Table .5. — The monthly mean length of alhacore, and {in parentheses) the estimated percentage of the total catch from each 

 locality by American Samoa-based vessels, shown by 5 degrees of latitude in the central South Pacific — Continued 



In spite of the widespi-ead knowledge among 

 fishermen that fisli are smaller in the more north- 

 ern waters (north of lat. 15° S.), they pereist in 

 fishing these waters during February-May because 

 of their belief that the higher catch rates will more 

 than compensate for the smaller sizes of fish. Fish- 

 ing effort was concentrated in these watere during 

 the early months of both 1964 and 1965. This type 

 of selection has undoubtedly contributed to the 

 general size decrease of albacore in the landings 

 in those two years. 



SEASONAL VARIATION IN FISH SIZES 



Although we can generalize that sizes of alba- 

 core change with latitude, seasonal variation in 

 sizes within the same areas is also considerable. 

 For example, the average sizes of albacore taken 

 from the area between lat. 6° and 10° S. were 

 smallest during February-May in both 1964 and 

 1965, as compared with the rest of the year (fig. 

 18). The decrease in size may be explainable to a 

 large extent by the fact that the vessels concentrate 

 their fishing north of 10° S. where the fish are 

 smaller; this effect on fish size is further aggra- 

 vated by the coincidental presence of smaller fish 

 in the area at that particular time of the year. 



DECREASE OF OLDER FISH IN STOCK 



Albacore taken in 1964 and 1965 were shorter on 

 the average than those caught in 1963 and perhaps 

 earlier (figs. 16 and 17 and table 5). This decrease 

 in size may be attributable, at least in part, to an 

 actual decrease of older, larger fish in the stock. A 

 decrease of this magnitude might be expected 



when a stock undergoes exploitation, because in- 

 creiising fishing mortality over the yeai-s increases 

 total mortality. As total mortality increases, the 

 average age and, thus, the average size of the stock 

 must decline. 



EFFECT OF ABUNDANT YEAR CLASS ON SIZE 

 COMPOSITION 



Nmnbers of larger fish of both sexes decreased 

 from September 1962 to about May 1965, while 

 smaller fish ( <90 cm.) tended to increase (figs. 19 

 and 20). Superimposed on this general increase of 

 smaller fish was an abrupt, exceedingly large in- 

 crease beginning around February 1964. This rise 

 of apparent abundance of smaller fish suggests the 

 possibility of the entry into the fishery of an abun- 



n — ] — I — 1 — r 



JFMAMJJASONDJFMAMJJASOND 

 I . I I . I 



Figure IS. — The monthly mean length of albacore (sexes 

 combined) taken by American Samoa-based vessels in 

 the area 6° to 10° S., 170° E. to 150° W., in 19^4 and 

 1965. 



66 



U.S. FISH AND WILDLIFE SERVICE 



