CONSULTANCY AND RESEARCH IN AQUACULTURE AND THE AQUATIC ENVIRONMENT

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• CONSULTANCY AND RESEARCH IN AQUACULTURE AND THE
  AQUATIC ENVIRONMENT

A Company in the NIVA-group
Environmental impacts of aquaculture
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Factors affecting impact

• Analysis of monitoring results from 168
  environmental surveys on 80 Salmon farm sites in
  Norway (Carroll, 2003) has shown that management
  practices as well as environmental factors play a
  strong role on the impact of sediments below the
  cages.
• For salmon production in cold waters, management
  practices such as years in operation (without
  fallowing) and feeding strategy were found to
  have greater influence on impact than
  environmental factors such as current speed and
  water depth.

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Use of models to test mitigation scenarios

• The MERAMOD model is designed to predict the
  solids deposition from seabass and seabream
  mariculture operations in the Mediterranean.
• The model uses site information on bathymetry,
  cage layouts, current speed and direction.
• For each cage at the site, feed input (FI) (i.e.
  ration) and species is specified. Using
  information on feed digestibility, water content
  and wastage (uneaten), the rates of discharge of
  faecal material and uneaten feed can be
  calculated.

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Scenario 1 shallow site versus deep site

• The majority of fish farm sites in the
  Mediterranean are located inshore in relatively
  shallow and protected areas. However, in Cyprus
  and Malta, farms are located at relatively
  exposed sites in deeper water. For continued
  growth of the industry, it will be necessary to
  develop new sites offshore in deeper areas.
  Scenario 1 tests the effect between cages sited
  in a shallow site (15 m) and a deep site (30 m)
  and compares the waste solids deposition.

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Scenario 1 shallow site versus deep site

• Model predictions of flux (g m-2 yr-1) showing
  the larger footprint area around the cages
  (centres shown as ?) at the deeper site. The
  deeper site also has lower flux (impact) below
  the cages as there are no dark areas shown

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Scenario 2 spacing between cages

• The development of aquaculture in the
  Mediterranean has progressed from the use of
  small square wooden cages used in the 80s to
  large round plastic cages or large square metal
  cages in the 90s. The mooring system for the
  large round cages is based on a fixed mooring
  grid to which individual round cages are attached
  which provides spacing between cages. This
  compares with the large square metal cages that
  are connected to each other by hinges and forms a
  relatively tight cluster of cages. Scenario 2
  tests the effect of round cages spaced out by 6 m
  against square tightly clustered cages on waste
  solids deposition. A depth of 30 m was used

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Scenario 2 spacing between cages

• Model predictions of flux (g m-2 yr-1) showing
  the difference in deposition footprint shape when
  tightly clustered square cages are replaced by
  circular cages spaced by 6 m.

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Scenario 3 large spacing between cages

• This scenario is similar to Scenario 2 but a
  larger spacing of 30 metres was used between the
  circular cages. This is to test the effect of
  round, largely spaced out cages against square
  tightly clustered cages. A depth of 30 m was used.

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Scenario 3 large spacing between cages

• Model predictions of flux (g m-2 yr-1) showing
  the significant difference in deposition
  footprint severity and extent when tightly
  clustered square cages are replaced by circular
  cages spaced by 30 m. For the spaced out cages,
  areas of lower flux are shown in between lines of
  cages which will tend to assist sediment
  processes.

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Scenario 4 effect of different species (feed
input slightly higher for bream due to SFR in
tables)

• Some farm sites could be more suitable for
  seabass than for seabream and visa versa due to
  seabass having a faster faecal settling velocity
  than seabream. This scenario tests the difference
  in impact on sediments depending on whether
  seabass or seabream are stocked in the cages. A
  shallow site (15 m) was used in this test, using
  faster settling velocities for bass.

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Scenario 4 effect of different species

• Model predictions of flux (g m-2 yr-1) showing
  the significant difference in deposition
  footprint shape between bass and bream cages.
• Higher flux (impact) is predicted below bass
  cages and wastes from bream are dispersed more
  widely.
• This indicates that bass should be placed in more
  dispersive, deeper areas of the site.

