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FINAL
REPORT
Comparative
fish
production
trials
in
copper
and
polymer
net
cages
in
Cahora
Bassa,
Mozambique
October
2014
Prepared
for
The
Copper
Development
Association
Africa,
Copalcor
(Pty)
Ltd.
and
Mozambezi
Fisheries
and
Aquaculture
by
Advance
Africa
Management
Services
cc.
Authors:
T.
Hecht
and
S.
Daniel
Reviewed
by
F.
Formanek

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Abstract
This
study
examined
the
comparative
efficacy
of
copper
alloy
cages
for
farming
of
Nile
tilapia
in
Lake
Cahora
Bassa.
Two
non-­‐replicated
trials
with
small
and
large
mesh
rigid
HDPE,
soft
polyethylene,
nylon
and
copper
net
material
were
undertaken
in
2013
and
2014.
The
experiments
were
undertaken
under
faming
conditions,
using
5
x
5
x
6
m
cages
with
an
effective
volume
of
125m3
.
In
all
trials
the
performance
of
the
fish
with
respect
to
weight
gain,
specific
growth
rate
and
condition
was
significantly
better
in
the
copper
cages
than
in
any
of
the
polymer
cages.
On
average,
yield
in
the
copper
cages
was
22.4%
higher
than
in
the
corresponding
polymer
cages.
The
lower
specific
growth
rate
of
the
fish
in
the
polymer
cages
was
a
consequence
aperture
occlusion
resulting
from
biofouling
by
filamentous
algae.
Aperture
occlusion
reduces
the
rate
of
water
exchange
resulting
in
lower
dissolved
oxygen
and
pH
levels
in
the
polymer
cages
relative
to
the
copper
cages.
In
some
instances
these
differences
were
statistically
significant.
Within
4
weeks
of
feeding
the
fish
in
the
cages
aperture
occlusion
in
the
polymer
cages
could
reach
levels
of
up
to
90%,
while
in
the
copper
cages
occlusion
levels
did
not
exceed
10%.
It
was
concluded
that
the
use
of
copper
cages
for
fish
production
in
a
sub
tropical
fresh
water
lake
in
comparison
to
polymer
net
pens,
has
the
following
advantages;
the
low
levels
of
aperture
occlusion,
relative
to
polymer
materials,
improves
water
exchange
and
provides
better
conditions
for
fish
growth.
The
improved
conditions
manifests
in
higher
fish
growth
rate,
better
condition
and
higher
yields.
The
alloy
material
precludes
the
use
of
predator
nets
and
maintenance
and
labour
requirements
are
reduced.

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Copper
was
needed
to
protect
the
fish
against
the
thieving
behaviour
of
these
guys
Getting
it
all
together
and
the
guys
being
artistic
with
COPALCOR
alloy
Nearly
done
and
then
out
into
the
lake.

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Introduction
The
aim
of
this
study
was
to
assess
the
efficacy
of
using
copper
alloy
mesh
cages
in
a
subtropical
lake
in
southern
Africa.
Specifically
the
study
was
designed
to
test
the
hypothesis
that
fish
growth,
and
hence
biomass
increase,
in
copper
alloy
cages
would
be
better
than
in
polymer
net
cages.
The
basis
of
the
hypothesis
is
that
the
copper
mesh
would
not
be
subject
to
bio-­‐fouling,
resulting
in
better
water
flow
through
the
net
pen
and
hence
higher
dissolved
oxygen
levels
in
the
water
column
within
the
alloy
cages,
which
would
manifest
in
superior
fish
growth.
The
growth
experiments
were
carried
out
at
Mozambezi,
a
Tilapia
farm
in
the
Chicoa
basin
of
Lake
Cahora
Bassa,
using
Nile
Tilapia
(Oreochromis
niloticus).
Lake
Cahora
Bassa
is
on
the
middle
Zambezi
River
in
Tete
Province
of
Mozambique
(15o
29’S
–
26o
00’S
x
30o
25’E
–
32o
44’E).
The
lake
was
created
in
1974
by
impounding
the
Zambezi
River
in
the
Cahora
Bassa
gorge.
The
lake
is
246
km
long
with
a
mean
width
of
10
km
and
an
estimated
shoreline
of
1,775
km.
The
surface
elevation
of
the
lake
is
314m
ASL
and
it
covers
a
surface
area
of
2,665
km
2
and
at
full
supply
level
holds
55.8km3
of
water.
The
lake
is
the
second
largest
man-­‐made
lake
along
the
Zambezi
River,
after
Lake
Kariba,
and
is
the
fourth
largest
reservoir
in
Africa
(Vostradovsky
1984).
The
lake
is
climatically
affected
by
three
seasons:
(1)
the
hot
rainy
season
from
November
to
April;
(2)
the
cool
and
dry
season
from
May
to
August
and
(3)
the
hot
and
dry
season
between
September
and
November.
The
lake
is
stratified
from
September
to
April.
Air
temperatures
range
from
a
minimum
of
14o
C
in
July
/
August
39o
C
in
October
with
the
mean
annual
temperature
between
26o
C
and
27o
C
(Vostradovsky
1984).
The
farming
of
Nile
Tilapia
in
Cahora
Bassa
is
a
recent
initiative
and
was
pioneered
by
Mr.
Kurt
Heyns,
the
owner
of
Mozambezi
Aquaculture
and
Fisheries.
There
are
no
other
aquaculture
operators
on
the
lake.
On
the
other
hand,
Tilapia
cage
culture
on
Lake
Kariba
(upstream
from
Cahora
Bassa
on
the
Zambezi
river)
is
a
well
established
industry
(AfDB
2011)
and
production
currently
exceeds
10
000
tonnes
per
annum.
Several
other
Tilapia
farms
on
Lake
Kariba
are
in
various
stages
of
development.
Once
all
farms
are
in
operation
it
is
anticipated
that
total
tilapia
production
in
Lake
Kariba
will
exceed
40
000
tonnes
of
fish
per
annum.
Cahora
Bassa
undoubtedly
has
the
same
production
potential
as
Lake
Kariba.
It
is
further
worth
mentioning
that
the
projected
production
volumes
would
by
no
means
satisfy
the
fish
deficit
in
the
region,
which
is
currently
estimated
at
around
240,000
tonnes
per
annum.
Zambia
alone
has
a
current
estimated
shortfall
of
fish
in
excess
of
70,000
tonnes
per
annum.
In
its
mission
to
increase
the
market
for
copper
products
the
International
Copper
Association
has,
since
around
1970,
been
promoting
and
supporting
the
use
of
copper
alloys
in
aquaculture.
The
principal
advantage
of
using
copper
alloys
is
that
the
release
of
cupric
ions
prevents
the
settlement
of
invertebrate
organisms
on
the
material
and
hence
is
less
prone
to
biofouling
(Dwyer
and
Stillman
2009).
Copper
alloys
are
not
immune
to
microfouling
but
colonization
of
macrofouling
organisms
is
much
restricted
(Michel
et
al.
2011).
Biofouling
impedes
the
flow
of
clean,
oxygenated
water
to
the
fish
being
cultured
and
provides
a
growth
environment
for
parasites
and
pathogens
that
can
infect
fish.
The
removal,
cleaning,
and
disposal
of
biofouled
nets
requires
care
to
avoid
adverse
impacts.

