concentric, half-mile-wide, semicircular segment of the scanning area ahead of the 

 vessel. If we trsinsform these to the number per unit area (a square mile is convenient) and ima- 

 gine that the unit area is approached repeatedly, then we have a series showing the number of 



— Most of this analysis was prepared immediately after GILBERT cruise 13 in order to 

 provide a basis for planning SMITH cruises 22, 23, and 24. On these latter cruises only 94 more 

 flocks were seen, which add little to the evaluation of many factors affecting the scouting. Con- 

 sequently the SMITH cruises are considered only where the data make a significant contribution. 



