1. Water
Quality and Coastal Hydrology. The farm will require
access to a source of fresh seawater and the capacity
to expel waste water (Following on site treatment)
to a point where short circuiting is not a problem.
Where possible the intake should be located in
a protected position, possibly on a tidal creek,
close to the mouth. It is generally an advantage
if the creek is fully flushed by the tide on each
tidal cycle so that water which has been in the
creek systems for some time, with possible low
oxygen levels is not taken in. The capacity of
the proposed outlet to dilute and assimilate the
waste water in a major consideration. Farm size
will be limited by the capacity of the receiving
waters for advection and dispersal of effluents.
There is an advantage in having an outlet on a
separate creek or a different branch of the same
creek to the intake, to eliminate the possibility
of short circuiting (The taking in of effluent).
A
creek inlet and outlet has advantages to an open
ocean inlet or outfall in that the structural works
required to give stability are less costly. Ocean
outfalls and intakes are also of greater concern
to regulating authorities.
Creeks
with steep tidal gradients are an advantage in
that the inflow and outflow are quicker causing
less concerns with captivation of effluents and
building concentrations of effluents and nutrients
within the creek system. Creeks with shallow gradients
are usually associated with wetlands which may
also create additional environmental concerns.
2. Existing
Flora and Fauna both within and around the proposed
site may be of concern. The site should be free
of habitat for endangered species. The opinion
of the local forestry agency and DEH The position
of the local authority as to tree preservation
policy should also be obtained. The DPI Fisheries
section should also be consulted as to the status
of the waterways as to Fish breeding and habitat
and any special protection that is afforded to
them.
3. Soil
Types including clay content, erosion potential
and Acid Sulfate Soil Potential. Soil type mapping
can be provided by Maddocks and Associates together
with 1:100,000 topographic mapping. The potential
for acid sulfate soils is usually linked to the
presence of mangroves although not all areas containing
mangroves are acid sulfate producing and some areas
that do not contain mangroves are acid sulfate
producing. The DPI can usually be approached for
an initial indication of acid sulphate potential
and will provide soil sampling and basic testing.
This service is usually provided at a relatively
low cost. Minimum mangrove disturbance should occur
in order to provide the required inlet and outlet.
Materials that are removed from mangrove areas
are expensive to dispose of and may have to be
placed in lime lined pits.
Soils
should generally be sandy clays with a low dispersive
nature. The sand content should be less than about
40% and the clay content should be more than 40%.
The presence of montmorillonite or similar minerals
with a dispersive nature can be assessed by the
pin hole test carried out on samples extracted
from the site. Earthworks on prawn farms generally
involves cuts of less than 1m deep and fills up
to 5m high. The required fill material volumes
for the narrow embankments are generally balanced
by broad shallow cuts over the entire pond area.
It is important to assess the soil type over the
whole of the proposed construction area to shallow
depth. The presence of underlying rock or weathered
rock at shallow depth is usually a problem as is
the presence of floaters. The presence of rock
or floaters will add to the cost of earthworks
and may be an indication of unwanted pond permeability.
Clays should be present to a depth of up to 1m
below the anticipated pond bottom. Clays which
are unduly stiff may have to be tyned and worked
in order to be compacted to form pond bottoms.
This will also add to earthworks costs. The presence
of adequately performing, existing farm dams on
the potential property is a good indication of
the presence of the right kinds of soils. Farm
dams should be inspected as should any puddles
on the site. cloudy water is generally an indication
of dispersive soils. Dispersive soils effect the
quality of the product and increase the difficulty
in treatment of effluent. Nitrogen and Phosphorus
link with fines carried in water. The extraction
of phosphorus and nitrogen will be a major concern
of the approving authorities and will have a major
financial bearing on the project.
Soils
should not be saturated for at least part of the
year. Saturated soils are difficult to compact.
It will be difficult to dry the soils when the
potential scale of the earthworks is considered.
It is an advantage if the natural moisture content
of the soils is close to optimum level as determined
by Standard methods. Earthworks will generally
be carried out by large scrapers (30 cubic yard
capacity). Wet soils will limit the period of the
year and the efficiency that these machines can
work.
Steeply
graded banks in the local creeks are a good indication
of the presence of the required soils. Sandy creeks
are a warning.
4. Topography
and Flood Levels. The site should have a gradual
grade towards the east (Assuming Queensland is
the envisaged state) to allow the ponds to drain
by gravity and avoid any necessity for costly pumping
of effluent. The grade should if possible be less
than 0.25%. The site should not have obvious high
or low areas with deviations from a selected plane
being minimal. Undulations increase the cost of
earthworks. The tidal intrusion into the site should
be shallow. If possible there should be a narrow
defined flood plane. the banks of the creek should
be sharp and all but very high tides be contained
within the creek. Large tidal movements which intrude
into the prawn farm may necessitate the requirement
for extensive erosion protection to outlet and
inlet channels. Tidal intrusion into inlet channels,
outlet channels and treatment ponds increases the potential
for predator problems.
5. Freshwater
Influences. The presence of significant creeks
or drainage paths crossing the site will cause
problems during construction and for later permanent
diversion. It should be remembered the proposed
scale of farm will cover an area of up to 5 square
kilometres and will have wall lengths of up to
1.5 kilometre. If the ponds are in a major catchment
the potential for erosion of soils in drains constructed
for diversion will greatly increase. Turbidity
will be a major concern of the local and state
authorities. During construction it will be required
to follow rigid guidelines and prior to construction
it will be necessary to submit a temporary erosion
control plan. The cost of these works may extend
to several hundred thousand dollars for a problem
site.
