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Factors affecting the early life stages of hatchery-reared greenback flounder (Rhombosolea tapirina, Gunther, 1862)

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Hart, Piers Reid (1994) Factors affecting the early life stages of hatchery-reared greenback flounder (Rhombosolea tapirina, Gunther, 1862). PhD thesis, University of Tasmania.

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Abstract

The major parameters affecting the viability of hatchery-produced eggs, larvae and
newly metamorphosed juveniles of the greenback flounder (Rhombosolea tapirina,
Gunther, 1862), were investigated in order to design a suitable method of production.
The production of good quality eggs was considered to be a major problem requiring
further investigation. A fertilisation rate of approximately 50% was obtained with
most batches of eggs. However, it was found that the broodstock matured normally,
even in a completely artificial environment with temperature and photoperiod control.
Consistent ovulation of mature eggs was induced artificially, using 'Ovaprim'.
Fertilisation of eggs was carried out successfully both dry and in seawater. A salinity
of 35-45%0 was found to be optimal for initial fertilisation with incubation able to be
carried out in salinities ranging from 15-45%0 with no effect on subsequent hatch rates.
The optimal temperature for incubation of eggs was 9-12°C and incubation at 6 or
21°C resulted in complete mortality. The time to 50% hatch ranged from 1,089 to
1,212 degree hours, depending on the incubation temperature. Fertilised eggs were
buoyant in salinities above 27-28700. At the tail-bud stage eggs could be handled
without affecting hatch rates.
The optimal temperature for incubation of larvae at the yolk-sac stage was found to be
15°C, as this temperature resulted in the fastest growth rates and the maximum length
at complete yolk absorption. First-feeding occurred at approximately 96 hours post
hatch (day 4) at 15°C, after complete yolk absorption, but before the absorption of the
oil droplet. The 'point of no return' occurred at day 6 post-hatch, and mortality of
unfed larvae occurred at day 8 post-hatch. Rearing of the larvae to metamorphosis, was carried out successfully in black
fibreglass tanks of 3, 25 and 1601 capacity, contained in recirculating systems and
provided with light intensities ranging from 300-1,699 lux. The optimal photoperiod
for maximum growth was found to be 18-24 hours light and the optimal temperature
was 18-20°C. Culture in salinities of 15%0 resulted in a slight increase in mortality,
compared to higher salinities.
Rotifers were found to be a suitable first-feed, with enriched instar II Artemia suitable
from day 9 post-hatch onwards, when the gape height reached 0.69 mm and the larvae
were 4.7 mm in length. The larvae commenced feeding at the water surface but moved
to the tank base at approximately day 15 post-hatch at 15°C. At this time they
developed a dark colouration. They became lighter at approximately day 20 post-hatch and developed the juvenile colouration at approximately day 25 post-hatch.
Metamorphosis was complete by day 30 post-hatch at a length of 12.4 mm and weight
of 25 mg, under optimal conditions. The stomach was fully formed by day 20 posthatch.
Enrichment of live feeds with artificial diets, rather than microalgae, resulted in
considerable improvements in the growth rate but a higher incidence of
malpigmentation in some trials.
Weaning was carried out successfully from day 23 post-hatch using a 10 day
changeover period from live food to artificial food. The weaning diets tested, and the
stocking density (between 5-20 individuals/I), had no effect on survival or growth
rates. Changeover periods of 10 or 20 days had no significant effect on growth or
survival, but a 5 day changeover period resulted in poor growth. Weaning before day
50 post-hatch resulted in the highest survival rates. Larvae showing the highest growth
rates prior to weaning were easier to wean, and therefore, larvae fed with live feeds
enriched with commercial diets (containing high levels of highly unsaturated fatty
acids), showed the best weaning response. Enrichment of live feeds with algae, for the
first 15 days of larval rearing, resulted in the maximum post-weaning survival and
growth rates.
Growth rates and food conversion rates after weaning were good, and it appears that
the most valuable market size (500 g), may be attainable in a 2 year growout period.
However, there is a problem with dark pigmentation on the blind sides of most fish
and a high incidence of skeletal deformity, possibly caused by a vitamin C deficiency
at some stage in the life cycle. Early maturation of 1 year old fish may also be a
problem if it results in reduced growth rates.

Item Type: Thesis (PhD)
Keywords: Flatfishes, Aquaculture
Copyright Holders: The Author
Additional Information:

Thesis (Ph.D.)--University of Tasmania at Launceston, 1995. Includes bibliographical references (leaves 187-215)

Date Deposited: 09 Dec 2014 00:01
Last Modified: 11 Mar 2016 05:55
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