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Typical
polymer
nets
can
become
biofouled
within
weeks.
Fish
farmers
must
therefore
change
polymer
nets
frequently,
clean
the
nets
in
situ,
or
use
antifouling
coatings
to
maintain
water
flow
(Dwyer
and
Stillman
2009).
If
any
of
these
mitigating
measures
is
not
applied
then
aperture
occlusion
(Figure
1a)
can
create
very
unfavourable
conditions
for
the
fish
with
catastrophic
consequences.
Hecht
et
al.
(2012)
clearly
showed
the
resistance
to
biofouling
of
various
copper
alloys
in
comparison
to
Nylon
and
HDPE
netting
in
Saldanha
Bay,
Pemba
in
Mozambique
and
in
the
Seychelles.
They
further
concluded
that
cage
farming
in
Saldanha
Bay
would
be
greatly
facilitated
by
copper
alloy
netting.
There
are
some
interesting
benefits
when
copper
alloy
is
used
to
avoid
fouling
compared
to
antifouling
coatings.
The
main
one
being
that
it
does
not
need
recoating
periodically
and
foregoes
the
time
and
effort
of
removal,
preparation,
reapplication,
and
disposal.
The
alloy
is
also
fully
recyclable
(Michelet
al.
2011).
Other
advantages
(Dwyer
and
Stillman
2009,
ICA
2010)
of
using
copper
alloy
in
fish
cage
culture
include;
• Improved
water
flow
through
cages,
• Improved
dissolved
oxygen
levels,
reduced
parasite
load,
reduced
infections,
lower
FCR,
• Reduces
net
fouling
that
serves
as
intermediate
habitat
for
parasites
and
disease
organisms
resulting
in
healthier
fish.
• Higher
yield
as
a
consequence
of
lower
mortality
(no
stressful
net
changes;
no
stress
from
predators)
• The
material
is
strong
and
predators
cannot
cause
damage
thereby
reducing
fish
losses
due
to
predation
and
rate
of
escape
of
fish
from
cages.
• Lower
maintenance:
no
net
changes;
no
net
cleaning
• Avoid
need
for
predator
net;
avoid
antibiotics
• Reduced
environmental
impact:
can
be
made
from
recycled
materials;
can
be
recycled
after
use;
no
nets
to
dispose
of.
• Potential
for
consumer
market
positioning
as
more
environmentally
appropriate
fish
production.
Despite
the
advantages
the
adoption
of
copper
alloy
netting
in
the
aquaculture
industry
has
been
slow.
However,
since
the
development
of
chain
link
woven
brass
the
use
of
copper
net
pen
cages
has
been
gaining
momentum.
The
chain
link
woven
material
is
flexible
and
highly
suitable
for
round
and
square
cages
(Figure
1b).
Currently,
chain
link
woven
brass
nets
are
used
in
cages
on
commercial
and
experimental
farms
in
Chile,
China,
Hawaii,
USA,
Tasmania,
Korea,
Japan
and
Scotland
for
various
species
including
seabass,
turbot,
yellowtail,
cobia,
trout
and
salmon,
amongst
others.
Several
different
alloys
have
been
developed
and
are
in
use
today.

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Figure
1.
(a)
Total
aperture
occlusion
by
seaweed
of
a
net
cage
in
Algoa
Bay,
South
Africa
(Photograph:
Gert
LeRoux.
(b)
Circular
fish
cage
with
copper
chain
link
woven
mesh,
Chile
(Photograph:
Langley
Gace).
Moreover,
unlike
copper
based
dispersive
antifouling
agents
very
little
copper
is
released
into
the
environment
from
the
copper
alloys
used
in
aquaculture.
After
immersion
in
seawater,
a
protective
oxide
layer
naturally
forms
on
the
metal
that
inhibits
corrosion,
giving
copper
alloy
mesh
materials
a
working
life
of
between
5
and
10
years,
depending
on
its
chemical
composition.
At
the
end
of
its
working
lifetime,
the
material
will
have
lost
only
a
fraction
of
its
initial
mass,
and
the
remaining
metal
can
be
completely
recycled
to
produce
new
net
material
(ICA
2010).
While
copper
alloy
nets
have
been
used
in
the
marine
environment
since
the
mid
1970s
there
is
no
record
of
using
them
in
freshwater.
Bio-­‐fouling
by
filamentous
algae
in
spring
and
summer
is
a
problem
for
cage
culture
in
mesotrophic
and
eutrophic
impoundments
in
the
sub-­‐topics
and
the
tropics
(pers.
observations).
Cahora
Bassa
is
no
exception.
Nutrient
loading
from
the
70
odd
rivers
that
feed
the
lake
is
seasonal,
occurring
from
November
through
to
April,
while
aerial
loading
and
nutrient
inflow
from
Lake
Kariba
is
of
a
perennial
nature.
Fouling
in
Lake
Cahora
Bassa
is
most
severe
in
the
first
2
to
3
meters
of
the
water
column,
where
after
it
is
less
severe
and
the
intensity
of
fouling
is
greater
in
spring
and
summer
than
in
autumn
and
winter
(K.Heyns,
pers.
comm.
2013).
The
fact
that
there
is
no
information
on
the
efficacy
of
copper
alloy
nets
in
freshwater
aquaculture
and
the
high
degree
of
biofouling
in
Cahora
Bassa
provided
the
motive
for
this
work.
Material
and
methods
All
the
juvenile
fish
for
the
experiments
were
provided
by
Mozambezi.
Spawning
and
monosex
fingerling
production,
using
methyltestosterone,
takes
place
in
well
managed
ponds
and
once
the
fish
reach
5-­‐6
g
they
are
transferred
to
nursery
cages
in
the
lake.
The
fish
are
reared
for
a
period
of
6-­‐8
months
and
are
then
harvested
at
around
450-­‐550g,
which
is
the
preferred
size
on
the
local
market.
Two
experiments
were
carried
out,
in
which
performance
parameters
of
fish
in
copper
alloy
cages
were
compared
to
polymer
net
cages
(Table
1).

