Modelling environmental impact of cages (Tropomod). The TROPOMOD model is a particle tracking model which simulates the dispersion of waste feed and waste faecal particles from fish cages. Using depth and current velocity data from environmental surveys and husbandry data such as cage layouts and feed ration from production surveys, TROPOMOD was used to predict flux of waste solids to the sea bed (grams waste feed and faeces m-2 sea bed day-1). This was then related to a level of impact on the sediment benthos. The model was used to examine the existing situation at the Sual and Panabo AquaParks and then test various scenarios for site optimisation and future production of the AquaParks.

1. TROPOMOD modelling for optimisation of
AquaParks
Chris Cromey – EMMA2 project modeller using data from Akvaplan
and BFAR survey teams
Modelling objectives for each AquaPark
• Predict benthic impact for the existing situation and validate model with
survey data
• Predict benthic impact for expansion/reorganisation
• Predict optimum location for Integrated Multi-Trophic Aquaculture units
• Estimate of production and recommendations from modelling viewpoint
Other objectives
• TROPOMOD educational software showing different scenarios of impact

2. TROPOMOD educational software

3. Currents move in different speeds and direction at
different depths. Faeces settle more slowly and so
are transported further away from cages
0
Current Velocity
Source
Milkfish waste
faeces settle
very slowly
Fine
Coarse
Medium
Waste feed particles settle quickly

4. Contour map of waste flux
Benthic
community
(grams waste feed and faeces m-2 d-1)
-2
-1
grams solids m bed d
75
Severe impact
(no animals)
75
15
High impact
(some effect)
Cages
15
1
Moderate impact
1

5. TROPOMOD – validation with
sediment traps
Cage
Current
Sea bed
1. Deploy traps
75 cm
2. Retrieve, filter, dry solids
H:D = 5:1
ratio
3. Calculate observed flux
(total waste feed and faeces in traps
= grams per m2 bed per day)
4. Compare with TROPOMOD model

6. TROPOMOD model validation – comparison of
observed and modelled flux
Comparisons between modelled and observed flux for Sual (Offshore Milkfish)
and Panabo (Inshore Milkfsh and Grouper) were satisfactory

7. TROPOMOD validation – relationships between
predicted flux and indicators of impact - Sual
Where
TROPOMOD
predicted high
flux, sediments
contained
higher % of
organic material

8. TROPOMOD
validation –
relationships
between predicted
flux and indicators of
impact - Panabo
Organic Carbon as
also found to be high,
where TROPOMOD
flux predictions were
high

9. Criteria for AquaPark zoning
Regular spacing
between cage
groups to allow
flushing
Regular spacing
between zones to
allow flushing
No cages near
restrictions to allow
current flow
Only small
Milkfish cages or
Grouper cages in
shallow areas
near to shoreline
Larger cages
(polar circles) in
deeper areas
away from
shoreline

10. Compare with the
current situation
Current flow
restrictions due to
high density of
cages, with no gaps

11. Criteria for AquaPark zoning - impact
Within each
zone
MODERATE
impact (green)
or less between
cage groups
MODERATE
impact (green)
or less between
zones
SEVERE impact (grey) under the cages was minimised

12. Suspended/raft culture (e.g.
oysters) - majority of the wastes
intersect the culture in the top 10
m; these wastes are mostly fine
and slow settling Milkfish faeces
Criteria for AquaPark
zoning – IMTA location
18%
5%
0-3m
3%
6%
3-6m
2%
6-9m
Reversing
current
<1%
9-12m
12-15m
25 m
10 m
0m
0m
10 m
25m
Distance from cage edge (m)
Seaweed culture - majority of
the plume containing dissolved
nutrients intersects the seaweed
culture in the top 5 m
Benthic culture (e.g. sea cucumbers) around 20% of wastes deposited within 10
m of the cages; not placed in very high
deposition area under cages

13. Sual AquaPark – Existing and reorganised
Less SEVERE impact under cages
between zones
MODERATE impact

14. Panabo AquaPark – Existing and offshore zones added
Good feeding scenario improves inshore areas
Offshore Milkfish zones in deeper areas results in good
dispersion

15. Panabo AquaPark – Existing and reorganised
Spacing between zones results in less impact and also more space for IMTA units

16. Criteria for AquaPark zoning – IMTA location
Seaweed culture – located 10+
m from cage groups along the
axis of group
IMTA culture was minimal
between cage groups to
allow flushing
Benthic culture – optimum
location at end of cage
groups, but not directly
underneath
Suspended culture – located 10+ m from
cage groups along the axis of group

17. Sual AquaPark and IMTA close to the cages
Seaweed
-2 -1
Flux (g m d )
Oysters
Benthic
Community
Severe impact
(no animals)
Sea cucmbers
75
Large seaweed and
oyster culture
zones were placed
either side of the
Milkfish culture
zone.
High impact
15
Moderate impact
1
Smaller IMTA
zones were placed
inbetween cage
groups.
Scale (m)
0
200
400
600
800

18. Panabo AquaPark and IMTA close to the cages
Large oyster culture
zones were placed
inshore of the
finfish zones.
Smaller IMTA
zones were placed
inbetween cage
groups.
Some space
available offshore,
but deep.

19. Recommendations for site optimisation
• Regular spacing of cages and good separation between zones enhances
current flushing through the AquaPark and reduces impact between culture
areas
• Good spacing between inshore zones (min. 200 m) and offshore zones
(min. 400 m)
• Within a zone, good and regular spacing between cage groups (100 m)
and between rows of cages (25 m)
• Target FCR should be 2.0:1, as with careful feeding of a good quality feed,
this allows a reduction in feed ration
• IMTA suspended culture (oysters and seaweeds) should be minimum 10 m
from cages
• IMTA units should be placed along the axis of current flow leaving corridors
for enhanced flushing and dispersion of nutrients