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Murdoch University (2003)

Biology of two species of sparid on the west coast of Australia

Sybrand Alexander Hesp

Titre : Biology of two species of sparid on the west coast of Australia

Auteur  : Sybrand Alexander Hesp

Université de soutenance : Murdoch University

Grade : Doctor of Philosophy PhD 2003

Résumé
Various aspects of the biology of the tarwhine Rhabdosargus sarba and western yellowfin bream Acanthopagrus latus were studied. The studies on R. sarba have focused on populations in temperate coastal marine waters at ca 32 degrees S and the lower reaches of an estuary (Swan River Estuary) located at the same latitude and in a subtropical embayment (Shark Bay) at ca 26 degrees S, while those on A. latus were conducted on the population in the latter embayment. A combination of a macroscopic and histological examination of the gonads demonstrated that R. sarba is typically a rudimentary hermaphrodite in Western Australian waters, i.e. the juveniles develop into either a male or female in which the ovarian and testicular zones of the gonads, respectively, are macroscopically undetectable. This contrasts with the situation in the waters off Hong Kong and South Africa, in which R. sarba is reported to be a protandrous hermaphrodite. However, it is possible that a few of the fish that are above the size at first maturity and possess, during the spawning period, ovotestes with relatively substantial amounts of both mature testicular and immature ovarian tissue, could function as males early in adult life and then change to females. Although R. sarba spawns at some time between late winter and late spring in Western Australia, spawning peaks later in the Swan River Estuary than in coastal, marine waters at the same latitude and Shark Bay, in which salinities are always close to or above that of full strength sea water, i.e. 35 0/00. While the males and females attain sexual maturity at very similar lengths in the Swan River Estuary and Shark Bay, i.e. L50s all between 170 and 177 mm, they typically reach maturity at an earlier age in the former environment, i.e. 2 vs 3 years old. Thus, length and consequently growth rate influence the timing of maturity rather than age. During the spawning period, only 9 % of the fish caught between 180 and 260 mm in nearshore, shallow marine waters had become mature, whereas 91 % of those in this length range over reefs were mature, indicating that R. sarba tends to move offshore only when it has become physiologically ready to mature. The L50s at first maturity indicate that the current minimum legal length in Western Australia (230 mm) is appropriate for managing this species. Oocyte diameter frequency distributions, stages in oocyte development, duration of oocyte hydration and time of formation of post-ovulatory follicles in mature ovaries of Rhabdosargus sarba in the lower Swan River Estuary (32 degrees 03’S, 115 degrees 44’E) were used, in conjunction with data on tidal cycles, to elucidate specific aspects of the reproductive biology of this sparid in an estuarine environment.

Mots clés : Tarwhine ; Rhabdosargus sarba ; western yellowfin bream ; Acanthopagrus latus

Présentation

Page publiée le 17 septembre 2004, mise à jour le 10 juillet 2017