ESTIMATING MAXIMUM FISHING DEPTH OF LONGLINE GEAR 

 WITH CHEMICAL SOUNDING TUBES 



By 



Joseph J. Graham 



Fishery Research Biologist 



Pacific Oceanic Fishery Investigations 



Honolulu, T. H. 



and 



Dorothy D. Stewart 



1/ 



Longline gear of the type employed in this 

 study was developed by Japanese fishermen to 

 capture fish swimnning at considerable depths 

 below the surface of the ocean (Shapiro 1950). 

 During the past 5 years this gear has per- 

 mitted an expansion of the Japanese high seas 

 tuna fishery, particularly in the South Pacific 

 and Indian Oceans. The longline has also been 

 used with successby American fishery research 

 organizations— to ascertain the abundance of 

 oceanic tunas. 



Attempts to determine the fishing depths 

 of longline gear have been made by a number 

 of investigator s using various methods. Murphy 

 and Shomura (1955) and associates made echo- 

 sounding measurennents of the depth of the 

 deeper portions of the longline with a Bendix 

 depth recorder. BuUis (1955) by u s e of the 

 Echograph depth recorder obtained excellent 

 traces of entire baskets of the mainline. Such 

 measurements are difficult to obtain with con- 

 sistency because of the large number of varia- 

 bles involved such as the state of the sea, the 

 type of equipment, experience of the operator, 

 condition of the equipment, tuning, etc. Murphy 

 and Shomura (1955) also determined distances 

 between buoys with the use of radar. A conn- 

 parison of theoretical depths, derived from 

 the buoy distances, withthose obtained 



— Fishery Research Biologist, Ocean 

 Research, U. S. Fish and Wildlife Service, 

 Stanford, California (formerly Fishery Aid, 

 U. S. Fish and Wildlife Service, Honolulu, 

 T. H. ). 



— The California Department of Fish and 

 and Game and the following Bureau of Conn- 

 mercial Fisheries organizations: Branch of 

 Exploratory Fishing and Gear Research, Divi- 

 sion of Industrial Research and Services, 

 Pascagoula, Miss., and Portland, Maine, and 

 the Pacific Oceanic Fishery Investigations, 

 Honolulu, T. H. 



simultaneously with the Bendix, showed a low 

 correlation; the Bendix readings being shallower 

 than the theoretical. The authors suggest that 

 this low correlation was caused by the lack of 

 precision in measuring line depth and buoy in- 

 tervals, and also by the influence of environ- 

 mental factors such as currents which might 

 have caused shearing and thereby shoaling of 

 the gear. Previous findings (Murphy and 

 Shomura 1953), secured by u s e of pressure 

 gauges attachedto the deeper hooks of the long- 

 line, showed that the gear was most likely 

 limited in i t s penetration of the thermocline, 

 which was probably the boundary between the 

 moving surface water and the stable deeper 

 water, at least in the equatorial region. The 

 use of pressure gauges was abandoned by 

 Murphy and Shonnura because of troublesome 

 technical difficulties inherent in the mechanism. 



Yoshihara (1954) gave the problem a 

 completely theoretical treatment by deriving 

 depths of individual hooks through a mathennati- 

 cal analysis. His analysis, though exacting, 

 was based on assumptions that certain con- 

 ditions were stable; i.e., that buoy distances 

 were uniform throughout a set of gear and that 

 the action of currents and winds upon the gear 

 did not vary. 



Finally, chemical sounding tubes, which 

 give a measure of maximum fishing depth, have 

 been used by various research agencies and 

 investigators (Anonymous 1940, Yoshihara 1954, 

 Anonymous 1955, Shomura and Otsu 1956, 

 Iversen and Yoshida 1957). The discussion 

 which follows is based on the results obtained 

 by the use of chemical sounding tubes by 

 Pacific Oceanic Fishery Investigations (POFI) 

 in conjunction with studies of the albacore tuna, 

 Germo alalunga (Bonnaterre), in the central 

 North Pacific Ocean. We found that the sounding 

 tube recorded the maximum depth of the longline 

 accurately and showed that the unit or basket of 

 gear was skewed from a catenary. However, 

 the skewness was not related to the environment 



