Fishery-Oceanographic Studies of Striped Marlin, 



Tetrapturus audax, in Waters off Baja California. 



I. Fishing Conditions in Relation to the Thermocline 



EIJI HANAMOTO 1 



ABSTRACT 



In this report, the author analyzed fishing conditions for striped marlin in waters off Baja California in 

 relation to the thermocline. The results were as follows: 



1. In subarea SW, bounded by lat. 15°-25°N and long. 115°-110°W, catch rates begin increasing from 

 about May and reach a peak between July and October. In subarea SE, bounded by lat. 15°-25°N and long. 

 110°-105°W, there appears to be a tendency for catch rates to be highest from July through October. In 

 subarea M, bounded by lat. 10°N to along the coast of Mexico and long. 105°-95°W, catch rates are highest 

 between May and July. 



2. From December through March there is good fishing in relatively narrow areas around the tip of Baja 

 California. In April, a good fishing ground appears off Manzanillo and in May this ground begins to expand 

 seaward. From June, the area of good fishing off the coast from Acapulco to Mazatlan begins to expand 

 seaward and the greatest expansion of grounds occurs off Baja California in September. In October, the 

 ground becomes narrow and is located farther east. 



3. The pattern of expansion and contraction of the shallow thermocline area coincides fairly closely with 

 the pattern of expansion and contraction of good fishing grounds. One of the factors related to this 

 phenomenon is that the formation of good fishing grounds off Baja California is considered to be related to 

 the shallow thermocline areas where there is a more abundant food supply. 



The waters off Baja California have been known 

 to be a good subsurface fishing ground for striped 

 marlin, Tetrapturus audax, ever since the Japanese 

 tuna longline fishery began fishing the area in 1963. 

 This same area is also a good surface fishing ground 

 for yellowfin tuna, Thunnus albacares, and skipjack 

 tuna, Katsuwonus pelamis. 



Although several studies have been carried out on 

 striped marlin in the eastern tropical Pacific (How- 

 ard and Ueyanagi, 1965; Kume and Joseph, 1969: 

 Shiohama, 1969), there has been relatively little 

 work done on the relationships between the fish and 

 the environment. The main purpose of this study is 

 to describe the formation mechanism of the striped 

 marlin fishing ground in this area through the exami- 

 nation of the monthly distribution of striped marlin, 

 seasonal variations in catch rates and size composi- 

 tions, and the relationship between fishing condi- 

 tions and the thermocline. 



'Kanagawa Prefectural Fisheries Experimental Station, 

 Jogashima. Miura-city. Kanagawa-pref.. Japan. 



MATERIALS AND METHODS 



In order to examine the seasonal variations in 

 catch rates of striped marlin, the data were sum- 

 marized by subareas as shown in Figure 1. These 

 subareas, SW. SE, and M, were designated on the 

 basis of similarities in trends in the monthly distribu- 

 tions of mean relative abundance of striped marlin. 



The source of data used in examining seasonal 

 variations in relative abundance (Figs. 2, 3, 4) was 

 the "Annual Report of Effort and Catch Statistics by 

 Area on Japanese Tuna Longline Fishery" for 

 1963-70 (Japan. Fisheries Agency, Research Divi- 

 sion, 1966-72). Numbers of hooks fished and fish 

 caught were summarized by month and by 5° 

 squares, and the monthly catch rate in each subarea 

 was calculated as follows: 



Catch rate in a subarea = (2C//2F/) x 100, 

 where C/ = number offish caught in the /th 5° square, 

 Fj = number of hooks fished in the /th 5° square. 



Monthly distributions of mean relative abundance 

 ( Fig. 5) were based on averages for the years 1966-70 



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