Determination of the biological parameters for managing the fisheries for Mulloway and Silver Trevally in Western Australia

The biological data required by fisheries managers to develop plans for conserving fish stocks have been obtained for Mulloway and Silver Trevally in Western Australian waters. The first species is commercially and recreationally important and the second is caught in large numbers by recreational fishers. We have produced, for managers in the Department of Fisheries Western Australia, data on crucial aspects of the biology of Mulloway and Silver Trevally. These include data on habitats, spawning periods, size and age compositions, growth and reproductive biology in the northern and southern parts of the distribution of these two species on the west coast of Australia. Although all of the results are relevant to developing management plans for conserving the stocks of these two important species, the following are of particular relevance. The females and males of Mulloway typically reach first maturity at lengths of about 930 and 880 mm, respectively. These lengths, which are usually attained at 5 - 6 years in age, are far greater than the minimum legal length (MLL) of 500 mm for the retention of this species. In the case of Silver Trevally, the average length of females at first maturity is 60 mm greater than the current MLL of 250 mm and thus the females are exposed to one year of fishing mortality before they are able to spawn. Thus, managers will need to assess whether the current MLLs for the above two species, and particularly for Mulloway, are appropriate for ensuring that the stocks of these heavily-fished species are conserved. Management policies will also need to take into account the schooling behaviour of Mulloway at spawning as this results in this species becoming an easy target for fishers at a crucial stage in its life cycle

Restocking the Blackwood River Estuary with the Black Bream Acanthopagrus butcheri

The results of this study show that hatchery-reared Black Bream can be used to enhance the stock of the population of this commercially and recreationally important species in the Blackwood River Estuary in which it has become depleted. An initial trial of different stains demonstrated that alizarin complexone was particularly effective for staining the otoliths (ear bones) of Black Bream. The mark on the otoliths, produced by this stain following immersion of hatchery-reared juveniles, was still visible to the naked eye 3.5 years later. Substantial numbers of the stocked Black Bream, which were introduced into the Blackwood River Estuary, were still living at the end of 3.5 years. On average, these individuals did not grow as rapidly as those in the wild population, and unlike the wild fish, not all stocked Black Bream attained maturity by 4 years of age. However, they still grew at a rate that was greater than that in some other estuaries and many did reach maturity by 4 years of age. The Black Bream is thus a particularly good candidate for restocking an estuary as it completes its life cycle within these systems in south-western Australia and consequently any stocked fish are unlikely to move into other estuaries in this region. The ease and relatively low cost of culture of Black Bream and its hardiness and restriction to its natal estuary make the restocking of Black Bream a feasible and economically-viable proposition. This study shows that restocking provides managers with a further and viable option for countering the effects of a decline in a stock of Black Bream in an estuary

Comparisons of the growth and gonadal development of Otolith-stained, cultured black bream, Acanthopagrus butcheri, in an estuary with those of its wild stock

Acanthopagrus butcheri was cultured from broodstock from a southwestern Australian estuary in which its abundance had declined. After marking their otoliths with alizarin complexone, the cultured juveniles were released into this estuary at 6 months of age. Regular sampling demonstrated that this stain persisted throughout the next 3.5 years and that, during the last two of those years, cultured fish dominated the black bream assemblage. Cultured fish grew slower than wild fish, with females reaching total lengths of about 182, 220, and 243 mm at 2, 3, and 4 years, respectively, compared with about 199, 248, and 286 mm in wild fish. However, cultured A. butcheri grew faster than wild A. butcheri in most other estuaries studied. By 3 years of age, essentially all wild fish, but only some restocked fish, had matured. Cultured females matured at a similar length but at an older age than wild females, and cultured males matured at both a greater length and older age than wild males. We conclude that cultured A. butcheri can be used to increase the abundance of a depleted stock, but the reason for the slightly reduced performance of cultured fish should be elucidated and overcome