ICES Journal of Marine Science, 58: 497–503. 2001
doi:10.1006/jmsc.2000.1033, available online at http://www.idealibrary.com on
Mitigating the environmental effects of mariculture through
single-point moorings (SPMs) and drifting cages
C. A. Goudey, G. Loverich, H. Kite-Powell, and
B. A. Costa-Pierce
Goudey, C. A., Loverich, G., Kite-Powell, H., and Costa-Pierce, B. A. 2001.
Mitigating the environmental effects of mariculture through single-point moorings
(SPMs) and drifting cages. – ICES Journal of Marine Science, 58: 497–503.
The production of finfish in cages causes a measurable impact on the nearby water and
seabed due to faeces production and uneaten feed. The impact depends on the specifics
of the culture operation and on the environment in which it is located. The most severe
impact has been associated with large, intensive operations in areas with inadequate
water circulation, where benthic habitats have been seriously degraded. The current
practice of mooring a cage, or a cage array, over a permitted site exacerbates
the problems. An alternative approach is to allow cages to move in response to the
environment. For example, the use of a single point mooring (SPM) would allow the
operation to maintain a ‘‘watch circle’’ where the position of the cage(s) depends on
the sum of the environmental forces. By spreading out the accumulation of organic
matter, one can prevent the local environment from being overwhelmed. Preliminary
analyses of the benefits of SPM indicate a two-fold to 70-fold reduction in deposition
of waste on the seabed, depending on mooring geometry and current type. Other
advantages are related to reduced anchoring costs, improved accessibility, and the
ability of having certain cages in the lead position with respect to currents and good
oxygen conditions. The concept of drifting cages is introduced as a further alternative
to minimizing impact on the seabed.
2001 International Council for the Exploration of the Sea
Key words: aquaculture, benthic impacts, cage mooring, single point mooring.
Received 16 October 1999; accepted 15 March 2000.
C. A. Goudey: MIT Sea Grant College Program, Room NE20-376, 3 Cambridge Center,
Cambridge, MA 02139, USA; tel: +1 617 253 7079; fax: +1 617 252 1615; e-mail:
[email protected]; G. Loverich: Ocean Spar Technologies, LLC., 7910 N.E. Day Road
West, Bainbridge Island, WA 98110, USA; H. Kite-Powell: Woods Hole Oceanographic
Institution, Marine Policy Center, Mail Stop 41, Woods Hole, MA 02543, USA;
B. A. Costa-Pierce, Mississippi-Alabama Sea Grant Consortium, PO Box 7000,
Ocean Springs, MS 39566-7000, USA.
Introduction
Scientific interest in the impact of nutrient pollution
from aquaculture has increased markedly since the 1980s
(Costa-Pierce et al., 1983; Rosenthal et al., 1987; ICES,
1998; Iwama, 1991; Cowey and Cho, 1991; DePauw and
Joyce, 1991; Makinen, 1991; Pullin et al., 1993), and
especially so as aquaculture has become one of the
world’s fastest growing industries (Davlin, 1991). Concerns about pollution, combined with real and perceived
water quality degradation, health concerns, and other
violations of the public trust, have fuelled vigorous
public and policy debates. Controversies have resulted in
adoption of regulations intended to preserve the integrity of natural ecosystems and to ameliorate the public’s
1054–3139/01/020497+07 $35.00/0
concerns. The increasing regulatory burden has been
identified as one of the main factors slowing the growth
of aquaculture.
Impact of intensive cage aquaculture on benthic ecosystems has been studied extensively (Brown et al., 1987;
Gowen and Bradbury, 1987; Ritz et al., 1989; Gowen
and Rosenthal, 1993; Johannessen et al., 1994). Impact
of sedimenting materials is related to the size, intensity, and management of farming operations, and to
the morphology, bottom topography, and physical
oceanography of the site.
Cairns (1977) defined assimilative capacity in an ecological sense as the ‘‘ability of an aquatic ecosystem to
assimilate a substance without degrading or damaging
its ecological integrity’’. Ecological integrity is defined as
2001 International Council for the Exploration of the Sea
498
C. A. Goudey et al.
Table 1. Summary of studies on the extent of environmental
impact caused by intensive aquaculture on benthic communities
(SC: species composition; MBI: measurable benthic impact;
measured from edge of farm/cages).
Reference
Mattsson and Linden, 1983
Brown et al., 1987
Gowen et al., 1988
Lumb, 1989
Kupka-Hansen et al., 1991
Weston, 1990
Johannessen et al., 1994
Extent
SC: <20 m
SC: <25 m
SC: <40 m
MBI: <50 m
SC: <25 m
SC: <100 m
SC: <250 m
the maintenance of ecological structure and functional
characteristics of that locale. Structural integrity is
determined from the numbers of organisms and how
they are ordered. An abnormal change (increase or
decrease) is interpreted as evidence of stress (Patrick,
1949).
