Browsing by Author "Merrett, Nigel Robert"

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    Comparative morphometry and breeding in the family istiophoridae in the equatorial western Indian ocean
    (university of Dar es Salaam, 1968) Merrett, Nigel Robert
    A tuna longline survey of billfish (family Istiophoridae) has been carried out from latitude 00° to 13°S, and from the coast of eastern Africa to longitude 45°E. during this survey, From September 1964 to October 1967, 265 specimens of istiophorids were caught, consisting of: 77 sailfish, Istiophorus gladius (Bloch) 1793; 156 striped marlin, Tetrapterus auday Philippi 1887; 7 shortbill spearfish, Tetrapterus angustirostris Tanaka 1914; 17 blue marlin, Makaira nigricans Lacepede 1803; and 8 black marlin, Makaira indica (Cuvier) 1831. On the basis of morphometric analyses involving the pelvic, dorsal and anal fins together with vertebral and fin ray counts, the co-specificity of the istiophorid fish in the equatorial western Indian Ocean with those from the Pacific Ocean evidence provides a useful picture in the consideration of co-specificity, it is conclusions on population structure. Distribution and seasonal abundance of the species under review are discussed in terms of catch per 100 hooks. Distribution is considered by depth range and degrees latitude, and seasonal abundance by monsoon periods. The evidence indicates that I.gladius, T.audax and M. indica are more abundant during the N.E. monsoon. The abundance of T. angustirostris and M. nigricans is shown to increase in the S.E. monsoon. It is shown that I. gladius and T. audax. For the remainder, however, too few data were available for similar meaningful computations. Size composition of the catches is shown by the analysis of length and weight frequencies, and preliminary estimayes of year classes present in the catches of T. audax are given. The sex ratio of the species is examined. The overall catches of I.gladius indicate a significant preponderance of males, although when considered by seasons this is not so. A significant preponderance of males, although when considered by seasons this is not so. A significant majority of female T. audax occurs both in the overall catches and those during the S.E. monsoon, but during the N.E. monsoon the ratio is 1:1. No reliable estimate is it present possible for the other species. The morphology of the urino-genital system is described for all species and it is shown to be uniform except for T. angustirostris. The gonads of this species are shown to be Y-shaped rather than bilaterally symmetrical as in the other istiophorids. The histology of the gonads is described and is related to the spermatogenic and cogenic cycles. 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 from the survey area. The relative gonad weight is also considered as a means of assessing the stage of godan development. This ratio is shown to have a significant correlation with maturity in females, but not in males. Evidence of godan examination indicates that no species spawns internsively in the area. From the same evidence it is concluded that the seasonal increase in istiophorid numbers in the survey area is primarily due to a feeding migration, rather than to breeding activity. The size at first maturity of the species investigated is considered and provisional observations made. It is emphasized that information is sparse and that further analysis must involve more detailed information. The fecundicy of females I. gladius, T. audax and T. angustirostris is examined by estimation from the ovaries of gravid specimens. It is suggested from the evidence that this increased with size in individuals of the same species.