obtained by "preasure gauges" (Murphy and 

 Shomura 1953b) and by using a Bendix Echo 

 Sounder (Murphy and Shomura 1955). Neither 

 of these nn e t h o d s was satisfactory, so other 

 means have been sought. Beginning with the 

 first longline cruise of 1955, glass sounding 

 tubes were used regularly. 



The sounding tubes which we have 

 employed are 24 inches in length, 1/8 inch in 

 diameter, and are coated on the inside surface 

 with a water soluble chemical. Before placing 

 the tube in the water, one of the sealed ends is 

 broken. As the tube is lowered pressure forces 

 water into the open end of the tube, dissolving 

 the chemical adhering to the inside surface over 

 a distance proportional to the depth. In opera- 

 tion the glass tubes were placed in metal tubes 

 which were attached to the branch lines (drop- 

 pers). At the time the gear was set one end of 

 the sealed tube was broken off and the tube was 

 placed into the holder with the open end down. 

 When the gear was retrieved the fishing depth 

 was obtciined by measuring the distance from 

 the closed end of the tube to the line formed by 

 the dissolved chemical. A close correspondence 

 has been found between the depth readings of 

 sounding tubes and of bathythermographs when 

 the two instruments were lowered simultane- 

 ously to the same depth and immersed for 5 

 to 10 hours (Shomura and Otsu 1956), 



The chemical sounding tubes disclosed 

 considerable variation in the depth of the deepest 

 hooks between sets of longline in the Line Islands 

 area. The reasons for this variation are not 

 understood but by examining each source of 

 variation independently we can offer possible 

 explanations for different gear behavior under 

 different circumstances. Variation in the depth 

 of different hooks within a single basket, varia- 

 tion in depth of the deepest hooks on representa- 

 tive baskets within a set, and variation in depth 

 of the deepest hooks between stations all merit 

 exajnination. 



At some stations sounding tubes were 

 placed on different hooks of individual baskets. 

 The depths recorded for the various hooks have 

 been plotted in figure 3. (Data in Appendix 

 table 7, Charles H. Gilbert cruise 20, stations 

 13, 15; cruise 25, stations 7, 10; Comnnonwealth 

 cruise 3, station 5; cruise 4, station 11). The 

 only fish taken on these test baskets that could 

 conceivably aifect the results was a brown shark 

 caught on hook number 3 ( Commonwe alth cruise 

 4). No explanation is offered for the number 10 

 hook reading of 100 fm. It appears from the 

 lines drawn in figure 3 that there was greater 

 variation in the depth of the deeper hooks than 



HOOK NUMBER 



10 II 



Figure 3. --Depth of various hooks on single 

 baskets of longline, as determined by sound- 

 ing tubes (data in Appendix table 7, Charles 

 H. Gilbert cruise 20, stations 13, 15; cruise 

 25, stations 7, 10; Commonwealth cruise 3, 

 station 5; cruise 4, station 11). 



of the shallower hooks. A streaming of the line 

 has been suggested as the reason for the lesser 

 depths reached by the deepest hooks in the area 

 just north of the Equator (Murphy sind Shomura 

 1955) and it appears reasonable that the strong 

 currents of this region could have produced this 

 effect. These results provide an exemplifica- 

 tion of the theoretical model of the effect of 

 altering buoy distance on the depth of hooks on 

 longline gear as shown by Murphy and Shonnura 

 (1953: fig. 5, p. 8). 



An examination of the depth of the deepest 

 hooks on baskets within individual sets (table 7, 

 Appendix) in the majority of cases shows no 

 important differences among baskets. The dis- 

 tance between buoys, varied by different setting 

 speeds, can affect the depth of the mainline and 

 in turn the hook depth. Unfortunately there is no 

 s imple, reliable method of accurately determin- 

 ing the distance between buoys. Murphy ajid 

 Shomura (1955) have made some measurements 

 by radar and found that usually the buoys were 

 900-1,100 ft. apart. While we did not measure 

 this interval during the 1955-56 cruises, dis- 

 tances between the buoys in some cases theore- 

 tically should be less than this in order to obtain 

 depth readings of 90-100 fm. with baskets com- 

 posed of 2 10-fm. nnainlines, 10-fm. float lines 

 and 3-fm. branch lines. 