In studies from coastal sites receiving wastes from fish
cages (cited above) or mussel rafts (Mattsson and
Linden, 1983), the addition of sedimenting materials has
been shown to decrease the vertical extent of the oxidized (oxygenated) sediment layer, and thus lowers
oxygen reduction (redox) potential. Lower redox potentials increase sedimentary oxygen demands (SODs) and
stimulate facultative microbial activities. As a result,
microbial metabolism shifts from aerobic to anaerobic,
and sulfur and methane reduction occur, increasing
oxygen depletion further by increasing sedimentary
chemical oxygen demand (COD). Where high rates of
solids and nutrient loading occur, microbial processes
accelerate to the point where oxygen-depleted sediments
go anaerobic and large amounts of bound, sedimentary
phosphorus are released. Bacterial reduction in anaerobic sediments under intensive cage cultures can be so
high as to cause outgassing of methane and hydrogen
sulfide (Braaten et al., 1983; Samuelsen et al., 1988).
In such situations, effects on biological communities
can be detrimental. Species diversity decreases with a
simplification of community structure and a large
increase in the numbers of a few opportunistic species
(Brown et al., 1987; Tsutsumi et al., 1991; Johannessen
et al., 1994). In enclosed basins, the benthos near
the cage site can be nearly wiped out owing to
deoxygenation.
A summary of the available research on the impact of
intensive aquaculture on the benthic environment
shows, however, that the pollution processes producing
detrimental changes in the benthos are limited to the
immediate vicinity of the operations (Table 1). While
Enell and Lof (1983) measured elevated SODs of
45–55 mg O2 m2 h 1 under salmon cages in comparison
with a control site of 16 mg O2 m2 h 1, these figures are
still within reported ranges of 14–376 mg O2 m2 h 1 for
natural waters (Veenstra and Nolen, 1991). Flushing,
resuspension and dispersion may prevent waste accumulations in many sites (Ackefors, 1986; Ackefors and
Enell, 1990). In addition, long-term studies have found
that, even for enclosed basins, reported sedimentary
oxygen depletions have been brief and could not be
attributed solely to aquaculture (Gowen and Rosenthal,
1993).
Deposition of organic materials from aquaculture
leaves an organic signature (a ‘‘memory’’) in the sediment, the extent of which can be read by coring. Organic
layers deposited under aerobic conditions are brown,
while layers deposited under anaerobic conditions are
black. In environments where there is a high degree of
terrestrial soil erosion or natural plankton deposition,
these deposited materials can form a ‘‘cap’’ over the
anaerobic layers because these materials were deposited
under highly aerobic conditions (Wilcox, 1994). Soil
erosion rates of 12 000 kg ha 1 are common in many
agricultural areas (Ellis et al., 1978) and, where these
enter water bodies, previous organic layers may be
buried. If a consequential amount of bioturbation of
sediments occurs, mineralization and aerobic microbial
activities increase (Costa-Pierce et al., 1983). Therefore,
the natural sedimentary profile could recover fairly
rapidly if large populations of animals that cause bioturbation exist in the areas of aquaculture development.
A variety of strategies has been studied and implemented to reduce the benthic effects of waste deposition
under aquaculture cages, including fallowing (site rotation programmes), establishing minimum separation
distances between farming sites, and others (ICES,
1998). In addition, better feed and feeding techniques
reduce waste production for a given production level.
Industry practices and regulatory requirements have
also been advanced in response to both the reality and
the perception of benthic impact. Determining the carrying capacity of an area is important in determining the
amount and type of fish farming that may be allowed.
However, in some cases, the issue of benthic impact
remains a limiting factor in the production capacity of a
cage system and further methods of amelioration are
needed. We discuss the potential for mitigating environmental impact by applying single point mooring (SPM)
instead of conventional anchor arrays.
Comparison of mooring systems
The traditional method of mooring a pen, or an array of
pens, is to use multiple anchors to hold the system in a
fixed location. The anchors are usually arranged
bi-axially, with at least four anchors associated with a
single pen and additional anchor pairs at pen intersections in a multi-pen array [Figure 1(a)]. Precise and tight
adjustment of multiple anchors is critical in maintaining
Mitigating the environmental effects of mariculture
499
(a)
51.00
267 m
15 m × 15 m × 10 m cages
Water depth = 28 m
65 m
Current~50 cm/sec Riser float
70 m
Screw anchors
281.23
(b)
15 m × 15 m × 10 m cages
Water depth = 28 m
Current~50 cm/sec
65 m
Riser Float
70 m
Screw anchors
145 m
Figure 1. Comparison of typical 12-cage array moored with (a) a multi-anchor system and (b) a single anchor.
the proper alignment of a pen array and in ensuring that
each anchor bears an appropriate load.
