were numerous. These small fish dominated the 

 summer catch, reached a peak in September, and 

 then slowly declined. By December, conditions were 

 similar to January. 



Erdman (1968) recorded weight frequencies for 263 

 males and 65 females taken in waters off Puerto Rico. 

 His data show that both males and females reach a 

 peak of abundance in September. 



Kume and Joseph (1969b) recorded the length- 

 frequency distribution for 3,595 blue marlin from the 

 eastern Pacific. The length ranged from 100 to 340 cm 

 (eye to fork) but most of the specimens fell between 

 150 and 250 cm. The frequency curve was bimodal; 

 the dominant mode occurred at about 200 cm. 



Ueyanagi et al. (1970) recorded the length composi- 

 tion of Atlantic blue marlin by sex, season, and area. 

 Their data show that in the North Atlantic the 

 predominant length for males was about 150 cm (eye 

 to fork) during May to October and about 170 cm dur- 

 ing November to April. The predominant length for 

 females was about 210 cm during May to October and 

 about 210 to 220 cm during November to April. In the 

 equatorial Atlantic the predominant length for males 

 was about 160 to 190 cm throughout the year, and for 

 females about 230 to 240 cm during May to October 

 and about 140 cm from November to April. In the 

 South Atlantic the predominant length for males was 

 about 240 cm from May to October and about 190 cm 

 from November to April. The predominant length for 

 females was about 210 to 290 cm from May to October 

 and from November to April there were two modes at 

 about 130 and 250 cm. Ueyanagi et al. (1970) data also 

 show that males were usually more numerous than 

 females in all areas and seasons. Of the 1,209 

 specimens studied by them 785 were males and 424 

 females. 



Ovchinnikov (1970) gave the size composition by 

 length and weight of 80 specimens of blue marlin from 

 the Atlantic but the material was not segregated by 

 sex. He states that the most common length ranges 

 from 180 to 220 cm and the most common weight from 

 50 to 80 kg (110 to 176 lb), but his table shows a mark- 

 ed mode at 190 to 200 cm and at 20 to 30 kg (44 to 66 

 lb). Obviously, the weights are erroneous in the table. 



Length and weight frequencies of total longline 

 catches of blue marlin from the equatorial western In- 

 dian Ocean were presented by Merrett (1971). Males 

 ranged from 150 to 200 cm in length (tip of lower jaw 

 to fork) and from 43 to 114 kg (95 to 250 lb) in weight. 

 Females ranged from 235 to 270 cm and from 191 to 

 270 kg (420 to 595 lb). 



Additional data on composition of stocks by size 

 and sex is given under 3.12 and 3.16. As already in- 

 dicated, there is no information on age composition. 



3.53 Responses to stimuli 



Environmental stimuli: As already discussed under 

 2.22 and 3.22, the blue marlin normally occur in 



waters warmer than 24°C, but it has been found at 

 surface temperatures as high as 30.5° C and as low as 

 21.7°C. 



As to responses to light, Ovchinnikov (1970) stated 

 that according to recent investigations, the epiphysis 

 and contiguous regions of the diencephalon, have a 

 very high light sensitivity in fishes. He also stated 

 that "... in all billfishes the glandular structure of the 

 diencephalon, the analogue of the epiphysis, is pow- 

 erfully developed . . . ." In the blue marlin, as well as 

 in the other istiophorids, the tissues overlying the 

 diencephalon are transluscent and admit light into 

 the brain cavity. The structures involved are simi- 

 lar to the pineal apparatus of tunas described by me 

 (Rivas, 1953). As indicated by Ovchinnikow, it is 

 assumed that the pineal apparatus of the blue marlin 

 reacts to light and may serve for orientation during 

 horizontal and vertical movements. 



There is no information available on the responses 

 of the blue marlin to mechanical and chemical 

 stimuli. 



Artificial stimuli: In the northern Gulf of Mexico 

 various natural and artificial baits are used in the 

 sport fishery for billfishes (Nakamura and Rivas, 

 1972). According to these authors, blue marlin prefer 

 striped mullet, Mugil cephalus, over ballyhoo, 

 Hemiramphus brasiliensis, and bonito strip. These 

 are the three baits most frequently used in the 

 northern Gulf where blue marlin have also been taken 

 on artificial lures (Kona head, rubber squid, etc.). In 

 taking a dead bait or artificial lure trolled at the sur- 

 face it is not known whether blue marlin are respond- 

 ing to taste, smell, color, size, shape, or action. It is 

 well known that, in the longline fishery, blue marlin 

 are taken on various types of dead bait. 



Electrical: No information available. 



4 POPULATION 

 4.1 Structure 



4.11 Sex ratio 

 See 3.16 and 3.52. 



4.12 Age composition 

 No information available. 



4.13 Size composition 



See 3.12 for size at maturity and 3.52 for length and 

 weight composition. 



The largest blue marlin from the Atlantic Ocean of- 

 ficially recorded by the International Game Fish 

 Association weighed 384 kg (845 lb) and measured 399 

 cm (157 inches) in total length with a girth of 180 cm 