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Two
types
of
small
mesh
polymer
nets
were
used,
viz.
soft
polyethylene
netting
and
a
rigid
HDPE
oyster
mesh
material
with
a
mesh
size
of
12mm
in
the
square.
Large
mesh
material
consisted
of
nylon,
polyethylene
and
HDPE
oyster
netting
with
a
mesh
size
of
18
mm
in
the
square.
The
small
and
large
mesh
copper
alloy
nets
had
mesh
sizes
of
9
x
15mm
and
15
x
20mm,
respectively
(Figure
2).
The
copper
and
the
HDPE
oyster
mesh
material
were
rigid
and
this
made
it
very
difficult
to
harvest
fish
in
comparison
to
the
polyethylene
and
nylon
cages.
Copalcor
(Pty)
Ltd.,
the
manufacturer
of
the
copper
alloy,
is
currently
exploring
chain
link
woven
material,
which
makes
harvesting
as
easy
as
in
soft
polymer
net
pens.
Table
1.
The
number
of
cages,
the
net
material,
mesh
size,
and
stocking
density.
Trial
Cages
Mesh
size
(mm2
)
Start
density
(Fish/cage)
1A
1x
Polyethylene
144
12
800
1x
HDPE
144
12
800
1x
Copper
135
12
800
1B
1x
HDPE
324
16
630
1x
Copper
300
16
630
2A
1x
HDPE
144
4
035
1x
Copper
135
4
035
2B
1x
Nylon
324
4
500
1x
Copper
300
4
500
All
the
cages
were
5
x
5
x
6m
deep
with
an
effective
volume
of
125m3
(Figure
3).
The
copper
alloy
nets
were
fitted
with
a
0.75m
skirt
made
of
8
mm
anchovy
netting
between
the
top
of
the
cage
and
the
water
surface.
Figure
2.
Two
mesh
sizes
of
rigid
copper
alloy
net
material.
The
woven
material
was
manufactured
by
Copalcor
(Pty)
Ltd.
in
Johannesburg.

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Figure
3.
The
5
x
5
x
6
m
cages
used
for
the
copper
alloy
trials
at
Mozambezi
in
Lake
Cahora
Bassa.
The
experimental
protocols
were
as
follows:
pH,
temperature
in
degrees
Celsius
and
dissolved
oxygen
(DO)
in
mg/L
were
measured
daily
at
09.00.
Every
second
month
a
sample
of
100-­‐200
fish
were
caught
from
the
cages
with
a
cast
net,
weighed
to
the
nearest
g
on
a
digital
balance
and
measured
to
the
nearest
mm
(total
length)
on
a
measuring
board,
where
after
they
were
returned
to
the
cages.
Specific
growth
rate
was
calculated
using
the
equation,
SGR
(%
body
wt.
Gain
/day)
=
(Logn
Final
fishwt.(g)
—
Logn
Initial
fishwt.(g))
x
100
Time
interval
(days)
On
average,
the
fish
were
fed
at
3%
body
weight
per
day.
The
daily
ration
was
adjusted
weekly
(based
on
calculated
fish
biomass).
The
quantity
of
feed
fed
per
day
was
recorded.
When
there
was
a
shortage
of
feed
then
the
daily
ration
in
each
cage
was
reduced
by
the
same
percent.
The
final
biomass
in
each
cage
was
calculated
by
multiplying
the
number
of
remaining
fish
in
the
cage
(initial
number
minus
mortalities)
by
the
mean
final
weight
of
the
fish.
Final
density
was
calculated
by
dividing
the
final
biomass
by
125
m3
,
which
was
the
effective
volume
of
all
cages.
Biofouling
was
expressed
on
a
scale
of
1
to
4,
where
1
=
0
–
9%
aperture
occlusion,
2
=
10
–
49%,
3
=
50
–
74%
and
4
=
75
-­‐
100%
aperture
occlusion.
The
condition
factor
(K)
of
the
fish
was
calculated
using
the
equation
K
=
100(W/TLb
),
where
K=Fultons
condition
factor
(Ricker
1975),
W
=
weight
(g),
TL
=
total
length
(mm)
and
b
=
exponent
of
the
length
weight
relationship.
The
Condition
Factor
K
allows
for
a
quantitative
comparison
of
the
condition
of
fish
within
a
population
or
between
populations.
The
length
weight
relationship
of
the
fish
was
described
by
the
equation
W
(g)
=
0.00006
TL
(mm)3.2634
(Figure
4).

