Aspects of the biology of billfish (lstiophoridae) from the equatorial western Indian Ocean*

Abstract

A tuna longline survey of billfish was carried out in the equatorial western Indian Ocean during 1964‐67, in which 265 specimens of five species (Istiophorus platypterus, Tetrapterus audax, T. angustirostris, Makaira indica and M. nigricans) were caught. The comparative morphometric evidence obtained is used in confirming the conspecificity of these istiophorids with Pacific Ocean forms. However, the insufficient depth and detail of the data allow only preliminary observations on populational differences. The distribution and seasonal abundance of the species are discussed in terms of percentage catch rate. The weight‐length relations of the species are examined. The size composition of the catches is shown by the analysis of length and weight frequencies, and tentative estimates of the year classes present in the catches of T. audax are given. An examination of the sex ratio of the species is made. Gonad maturation indices for males and females are established, based on both macroscopic and histological evidence. The indices are used to study breeding in the species caught in the survey area. Relative gonad weight is also considered as a means of assessing the stage of gonad development. The results of the gonad examination indicate that little spawning occurs in the area, and it is suggested that seasonal abundance of certain species is related to feeding. Preliminary observations on the size at first maturity are made, and the fecundity of female I. platypterus, T. audax, and T. angustirostris is examined.