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Scenario 5 effect of locating seabass in deeper
and more dispersive sites

• As the findings in Scenario 4 indicate that it
  may be better to place seabass in more dispersive
  sites, Scenario 5 tests the effect of locating
  bass in deeper more dispersive areas to take
  account of the higher faecal settling rates. In
  scenario 5a, a depth of 30 m was used to test the
  effect of bass in deeper sites. In scenario 5b,
  a depth of 30 m was also used but the current was
  increased by 50 to represent a more dispersive
  site

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Scenario 5 effect of locating seabass in deeper
and more dispersive sites

• Model predictions of flux (g m-2 yr-1) showing
  the difference in the deposition footprint when
  bass are moved to deeper and more dispersive
  sites. The effect of depth is seen by comparing
  Figure 4a and Figure 5a the effect of higher
  current is seen by comparing Figure 5a and 5b.
  This indicates bass should be located in deeper
  and/or more dispersive areas

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Scenario 6 test efficient FCR and less
efficient FCR

• Food conversion rate in seabass and seabream
  farms varies between 1.41 and 2.21 depending on
  the feeding strategy and close feed management.
  This overfeeding leads to feed wastage and
  potential higher environmental impact. Scenario 6
  tests the effect between cages with a FCR of
  1.61 (FI 111.6 kg cage-1 d-1) and 2.01 (FI
  139.5 kg cage-1 d-1). A depth of 15 m was used.

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Scenario 6 test efficient FCR and less
efficient FCR

• Model predictions of flux (g m-2 yr-1) showing
  the difference in deposition for different values
  of FCR. The darkest area representing high flux
  (impact), covers more area underneath the cages
  with the less efficient FCR scenario

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Scenario 7 feeding method

• The majority of farms in the Mediterranean still
  use hand feeding of fish rather than automatic
  feeding. This results in less frequent feeding of
  larger portions. Scenario 7 tests the effect
  between undertaking hand feeding twice a day and
  automatic feeding. Hand feeding was undertaken
  twice per day (am and pm) with 70 of ration (and
  defecation) in the morning feed. Automatic
  feeding was constant feeding and defecation
  between 0900 and 1600 local time. These can be
  compared with scenario 1b. A depth of 30 m was
  used.

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Scenario 7 feeding method

• Model predictions of flux (g m-2 yr-1) showing
  little difference in deposition for the different
  feeding methods. If the model was used to
  examine the effect of feeding method over a
  shorter period and at a site where a strong
  diurnal pattern of wind occurs, a difference
  might be more obvious.

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Scenario 8 low and high stocking density

• Scenario 1b uses cages with a stocking density of
  12 kg m-3. To test the effect of stocking
  density, stocking density in scenario 8a was
  reduced to 6 kg m-3 and increased to 20 kg m-3 in
  scenario 8b and the predictions compared. A depth
  of 30 m was used.

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Scenario 8 low and high stocking density

• Model predictions of flux (g m-2 yr-1) showing a
  significant difference between deposition
  footprints for low and high stocking density.
  The high stocking density will cause a high level
  of impact underneath the cages.

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Conclusions

• Greater dispersion of wastes resulting in low
  severity and high extent of the deposition
  footprint occurs where sites are deeper (scenario
  1), cages are highly spaced (scenario 2, 3) and
  bream is farmed (scenario 4) (Table 1). In
  particular, where 30 m spacing of cages is used,
  the severity of impact underneath the cage groups
  is reduced by four times. Scenarios with a high
  FCR (scenario 6) and stocking density (scenario
  8) resulted in higher severity and extent of
  footprint, an undesirable situation. Scenario 5
  showed bass are more suitable for deep dispersive
  sites as this results in a low severityhigh
  extent situation. No real difference was
  detected for feeding method (scenario 7),
  particularly for the extent of the deposition
  footprint.

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Conclusions

• These scenarios show deeper, dispersive sites
  result in less severe impact over a larger area.
  In addition, spacing out of cages reduces
  predicted deposition markedly especially where a
  large spacing is used. The modelling also
  suggests bass potentially have more impact than
  bream as a result of faster faecal settling
  velocities, despite the slightly lower feed input
  used for bass. Therefore, bass should be sited in
  deeper more dispersive sites or, where farmed at
  the same site as bream, bass should be placed in
  the outer (deeper) areas of the farm.
  Consideration should be given to modifying
  management practices to reflect this.

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Conclusions

• The effect of inefficient feeding and high
  stocking density is clear. A more severe impact
  over a larger area will result, with a higher
  probability of problems with sediment and fish
  health.
• Little difference was found between the scenarios
  where feeding method was tested. However, where a
  strong diurnal pattern of wind and circulation
  exists at a site, the effect of feeding larger
  portions by hand in two feeding events may result
  in periods of higher deposition. This would
  mainly be a result of a larger feeding event in
  the morning coinciding with lighter winds and
  therefore less potential for dispersion.

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