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Figure
4.
The
relationship
between
length
(mm)
and
weight
(g)
for
Oreochromis
niloticus
at
Mozambezi,
Mozambique
(n=1215).
The
cost
of
the
copper
alloy
material
did
not
allow
for
replication.
For
this
reason
we
ran
two
independent
trials.
In
all
instances
the
experiments
in
the
small
and
the
large
mesh
cages
could
not
be
started
simultaneously
because
of
a
lack
of
either
small
or
larger
fish.
Statistical
analysis
All
statistical
analyses
were
performed
using
StatSoft
Statistica
10
software.
Data
were
tested
for
normality
or
equality
of
variance
using
Lévene’s
test.
A
One
Way
Analysis
of
variance
(ANOVA)
was
used
to
test
for
differences
between
data
and
if
significant
differences
(p
<
0.05)
were
observed,
Tukey’s
HSD
post-­‐hoc
test
was
used
to
show
where
the
differences
occurred
(Zar,
2009).
Second
or
third
order
polynomial
equations
were
fitted
to
the
data
to
illustrate
trends
in
growth
and
or
biomass
gain.
Results
Trial
1A
(small
mesh)
The
small
mesh
cages
were
stocked
with
fish
on
15
May
2013
and
the
experiment
was
planned
to
continue
for
6
months,
until
15
November.
Due
to
a
misunderstanding
an
unknown
quantum
of
fish
was
harvested
from
the
cages
on
7
October
2013.
This
meant
that
the
mortality
and
biomass
data
with
which
to
adjust
the
daily
ration
from
then
onwards
could
no
longer
be
applied.
The
collection
of
these
data
was
therefore
stopped
and
the
analysis
for
the
performance
parameters
was
curtailed
to
the
period
15
May
to
17
September
2013.
The
remaining
fish
in
the
copper
and
polyethylene
cages
were
fed
to
satiation
on
a
daily
basis
and
their
growth
was
monitored
up
to
15
November
2013.
It
should
be
noted
that
there
was
a
significant
difference
between
the
initial
weight
of
the
fish
in
the
three
cages,
with
the
heaviest
fish
(11.2g)
in
the
rigid
oyster
mesh
cage
and
the
smallest
fish
in
the
copper
cage
(7.7g).
The
results
of
the
trial
are
summarised
in
Table
2.
Figures
5
and
6
show
the
growth
of
the
fish
in
weight
and
the
increase
in
biomass
during
the
experiment.
The
fish
length
data
show
that
the
fish
in
the
copper
cage
caught
up
with
the
fish
in
the
HDPE
cage
and
at
the
end
of
the
experiment
there
was
no
significant
difference
in
length
between
the
fish
in
the
copper
cage
and
the
HDPE
cage
and
between
the
HDPE
and
polyethylene
cage,
but
the
fish
in
both
the
HDPE
and
the

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copper
cage
were
significantly
larger
than
the
fish
in
the
polyethylene
cage.
However,
in
terms
of
weight
gain,
the
fish
in
the
copper
cage
had
gained
significantly
more
weight
than
those
in
the
HDPE
and
polyethylene
cages.
The
condition
as
well
as
the
specific
growth
rate
of
the
fish
was
also
significantly
better
in
the
copper
cage
than
in
the
two
other
cages.
The
mortality
rate
of
the
fish
in
the
three
cages
was
low
and
ranged
from
3.23
to
3.75%.
The
FCR
of
the
fish
in
all
three
cages
was
excellent
and
ranged
between
1.22
in
the
copper
cage
to
1.47
in
the
polyethylene
cage.
Table
2.
Experimental
results
for
Trial
1A.
Trial
1A
Trial
1A
extended
Begin
and
end
date
15
May
to
17
Sep
2013
to
15
Nov
Production
parameter
Poly
SM
HDPE
SM
Copper
SM
Poly
SM
Copper
SM
Initial
number
of
fish
12
800
12
800
12
800
Initial
length
(mm)
80
85
81
Final
length
(mm)
226
233
235
261
281
Length
increase
(mm)
146b
148ab
154a
182
200
Initial
weight
(g)
9.13c
11.15b
7.65a
Final
weight
(g)
241
270
289
391a
502b
Weight
gain
(g)
232b
259b
281a
381a
495b
Specific
growth
rate
(g/day)
2.58b
2.51b
2.88a
1
Biomass
gain
(Kg)
2
869
3
292
3
465
Difference
in
final
biomass
gain
compared
to
copper
cage
-­‐595
-­‐254
Mortality
(%)
3.25
3.75
3.23
2
FCR
1.47
1.3
1.22
3
Condition
factor
0.89b
0.91b
0.95a
2.59a
3.24b
Initial
density
(kg/m3
)
0.94
0.94
0.94
Final
density
(kg/m3
)
25
27
29
Different
superscripts
indicate
statistical
differences
at
P<0.05.
1
Biomass
gain
=
Final
biomass
-­‐
Initial
biomass
2
FCR
=
Dry
food
fed
/
Biomass
gain
3
Condition
factor
=
100*(Final
mean
weight/
Final
mean
length
b
),
where
b
is
the
exponent
of
the
length
weight
relationship.
The
significant
difference
in
weight
gain
translated
into
the
greater
biomass
in
the
copper
cage
at
the
end
of
the
experiment.
The
results
also
show
very
clearly
that
the
early
advantage
in
weight
of
the
fish
in
the
HDPE
cage
was
overcome
within
a
period
of
2
months.
After
4
months
the
final
biomass
in
the
copper
cage
exceeded
the
biomass
in
the
HDPE
and
polyethylene
cages
by
254
and
595
kg,
respectively.
This
is
highly
significant
from
a
farming
perspective.

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Figure
5.
Growth
of
Nile
Tilapia
in
small
mesh
copper,
hard
HDPE
oyster
mesh
and
soft
polyethylene
cages
at
Mozambezi,
Lake
Cahora
Bassa
from
15
May
to
17
September
2013.
Figure
6.
Biomass
increase
of
Nile
Tilapia
in
small
mesh
copper,
rigid
HDPE
oyster
mesh
and
soft
polyethylene
cages
at
Mozambezi,
Lake
Cahora
Bassa
from
15
May
to
17
September
2013.
The
growth
of
the
fish
in
the
polyethylene
and
copper
cages
for
the
6
month
period
from
mid
May
to
mid
November
is
shown
in
Table
2
and
illustrated
in
Figure
7.
By
mid
November
the
fish
in
the
copper
cage
were,
on
average
111g
heavier
than
those
in
the
polyethylene
cage.