The
potential for collection and ponding of fresh water
close to but outside the potential site should
be possible. Optimum conditions for prawn growth
may be at levels of salinity which are lower than
ambient sea water concentration. The close proximity
of a potential storage to the prawn pond construction
area will allow the ready borrowing of materials
from the water storage for the pond construction.
6. Neighbouring
land Uses are often given priority when a conflict
is created with a new use. The proponent of the
new use is often required to provide buffer strips
and minimise the possibility of off site effects
beyond what would normally be called for. All this
adds to cost. Low order neighbouring land uses
such as grazing is an advantage. Higher order crop
production and associated insecticide and fertiliser
usage is undesirable.
7. Existing
Water Uses. Possible interference with groundwater
should be considered. Effects such as salination
are a possibility. Where a possibility for disturbance
to natural regimes exists expensive groundwater
studies may be required by the regulating authorities.
The presence of water extraction permits or bores
should be checked through the Department of Natural
Resources. The presence of any aquifers should
be investigated.
If
a natural stream is to be dammed as part of the
effort to store fresh water then the effect of
cutting of the downstream flow should be investigated.
Riparian flow is often a right of downstream users.
Where a defined stream is interfered with then
it will be required to obtain a licence from the
DNR. this will require advertising and an appeal
and objection process will apply.
8. Proximity
of Marine Parks, Fish Habitat Zones and World Heritage
Property. The positioning of a large aquaculture
project in these areas will increase the level
of environmental assessment required when compared
with a site outside such zones.
9. The
potential impact of the project on environmental
values and biodiversity. Will the construction
interfere with significant environments such as
coastal wetlands or mangrove systems.
10. Historical
Land Uses. The presence of aboriginal sites will
have to be independently assessed during the environmental
impact process. the discovery of sites may lead
to land claims. The impact of the Wik decision
should be considered. If it is required to obtain
a permit to occupy crown land for inlet and outlet
works Section 86 applications will be required.
It is also a requirement that any works at present
being considered for crown lands be above ground
level, canals can not be excavated. This requirement
of a site with marginal elevation may have the
effect of raising the pond levels by several meters
with consequent effects on the cost of earthworks.
11. Prevailing
Winds. Ponds should be able to be aligned across
prevailing winds to reduce wave set up and associated
pond erosion
FACTORS
EFFECTING GROWTH RATE AND PRODUCTIVITY
The
growth rate of prawns is effected by many variables
e.g. temperature, pond age & stocking
density. All of these factors can be taken into
account to provide an accurate prediction on the
production of the new site and then calculate the
possible profitability of the project. This can
be done using the Jackson and Wang Growth Model.
The growth model was derived from the Gompertz
model and a 6.5 year study of 48 1 Ha ponds with
a density of 32-35 prawns/m². This model shows
us how prawns react to variables and how we can
evaluate the prospects of new sites.
Temperature
has been shown to have a major effect on the growth
rates of the Paenaeus Monodon. The effect is quite
markable over the growing time of the prawns e.g.
in a pond of approximately 1 Ha the growth of prawns
reach an average of 34 grams in 180 days at 30°C
where as in the same time frame they only average
15 grams at 20°C. Temperatures of 23°C
to 30°C were found to provide the largest growth
rate. Any temperature above or below this range
caused a decrease in the growth rate of the prawns.
Another
major effect on the out come and time period of
the crops is the stocking density of the prawns
however the stocking density does not remain constant
due to mortality. The higher the mortality rate
the lower the prawn density and the higher the
consequent growth rate of the individual prawns.
e.g. at 100% survival the size of the prawns after
180 days are found to be an average of 31.6 grams
and at only 29% survival they average at 36.6 grams
after 180 days. A high mortality that occurs early
in the growing period will have an increased effect
on the growing rate of the prawns, on the other
hand high mortality late in the growing process
will have a very small effect on the rate of growth.
There is no accurate way to calculate the mortality
of a crop during the growing period it can only
be approximately calculated. The only way to accurately
calculate the mortality rate is at the time of
the harvest.
Pond
age also has an effect on the growth rate of the
prawns. The growth rate of the first few crops
is reasonably unchanging although as the ponds
age the growth rate and productivity of the ponds
can decrease but if the management of the farm
and the feeding methods and quantity are carefully
observed the growth rate of the prawns will improve
with each crop. It has been shown over the years
that improved methods and management have increased
the growth rate and production of the Paenaeus
Monodon.
Other
variables such as Dissolved Oxygen, pH and Salinity
are important factors. These are difficult to account
for because these conditions change rapidly through
the day.
Reference.
1. Jackson, C.J. and Y.Wang 1997. Modelling growth
rates of Penaeus monodon in intensively managed
ponds: effects of temperature, pond age and stocking
density.
Opportunity
No. 1
Proposed
100 Ha aquaculture (prawns) in Far North
Queensland. All approvals obtained. The project
is to be offered on the Australian Stock Exchange.
Properties are to be available by November.
Opportunity
No. 2
Proposed
250 Ha Aquaculture (prawns) North Qld.
Early stages of approval process. Expressions of
interest accepted.
