Abstract
Summary Sexual differences in the distribution pattern, sex ratio and the gonad condition of four istiophorid billfishes were investigated in three different tropical areas of the eastern North Pacific Ocean from September to November 2004. Sex ratios of striped marlin Kajikia audax were equal (female : male = 11 : 11) in the open-ocean area (16–18°N, 118–134°W), biased to males (32 : 62) in the near-continental area (13–16°N, 103–107°W), and biased to females (19 : 6) in the near-equatorial area (5°N, 104–120°W). Sex ratios of sailfish Istiophorus platypterus in the near-continental area and shortbill spearfish Tetrapturus angustirostris in the open-ocean area were not biased from 1 : 1 (43 : 36, 11 : 13), and that of blue marlin Makaira nigricans in the near-equatorial area was biased to females (14 : 1). Reproductively active females were found for striped marlin, sailfish and shortbill spearfish. Striped marlin showed a specific spawning area (the near-continental area), and sexual dimorphism in its temporal distribution pattern, which was presumably related to reproduction. On the other hand, the limited distribution with evidence from spawning of sailfish and shortbill spearfish imply that these species complete their life history within limited areas.
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