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Figure
7.
Growth
of
Nile
Tilapia
in
small
mesh
copper,
HDPE
oyster
mesh
and
soft
polyethylene
net
cages
at
Mozambezi,
Lake
Cahora
Bassa
from
15
May
to
17
November
2013.
The
reason
for
the
slower
growth
rate
and
smaller
final
weight
of
fish
in
the
polyethylene
cage
was
most
likely
caused
by
aperture
occlusion
of
the
net,
resulting
in
a
reduction
of
water
flow
through
the
cage
and
hence
lower
oxygen
levels.
After
4
months
in
the
water
from
mid
May
to
mid
September
the
apertures
were
already
almost
completely
clogged
(Figure
8)
and
this
would
have
prevented
adequate
water
exchange
in
the
upper
2.5
metres.
Figure
8.
Percent
aperture
occlusion
in
small
mesh
polyethylene
(Stage
4)
and
copper
cages
(Stage
1)
after
4
weeks
(Trial
1A).
The
ambient
environmental
conditions
in
the
lake
and
in
the
cages
during
Trial
1A
are
illustrated
in
Figures
8,
9
and
10.
There
were
no
significant
differences
in
the
pH
levels
within
the
various
cages
and
between
any
of
the
cages
and
the
lake.
However
it
was
of
interest
to
note
that
the
pH
level
in
the
lake
was
always,
except
for
a
single
occasion,
higher
than
in
the
cages.
This
is
caused
by
respiratory
CO2
excretion
by
the
fish
which
results
in
a
decrease
in
pH.
Dissolved
oxygen
was
always
highest
in
the
lake
but
not
significantly
different
to
the
DO
levels
in
the
cages
(P=0.55).
Water

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temperature
declined
from
24.7o
C
in
May
to
22o
C
in
July
and
then
started
rising
again
at
the
onset
of
spring
in
September.
The
lowest
temperature
of
22o
C
is
the
norm
for
the
lake
in
July
and
August.
Figure
8.
pH
levels
in
the
cages
and
the
lake
during
the
period
15
May
to
10
September
2013.
Figure
9.
Dissolved
oxygen
levels
in
the
cages
and
the
lake
during
the
period
15
May
to
10
September
2013.
Figure
10.
Water
temperature
in
the
cages
and
the
lake
during
the
period
15
May
to
10
September
2013.

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Trial
1B
(large
mesh)
Trial
1B,
using
the
larger
mesh
copper
and
rigid
HDPE
nets
(see
Table
1),
started
on
9
July.
As
for
Trial
1A
an
unknown
quantity
of
fish
was
mistakenly
harvested
from
each
cage
in
early
October.
The
same
remedial
procedure
was
followed
as
in
Trial
1A
and
length
and
weight
data
were
obtained
in
mid
October
and
at
the
end
of
the
planned
6
month
experimental
period
on
17
November
2014.
The
results
of
the
trial
are
summarised
in
Table
3.
Figure
10
shows
the
growth
rate
of
the
fish
in
the
copper
and
HDPE
cages
from
mid
May
to
mid
November
and
Figure
11
shows
the
increase
in
fish
biomass
in
the
cages
from
mid
May
to
mid
September.
Table
3.
Experimental
results
for
Trial
1B.
Trial
1B
Trial
1B
extended
Begin
and
end
date
9
Jul
to
15
Sep
2013
to
17
Nov
2013
Production
parameter
HDPE
LM
Copper
LM
HDPE
LM
Copper
LM
Initial
number
of
fish
16
630
16
630
-­‐
-­‐
Initial
length
(mm)
154
153
-­‐
-­‐
Final
length
(mm)
226
238
267
282
Length
increase
(mm)
72a
85b
113c
129d
Initial
weight
(g)
82
82
-­‐
-­‐
Final
weight
(g)
250
302
427
516
Weight
gain
168a
220b
345c
435d
Specific
growth
rate
(g/day)
-­‐
-­‐
1.23a
1.40b
1
Biomass
gain
(Kg)
2
745
3
619
-­‐
-­‐
Difference
in
final
biomass
gain
(kg)
compared
to
copper
cage
-­‐874
-­‐
-­‐
Mortality
(%)
1.12
0.78
-­‐
-­‐
2
FCR
1.11
1.08
-­‐
-­‐
3
Final
Condition
factor
0.93
a
0.94
a
2.77
b
3.32
b
Initial
density
(kg/m3
)
12.1
12.1
-­‐
-­‐
Final
density
(kg/m3
)
33
40
-­‐
-­‐
Different
superscripts
indicate
statistical
differences
at
P<0.05.
1
Biomass
gain
=
Final
biomass
-­‐
Initial
biomass
2
FCR
=
Dry
food
fed
/
Biomass
gain
3
Condition
factor
=
100*(Final
mean
weight/
Final
mean
length
b
),
where
b
is
the
exponent
of
the
length
weight
relationship.
The
data
show
that
the
increase
in
length
of
the
fish
in
the
copper
cage
was
significantly
greater
than
for
those
in
the
HDPE
cage
by
15
September
and
at
the
end
of
the
experiment.
The
same
pattern
was
evident
for
the
gain
in
weight.
The
growth
of
the
fish
during
the
period
9
July
through
to
17
November
is
shown
in
Figure
9.
The
higher
length
and
weight
gains
are
a
reflection
of
the
significantly
higher
specific
growth
rate
of
the
fish
in
the
copper
cage.
There
was
no
significant
difference
in
the
condition
of
the
fish
in
the
two
cages
in
September
as
well
as
in
November.
Mortality
in
the
HDPE
cage
(1.12%)
was
slightly
higher
than
in
the
copper
cage
(0.78%).
The
FCRs
in
both
cages
was
nearly
the
same.

