Relationship between reproductive biology and age composition and growth in Urolophus lobatus (Batoidea : Urolophidae)

The aim of the present study was to use samples, collected by trawling thro ughout the year along the lower west coast of Australia, to determine the r eproductive biology, size and age compositions, and growth rate of Urolophu s lobatus, a member of a speciose elasmobranch family (Urolophidae) for whi ch there was previously only a small amount of such data. U. lobatus copula tes about 3 months prior to the time when females ovulate, which implies th at sperm is stored in the oviducal gland of the female in the intervening p eriod. Gestation lasts for 10 months, which is relatively long for a specie s 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 th is species, respectively. The trends exhibited by the marginal increments o n 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 ag e this species. Maturity was attained by about 70% of females and 50% of ma les 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 typ ically first reach maturity at similar to 200 and similar to 160 mm, respec tively, and attained maximal disc widths of 277 and 237 mm, respectively. F emales reached maximum ages of 15 years from conception and 14 years from b irth, with the corresponding values for males being 13 and 12 years respect ively. Since such a large amount of growth occurs during gestation, von Ber talanffy 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 wit h elasmobranchs. The growth curves drawn through the points for the disc wi dth-at-age of the older female and male fish gave a slightly better fit whe n using, as age 0, the parturition date rather than the conception date. Ho wever, the asymptotic disc widths derived for females and males using the c onception date, that is, 241.3 and 202.9 mm, were each still only 7.6 mm le ss than those derived using the birth date. Moreover, the fit of the line d rawn 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 b ackwards extrapolation of the growth curve using the, birth date as age 0.