algae and plankton, to develop. The ponds were 

 not constructed to permit complete drainage and 

 proper harvesting of the fish crop. Seining re- 

 quired a rather large labor force and was not 

 very efficient since the fish tended to burrow in 

 the mud or to hide in the spawning beds (fig. 3) 



v«. ^-^^%-:^-:T^.- 



Figure 3. --Spawning beds or depressions 

 produced by male tilapia. 



place directly in the bait boxes of the sampans 

 while they were docked in Kewalo Basin, a salt- 

 water harbor. The bait-holding compartments of 

 these vessels have no forced circulation system 

 but open directly to the sea through ports in their 

 bottoms which may be plugged if desired. The 

 boxes were filled with fresh water prior to the 

 introduction of the fish. The fresh water was 

 then gradually replaced with sea water by open- 

 ing the ports in the bottom of the box and reducing 

 the inflow of fresh water. A complete exchange 

 could be produced in about 10 or 12 hours. 



In observing the fish during acclimatization 

 we saw few signs of distress until the salinity 

 reached 22 to 24%.. At this point the fish be- 

 came sluggish in their movements and tended to 

 congregate at the surface. If the salinity were 

 increased rapidly beyond this level mortality 

 was likely to occur, but if the fish were held for 

 perhaps 2 hours at this salinity they could then 

 be moved through increasing salinities to pure 

 sea water with little loss. 



Brock and Takata { 1955 ) state that young 

 tilapia may be transferred directly from fresh 

 water to water of 18%o with no ill effects. We 

 have not as yet carried out experiments to de- 

 termine the most efficient method for effecting 

 the acclimatization, but it is our belief that no 

 serious problems are involved and that the pro- 

 cess can be accomplished with little loss of bait 

 or of vessel time. 



or other depressions in the pond bottom. Fish 

 of all sizes were obtained in each haul so that 

 sorting, although greatly simplified by means of 

 the "kowari", was still a problem. A crew of 

 seven men was used in each day's seining oper- 

 ations on the No. 6 pond. At an approximate 

 wage of $15 per man-day the 436 pounds of bait 

 obtained cost $1.20 a pound for labor alone. It 

 was evident that tilapia culture for the object of 

 producing large quantities of bait-size fishcould 

 only be a profitable enterprise under more 

 efficient and more fully controlled conditions. 



ACCLIMATIZATION OF TILAPIA 

 TO SEA WATER 



Since the tilapia had been reared in fresh or 

 slightly brackish water they required a short 

 conditioning period before being subjected to the 

 high salinity of the open ocean. This acclima- 

 tization process was achieved by two methods. 

 One involved the use of shoreside tanks provided 

 withfresh- and sea-water inlets. Over a 10-hour 

 period the flow of fresh water was reduced while 

 sea water was introduced in increasing amounts. 

 In the other method, the acclimatization took 



RESULTS OF SEA TESTS 



During the summer of 1956, the effectiveness 

 of tilapia as skipjack bait was examined on 14 

 vessel-days at sea in waters off Oahu, employi ng 

 tilapia seined in the operations described above. 

 All tests were observed and directed by person- 

 nel of Hawaiian Tuna Packers and the Fish and 

 Wildlife Service. The government research 

 vessel Charles H. Gilbert was used on 2 days. 

 The remainder of the tests were conducted 

 aboard sampans of the local skipjack fleet; those 

 vessels taking part are listed in table 2. We are 

 glad to acknowledge our indebtedness to the 

 vessel owners and captains for their helpful 

 participation in these studies. 



Since it had been established by Brock and 

 Takata (1955) that tilapia could be used to catch 

 skipjack if the schools were first chummed-up 

 with nehu, it was the prime objective of these 

 present tests to determine the ability of tilapia 

 to attract and hold schools in a fishing position 

 without support from nehu. We hoped to observe 

 and compare the response of skipjack schools to 

 both nehu and tilapia when used independently. 