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The
50g
difference
in
the
mean
final
weight
of
the
fish
on
15
September,
which
is
reflected
by
the
significantly
higher
SGR,
resulted
in
874
kg’s
more
biomass
in
the
copper
cage
after
the
2.5
month
growth
period
from
9
July
to
17
September
2013
(Figure
10).
Figure
9.
Growth
of
Nile
Tilapia
in
large
mesh
copper
and
HDPE
oyster
mesh
cages
at
Mozambezi,
Lake
Cahora
Bassa
from
15
May
to
17
November
2013.
Figure
10.
Biomass
increase
of
Nile
Tilapia
in
copper
and
HDPE
oyster
mesh
large
mesh
cages
at
Mozambezi,
Lake
Cahora
Bassa
from
14
July
to
15
September
2013.
The
environmental
conditions
in
the
cages
and
the
lake
(Figures
11,
12
and
13)
provided
corroborating
evidence
for
the
improved
growth,
FCR
and
condition
of
the
fish
in
the
copper
cage
in
comparison
to
those
in
the
HDPE
cage.
While
there
was
no
significant
difference
in
DO
levels
between
the
lake
and
the
copper
cage,
these
values
were
significantly
higher
than
DO
levels
in
the
HDPE
cage
(p<0.004).
The
significantly
lower
DO
levels
in
the
HDPE
cage
suggests
a
much
lower
water
exchange
rate.
This
supposition
is
supported
by
the
high
level
of
biofouling
on
the
rigid
HDPE
material
shown
in
Figure
14
in
comparison
to
the
copper
mesh.
The
lower
water
exchange
rate
is
also
the
reason
for
the
significantly
lower
pH
in
the
HDPE
cage
in
comparison
to
the
copper
cage
and
the
lake,
between
which
the
difference
was
not
significantly
different.
The
temperature
during
this
trial
was
just
above
22o
C.

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Figure
11.
pH
levels
in
the
cages
and
the
lake
during
the
period
9
July
to
15
September
2013.
Figure
12.
Dissolved
oxygen
in
the
cages
and
the
lake
during
the
period
9
July
to
15
September
2013.

17. Advance
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Figure
13.
Water
temperature
in
the
cages
and
the
lake
during
the
period
9
July
to
15
September
2013.
Figure
14.
Biofouling
on
strips
of
large
mesh
rigid
HDPE
oyster
(left),
copper
(centre)
and
polyethylene
(right)
netting
suspended
in
the
water
column
from
9
July
to
16
September
2013.
Aperture
occlusion
on
the
HDPE
and
the
polyethylene
material
was
Stage
4,
while
occlusion
on
the
copper
material
was
Stage
1.

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Trial
2A
(small
mesh)
Trial
2A
began
on
15
April
and
was
terminated
on
17
August
2014.
Table
1
shows
the
specifications
of
the
two
cages.
The
results
of
the
trial
are
summarised
in
Table
4.
Figure
15
shows
the
growth
rate
of
the
fish
in
the
copper
and
HDPE
cages
from
mid
April
to
mid
August
and
Figure
16
shows
the
increase
in
fish
biomass
in
the
cages
for
the
same
time
period.
The
data
show
that
the
increase
in
length
and
weight
gain
in
the
copper
cage
was
both
significantly
higher
than
in
the
polyethylene
cage.
The
higher
length
and
weight
gains
are
reflected
by
the
significantly
higher
specific
growth
rate
of
the
fish
in
the
copper
cage.
There
was
no
significant
difference
in
the
condition
of
the
fish
in
the
two
cages
at
the
end
of
the
experiment.
Mortality
was
negligible,
at
an
average
of
around
2.5%
over
the
4
month
experimental
period.
The
FCR
in
both
cages
was
almost
the
same.
Table
4.
Experimental
results
for
Trial
2A.
Trial
2a
Begin
and
end
date
15
Apr
to
17
Aug
2014
Production
parameter
HDPE
SM
Copper
SM
Initial
number
of
fish
4
035
4
035
Initial
length
(mm)
115
115
Final
length
(mm)
200
215
Length
increase
(mm)
85a
100b
Initial
weight
(g)
34
34
Final
weight
(g)
157
201
Weight
gain
(g)
123a
167b
Specific
growth
rate
(g/day)
1.89a
2.35b
1
Biomass
gain
(kg)
481
655
Difference
in
final
biomass
gain
(kg)
compared
to
copper
cage
-­‐174
Mortality
(%)
2.31
2.66
2
FCR
1.45
1.43
3
Condition
factor
0.89a
0.9a
Initial
density
(kg/m3
)
1.1
1.1
Final
density
(kg/m3
)
5
6
Different
superscripts
indicate
statistical
differences
at
P<0.05.
1
Biomass
gain
=
Final
biomass
-­‐
Initial
biomass
2
FCR
=
Dry
food
fed
/
Biomass
gain
3
Condition
factor
=
100*(Final
mean
weight/
Final
mean
length
b
),
where
b
is
the
exponent
of
the
length
weight
relationship.

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Figure
15.
Growth
of
Nile
Tilapia
in
small
mesh
copper
and
HDPE
cages
at
Mozambezi,
Lake
Cahora
Bassa
from
15
April
to
17
August
2014.
Figure
16.
Biomass
increase
of
Nile
Tilapia
in
small
mesh
copper
and
HDPE
cages
at
Mozambezi,
Lake
Cahora
Bassa
from
15
April
to
17
August
2014.
The
dissolved
oxygen
levels
and
the
pH
in
the
cages
and
the
lake
showed
similar
patterns
(Figures
17
and
18)
as
observed
in
Trials
1A
and
1B.
Both
pH
and
DO
were
highest
in
the
lake
and
lowest
in
the
polymer
mesh
cages.
This
persistent
pattern
adds
weight
to
the
argument
that
the
lower
pH
and
DO
levels
are
a
consequence
of
restricted
water
flow
through
the
cages
caused
by
aperture
occlusion
as
a
consequence
of
the
growth
of
filamentous
algae
on
the
polymer
nets.
The
temperature
(Figure
19)
during
the
experiment
declined
from
26.4o
C
in
April
to
around
22o
C
in
August.

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Figure
17.
pH
levels
in
the
cages
and
the
lake
during
the
period
April
to
August
2014.
Figure
18.
Dissolved
oxygen
levels
in
the
cages
and
the
lake
during
the
period
April
to
August
2014.
Figure
19.
Temperature
in
the
cages
and
the
lake
during
the
period
April
to
August
2014.

