Relationship between reproductive biology and age composition and growth in Urolophus lobatus (Batoidea: Urolophidae)
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The aim of the present study was to use samples, collected by trawling throughout the year along the lower west coast of Australia, to determine the reproductive biology, size and age compositions, and growth rate of Urolophus lobatus, a member of a speciose elasmobranch family (Urolophidae) for which there was previously only a small amount of such data. U. lobatus copulates about 3 months prior to the time when females ovulate, which implies that sperm is stored in the oviducal gland of the female in the intervening period. Gestation lasts for 10 months, which is relatively long for a species that does not grow to a large size. This feature is reflected in the fact that the mean disc width at parturition (105 mm) is equivalent to as much as 44% and 52% of the asymptotic disc widths of the females and males of this species, respectively. The trends exhibited by the marginal increments on vertebral centra demonstrated that each of the translucent zones in these centra is laid down annually and that their numbers can thus be used to age this species. Maturity was attained by about 70% of females and 50% of males at the end of the 4th and 3rd years after conception, respectively, or at just over 3 and 2 years after birth, respectively. Females and males typically first reach maturity at ˜200 and ∼160 mm, respectively, and attained maximal disc widths of 277 and 237 mm, respectively. Females reached maximum ages of 15 years from conception and 14 years from birth, with the corresponding values for males being 13 and 12 years, respectively. Since such a large amount of growth occurs during gestation, von Bertalanffy growth curves were constructed using the date of conception, that is, the fertilisation date, as age 0, as is usually the case with teleosts, as well as employing the birth date as age 0, as is typically the case with elasmobranchs. The growth curves drawn through the points for the disc width-at-age of the older female and male fish gave a slightly better fit when using, as age 0, the parturition date rather than the conception date. However, the asymptotic disc widths derived for females and males using the conception date, that is, 241.3 and 202.9 mm, were each still only 7.6 mm less than those derived using the birth date. Moreover, the fit of the line drawn through the points for the disc width-at-age of fish during gestation was better using the conception date as age 0 than would be achieved by a backwards extrapolation of the growth curve using the birth date as age 0.
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