Sexual Difference in the Migration Pattern of Blue Marlin, Makaira nigricans, Related to Spawning and Feeding Activities in the Western and Central North Pacific Ocean

Abstract

The reproductive condition and stomach contents of blue marlin, Makaira nigricans Lacepede, 1802 (n = 645), were quantitatively investigated in three different regions of the North Pacific Ocean between 2003 and 2009. Males strongly dominated (females:males = 34:439) in Region III (4°N-21°N, 131°E-154°W) throughout the year, and eight females (28%) had ovaries in the maturing or spawning stage. Although the sampling months were limited to September-November in Region II (18°N-32°N, 171°W-140°W), the sex ratio was more similar (females:males = 28:26) and there was no evidence of spawning. Only females (n = 100) were observed in Region I (33°N-36°N, 135°E-140°E) from July to September, the main season when blue marlin occur off the coast of Japan, and no females had ovaries in the maturing or spawning stage. Stomach-content analysis revealed that the feeding intensity of females was higher in Region I than in the other two regions. These results suggest that blue marlin prey items may be more abundant at non- spawning areas in Region I, to which female blue marlin migrate for feeding. In contrast, lower feeding intensities and evidence of spawning in Region III suggest that blue marlin prey may be scarce in spawning areas. Male blue marlin tend to remain and wait for females in spawning areas, forgoing a feeding migration.