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Trial
2B(
large
mesh)
Trial
2B
began
on
15
February
and
was
terminated
on
15
August
2014
(6
months).
The
specifications
of
the
cages
are
provided
in
Table
1.
The
results
of
the
trial
are
summarised
in
Table
5.
Figure
20
shows
the
growth
rate
of
the
fish
in
the
copper
and
nylon
cages
from
mid
April
to
mid
August
and
Figure
21
shows
the
increase
in
fish
biomass
in
the
cages
for
the
same
time
period.
The
data
show
that
the
increase
in
length
and
weight
gain
in
the
copper
cage
was
significantly
higher
than
in
the
nylon
cage.
The
higher
length
and
weight
gains
are
a
reflection
of
the
significantly
higher
specific
growth
rate
of
the
fish
in
the
copper
cage.
There
was
no
significant
difference
in
the
condition
of
the
fish
in
the
two
cages
at
the
end
of
the
experiment.
Mortality
was
slightly
higher
than
during
the
other
trials
at
around
4%
over
the
6
month
experimental
period.
The
FCR
in
both
cages
was
almost
identical
at
1:2.8.
Table
5.
Experimental
results
for
Trial
2B.
Trial
2B
Begin
and
end
date
15
Feb
to
16
Aug
Production
parameter
HDPE
SM
Copper
SM
Initial
number
of
fish
4
500
4
500
Initial
length
(mm)
152
152
Final
length
(mm)
246
255
Length
increase
(mm)
94a
103b
Initial
weight
(g)
61
61
Final
weight
(g)
323
365
Weight
gain
(g)
262a
304b
Specific
growth
rate
(g/day)
1.15a
1.25b
1
Biomass
gain
(kg)
1124
1297
Difference
in
final
biomass
gain
(kg)
compared
to
copper
cage
-­‐173
Mortality
(%)
3.82
4.27
2
FCR
2.82
2.8
3
Condition
factor
0.92a
0.92a
Initial
density
(kg/m3
)
2.2
2.2
Final
density
(kg/m3
)
11
13
Different
superscripts
indicate
statistical
differences
at
P<0.05.
1
Biomass
gain
=
Final
biomass
-­‐
Initial
biomass
2
FCR
=
Dry
food
fed
/
Biomass
gain
3
Condition
factor
=
100*(Final
mean
weight/
Final
mean
length
b
),
where
b
is
the
exponent
of
the
length
weight
relationship.

22. Advance
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Figure
20.
Growth
of
Nile
Tilapia
in
large
mesh
copper
and
nylon
cages
at
Mozambezi,
Lake
Cahora
Bassa
from
15
February
to
16
August
2014.
Figure
21.
Biomass
increase
of
Nile
Tilapia
in
large
mesh
copper
and
nylon
cages
at
Mozambezi,
Lake
Cahora
Bassa
from
15
February
to
16
August
2014.
Figures
22
to
24
show
ph,
DO
and
temperature
date
for
the
duration
of
the
experiment.
During
the
experiment
the
temperature
decreased
from
a
summer
maximum
of
around
28.7o
C
to
the
average
winter
temperature
of
22o
C.
Once
again,
the
pH
and
DO
levels
were
highest
in
the
lake
followed
by
the
copper
and
then
the
nylon
cage.
The
DO
level
in
the
nylon
cage
was
significantly
lower
than
in
the
lake
and
the
copper
cage
(P>0.038)
and
there
was
no
significant
difference
in
the
DO
levels
in
the
lake
and
the
water
in
the
copper
cage.
This
confirms
the
supposition
made
earlier
that
there
would
have
been
a
greater
water
exchange
through
the
copper
cage
than
through
the
nylon
cage,
which
manifests
in
the
better
performance
of
the
fish
in
the
copper
cages.

23. Advance
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Figure
22.
Dissolved
oxygen
levels
in
the
cages
and
the
lake
during
the
period
February
to
August
2014.
Figure
23.
pH
levels
in
the
cages
and
the
lake
during
the
period
February
to
August
2014.
Figure
24.
Temperature
in
the
cages
and
the
lake
during
the
period
February
to
August
2014.

