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The life-history ecology of Platycephalus bassensis and Nemadactylus macropterus


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Jordan, AR 1998 , 'The life-history ecology of Platycephalus bassensis and Nemadactylus macropterus', PhD thesis, University of Tasmania.

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The ecology of all life-history stages of two species of demersal fish, sand flathead
(Platycephalus bassensis) and jackass morwong (Nemadactylus macropterus) found
in coastal and continental shelf waters of southern and eastern Tasmania was
investigated to determine the spatial and temporal patterns of spawning distribution
and larval transport, recruitment, abundance and distribution, and size and age
composition. The seasonal and interannual variations of the hydrography of shelf
waters are described and the influence of such variability on the life-history stages
Spawning in P. bassensis occurred for an extended period of up to six months
between October and March in estuaries, coastal embayments and shelf waters of
southern and eastern Tasmania. The distribution of spawning fish, larvae and
patterns of hydrography indicate that spawning on the shelf is primarily inshore.
Larval development of P. bassensis is described. Larvae are concentrated in midwater
which retain larvae inshore as cross-shelf subsurface currents are
predominantly onshore.
Spawning in N macropterus occurred between early January and late April in midand
outer-shelf waters. Ichthyoplankton data indicate N macropterus larvae are
concentrated in surface waters, with few larvae caught during subsurface sampling of
shelf waters over three years, despite large interannual differences in the extent of
vertical mixing and stratification. The surface distribution of larvae appears to be a
strategy to maximise offshore transport as movement of surface waters of the shelf
are generally offshore. However, large interannual variations in the influence of
subtropical and subantarctic waters on the shelf are described and the influence of
such variations on larval transport assessed.
Significant seasonal and spatial variations in abundance of mature P. bass ens is were
evident, the variations attributed primarily to the seasonal movement of fish between
shelf and nearshore waters. Overlying the seasonal trends in abundance were
interannual variations that were at least au order of magnitude in difference. Mature
P. bassensis were generally more abundant on the inner-shelf, with little evidence of
size-structuring with increasing depth. Settlement occurred over au extended period
in summer with juveniles showing a strong preference for unvegetated habitats in nearshore waters, compared to beds of the seagrass, Heterozostera tasmanica.
However, mature P. bassensis in nearshore waters showed no preference between
vegetated and unvegetated habitats. There is evidence of a seasonal movement of
these nearshore fish out onto the shelf close to the size at maturity.
In contrast, N macropterus on the shelf showed size-structuring between depths and
regions, with juveniles dominating the inner-shelf in both regions surveyed, while the
mid- and outer-shelf of the east coast was dominated by mature fish. Settlement
from the pelagic post-larval phase occurred in spring and early summer at between 7
to 9 em and 9 to 12 months old. Storm Bay appears to be principally a nursery are;t
for the species with migration from the region occurring upon maturity. In addition,
the size-structuring by depth was one of the main factors attributed to the significant
variations in abundance across the shelf. Significant seasonal variations in
abundance were also apparent, which is attributed to the seasonal movement of fish
from south-eastern Tasmanian shelf waters.
The age, growth and spatial and interannual trends in age composition of
P. bassensis and N macropterus were examined using transverse sections of sagittal
otoliths. The first annual increment was defined in both species by examination of
the progression of otolith radius and length of the 0+ cohort. Sex specific growth
curves are presented for both species. Maximum ages of P. bassensis was 17 years
for males and 13 for females, while N macropterus reached 41 years for males and
30 for females. The age composition of both species was dominated by 4 to 7 years
olds with considerable recruitment variability evident with a strong year-class in
1986 for P. bass ens is and in 1988 for N macropterus. The relationship between the
life-history strategies of P. bassensis and N macropterus and recruitment variability
is discussed.

Item Type: Thesis - PhD
Authors/Creators:Jordan, AR
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Copyright 1998 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
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