subpopulation of the Atlantic fish may remain at the 

 Cape during winter on account of the rich food avail- 

 able. The pattern of distribution in summer, how- 

 ever, suggests that there is limited scope for genetic 

 interchange between the two populations. This adds 

 support to the concept of only one worldwide 

 species of blue marlin, but with certain features of a 

 clinal nature. The possibility that the length of the 

 pectoral fin in T. angustirostris varies as a cline 

 across the Indo- Pacific has been advanced (Penrith, 

 1964; Merrett, 1971). It is possible that the degree of 

 development of the lateral line system in the blue 

 marlin is similar, but more marked, since the geo- 

 graphic range is greater, and the Cape of Good 

 Hope, while not a barrier to this species, probably 

 tends to minimize the degree of genetic interchange, 

 and thereby accentuates the development of minor 

 differences. 



The catch rates for the category white/striped mar- 

 lin for summer and winter are shown in Figure 4. 

 From the catch statistics the two species cannot be 

 separated. In view of the records from the same 

 source of black marlin in the Atlantic it must be 



assumed that occasional striped marlin will also 

 occur in the Atlantic. In summer it can be seen that 

 the Atlantic fish (white marlin) are present all down 

 the west coast, and in the southwest Indian Ocean 

 (striped marlin) high catch rates are general. In 

 winter there are still fish east of long. 20°E but the 

 catch rates have dropped, whereas west of this point 

 the fish have disappeared and are present only in 

 small numbers north of lat. 30°S. Although the dis- 

 tribution for autumn is not shown, it is essentially the 

 same as winter. This confirms the results of the 

 much more limited local South African fishing, 

 namely that these species are present in the Cape of 

 Good Hope area only in summer. 



Broadbill swordfish were taken by the Japanese 

 boats at all times of the year in the area. This species 

 was also common farther south than the other 

 species, being taken occasionally south of lat. 40°S. 

 Catch rates for this species were in general higher 

 than for the other species, but were apparently lim- 

 ited by the subtropical convergence. 



In the Japanese statistics the spearfishes are not 

 differentiated from the sailfish. It is not possible to 



* * \ * V * 



• • • • • • • 



• • • • V V**^ 



L «/ • • • 



\ y • • m 



\ / 



T • • ^^^ • • • • • • 



• • • • • • « 



Figure 3. — Distribution of W. nigricans around the southern tip of Africa by quarter of the 

 year. Catch rates (per 100 hooks) represented by number of dots in each 5° square 

 (one— <0. 001, two— 0.001-0.004, three— 0.005-0.009, four— 0.01-0.04, five— 0.04-0.1, 

 six— >0.1). 



180 