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Discussion
and
conclusion
Farm
conditions
sometimes
do
not
allow
for
experimental
rigidity
and
hence
some
measure
of
flexibility
was
required.
Feed
shortage
was
compensated
for
by
reducing
the
rate
at
which
the
fish
were
fed.
For
this
reason
it
is
not
possible
to
put
much
weight
on
the
FCR
data.
It
was
also
not
possible
to
start
the
trial
in
the
small
and
large
mesh
cages
at
the
same
time.
Starting
dates
were
dependent
on
the
availability
of
suitable
sized
fish
for
either
the
small
or
the
large
mesh
nets.
Moreover,
as
mentioned
earlier
it
was
not
possible
to
run
replicates
because
of
the
high
cost
of
the
copper
alloy
material.
Never
the
less,
and
fully
recognising
the
statistical
pitfalls
of
drawing
conclusions
on
non-­‐replicated
trials,
the
fish
in
the
copper
cages
in
all
instances
outperformed
the
fish
in
all
the
other
cages
by
significant
margins.
In
all
the
trials
the
SGR
of
the
fish
in
the
copper
cages
was
significantly
greater
than
in
any
of
the
polymer
cages.
Depending
on
initial
size
and
temperature
the
SGR
in
the
copper
cages
exceeded
the
specific
growth
rate
in
the
polymer
cages
by
an
average
of
12.6%
per
day
(range
8
–
19.6%
per
day).
From
a
farming
perspective
this
translates
into
a
much
higher
final
biomass
of
harvestable
fish.
On
average,
yield
in
the
copper
cages
exceeded
that
in
the
polymer
cages
by
22.4
%
(range
8
–
36%
or
between
173
and
874
kg’s).
The
only
experiment
that
ran
during
a
period
of
increasing
temperatures
from
July
(22o
C)
through
to
November
(28o
C)
was
Trial
1B.
By
using
the
mid-­‐November
final
weights
of
the
fish
in
Trial
1B
and
assuming
that
the
mortality
rates
would
have
remained
the
same
as
those
recorded
in
the
first
half
of
the
trail,
it
was
possible
to
calculate
the
theoretical
yield
(biomass
gain)
in
each
cage.
The
difference
in
yield
between
the
copper
and
the
HDPE
cage
in
Trial
1B
would
have
been
1.4
tonnes.
Expressed
as
a
percent
this
means
that
the
copper
cages
outperformed
the
polymer
cages
by
29.4%.
This
number
falls
well
within
the
range
of
greater
percent
yields
in
the
copper
cages
(see
above).
However,
depending
on
the
rate
of
fouling
all
polymer
nets
had
to
be
cleaned
with
a
broom
at
regular
intervals.
This
was
not
necessary
for
the
copper
alloy
material.
In
general,
as
soon
as
the
young
fish
(5
g
onwards)
are
put
into
cages
in
the
lake
they
grow
much
better
than
in
the
nursery
ponds.
It
is
mainly
for
this
reason
that
small
mesh
polymer
net
pens
are
used.
However,
depending
on
the
rate
of
fouling
the
small
mesh
as
well
as
the
larger
mesh
net
pens
had
to
be
cleaned
with
a
broom
at
regular
intervals.
This
practise
is
possible
at
this
stage
of
the
farms
development.
As
the
farm
expands
it
would
be
very
difficult
to
keep
pace
with
having
to
provide
optimal
growing
conditions
for
the
fish
by
cleaning
or
replacing
the
nets.
As
in
Lake
Kariba
it
would
not
be
possible
to
farm
fish
in
cages
on
a
large
scale
in
Cahora
Bassa
without
the
use
of
anti-­‐predator
nets
(Figure
25).
Predators
here,
as
in
Kariba,
include
birds
particularly
cormorants,
tiger
fish
(Hyrdocynus
vittatus)
(Figure
26)
and
Nile
crocodiles.
Predator
nets
would
also
be
subject
to
bio-­‐fouling
and
would
also
have
to
be
removed
and
cleaned
at
certain
intervals.
On
the
other
hand,
copper
alloy
netting
precludes
the
need
for
predator
nets.

25. Advance
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Figure
25
(left)
and
26
(right).
Crocodile
attack
on
Tilapia
net
pen
at
Mozambezi,
Cahora
Bassa
and
Norm
with
Tiger
fish
(Hydrocynus
vittatus).
The
benefits
of
using
copper
alloy
mesh
cages
in
a
sub
tropical
fresh
water
lake
and
the
advantages
over
polymer
nets
can
be
summed
up
as
follows:
• Bio-­‐fouling
is
negligible,
resulting
in
improved
water
exchange
and
better
conditions
for
fish
growth.
• Higher
growth
rate
of
fish
• Fish
are
in
a
better
condition
(greater
weight
for
a
given
length)
• Higher
yields
• Protection
against
predators
• Lower
maintenance
requirements
(no
need
to
clean
or
replace
nets)
• Reduced
labour
requirement
References
AfDB
Group.
2011.
Lake
Harvest
Aquaculture
Expansion
Project.
African
Development
Bank
Group.
13p.
Dwyer,
R.
L.
and
Stillman,
H.
2009.
Environmental
Performance
of
Copper
Alloy
Mesh
in
Marine
Fish
Farming:
The
Case
for
Using
Solid
Copper
Alloy
Mesh.
International
Copper
Association.
18pp.
Hecht,
T.,
Daniel,
S.
and
Formanek,
F.
2012.
A
comparative
assessment
of
bio-­‐fouling
on
copper
alloy
chain
link
mesh,
nylon
and
polyethylene
netting:
A
contribution
to
the
development
of
cage
aquaculture
in
southern
Africa
and
Western
Indian
Ocean
Region.
Advance
Africa
Report
to
CDA
Africa.
52p.
ICA,
H.
2010.
Copper
alloys
for
marine
aquaculture.
International
Copper
Association.
2pp.

26. Advance
Africa
Management
Services
Page
26
of
27
Michel,
J.H.,
Mitchels,
H.T.
and
Powel,
C.A.
2011.
An
Assessment
of
the
Biofouling
Resistance
and
Copper
Release
Rate
of
90-­‐10
Copper-­‐Nickel
Alloy.
Paper
11352,
NACE
International
Corrosion
Conference
and
Expo.
2011.
Ricker,
W.E.
1975. Computation
and
interpretation
of
biological
statistics
of
fish
populations.
Bull.
Fish.
Res.
Bd
Can.
191:
1-­‐382.
Vostradovsky,
J.
(1984).
Fishery
investigation
on
Cahora
Bassa
Reservoir
(March
1983
–
May
1984).
A
report
prepared
for
the
research
and
development
of
inland
fisheries
project.
FAO,
Rome;
FAO/GCP/066/SWE;
Field
Document
11.
Zar,
J.H.
2009.
Biostatitical
Analysis
5th
Ed.
Pearson.
960p.
Acknowledgements
The
study
was
funded
by
Copalcor
(Pty
Ltd)
and
Mozambezi
Fisheries
and
Aquaculture,
through
the
Copper
Development
Association
Africa.
On
a
personal
basis
we
should
like
to
thank
the
owner
of
Mozambesi,
Mr
Kurt
Heyns
and
his
very
able
team
on
the
shores
of
Lake
Cahora
Bassa
for
their
assistance
and
hospitality
to
bring
it
all
together,
Messrs
Gordon
Grant
and
Derick
Coetzee,
Managing
Director
and
Sales
&
Marketing
Director
of
Copalcor
(Pty)
Ltd.,
respectively
for
their
wisdom
in
seeing
the
potential
of
their
alloy,
Rudolf
Kruger,
the
Quality
Assurance
Manager
at
Copalcor
for
his
scientific
and
technical
assistance
and
finally
Evert
Swanepoel,
the
Director
of
the
Copper
Development
Association
Africa
for
his
encouragement
and
support.

27. Advance
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27
Kurt
Heyns
(owner
of
Mozambezi),
Margie
Paterson
(Hatchery
/
Nursery
Manager)
and
the
cage
farming
team.