The strength requirements of the anchoring components can be easily calculated from prior knowledge of
array design, local environmental conditions, and the
level of pre-tensioning. Because of variations in fetch,
prevailing winds, and hydrography, the size and length
of individual anchors may vary significantly. The overall
loads depend on the orientation of the array, the drag
characteristics of individual pens, and the cumulative
effects of currents, wind, and waves. Maximum total
loads often occur when the array is being subjected to
environmental forces against its long axis.
The adoption of SPM [Figure 1(b)] is a significant
departure from the traditional approach in aquaculture
but represents the favoured method of anchoring most
other floating marine systems, if exact positioning is not
a must (e.g. drilling operations). SPM greatly reduces
the complexity of anchoring and, more importantly,
reduces the maximum total loading on the system when
the anchor extends in the direction of the array’s
major axis.
SPM does introduce special requirements. The total
loading is concentrated on one mooring line system and
the anchor itself must be capable of resisting pull in any
direction. In addition, the system must include a swivel
to prevent the twisting of the mooring line, which can be
troublesome if a power cable or other umbilical has to
be connected from shore. The anchor loads can be
distributed along the front of the pen array with a bridle
arrangement to the attachment points normally used for
individual anchor lines.
The appropriate type of anchor depends on bottom
type. A dead-weight anchor may be used but attention
must be given to prevent fouling of the mooring cable
under the anchor, particularly if scouring is likely. A pile
or a helical anchor may be advantageous if the sediment
allows (Taylor, 1991). The use of a cluster of opposing
drag-embedment anchors can also be considered, but,
again, anchor-cable fouling may present a risk.
The selection of anchor cable length versus water
depth (i.e. the scope ratio) is an important consideration
for mooring performance, because a short scope
increases the dynamic wave-induced loading experienced
by the anchor standpoint. The scope ratio also affects
the area needed to accommodate the swing of the
system. Figure 1(b) shows an SPM that uses a helical
anchor array.
Predicting the discharge footprint of SPM
Numerous methods can be found in the literature for
predicting the accumulation of solids under a fish pen
(Hargrave, 1994). The methods typically require
assumptions about the sinking rates of uneaten feed
pellets and faeces, and the results are based on the
particle trajectories as influenced by water depth and the
ambient current. In all cases, these predictive models
assume the cage (array) is in a fixed location and the
strongest deposition is predicted directly under the cage.
Without reference to a specific site with a known
current regime, the implication of SPM on seabed solids
accumulation is best understood from a simple geometric standpoint. Under the influence of normal tidal
currents, a cage array will orient to the current when it is
strong enough to overcome other, less predictable influences. Tidal currents are generally reciprocal, with dominant ebb and flood directions. There may also be a
rotary component providing a non-zero current at what
would otherwise be slack water. We explore two simplified cases: a pure and constant rotary current and a
pure reciprocal current with a magnitude that varies
sinusoidally.
In the simplified case of a rotary current of constant
magnitude, the pen array maintains a watch circle,
pivoting around the anchor at a fixed distance. In this
case, the reduction ratio of accumulated solids is dependent on the distance from the SPM anchor. By relating
the circumference of the watch circle to the width of the
cage, the increase in area over which the solids are
disbursed can be estimated. The reduction can be
expressed as
Reduction factor
C. A. Goudey et al.
80
70
60
50
40
30
20
10
(a)
120 m cable
80 m cable
40 m cable
0
Reduction factor
500
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
50
100
150
Radius
200
250
10
20
30
Wander time (%)
40
50
(b)
0
Figure 2. Estimated reduction in accumulated solids through
application of SPM: (a) in relation to distance from anchor for
a 20100 m pen array under conditions of a pure rotary
current (ranges that may be achieved for different cable lengths
are indicated); (b) in relation to relative wander time under
conditions of a reciprocal current.
qf/qspm2r/w
(1)
where qf =the accumulation of solids under a fixed
system, qspm =the estimated accumulation of solids
using an SPM, w=the width of the pen or the pen array,
and r=the distance from the anchor. Given that any r of
interest is greater than the anchor cable length and
typically much larger than w, the potential for significant reductions in accumulated solids with SPM in a
rotary current may be estimated. Figure 2(a) is a plot of
predicted reductions for three different mooring cable
lengths for a 20-m wide by 100-m long array swinging on
an anchor cable of three different lengths. This simplified
situation indicates that accumulation reductions are
greater with a longer anchor cable and are further
reduced under the outer portions of the array’s watch
circle. It should also be noted that accumulations can be
further reduced by periodically altering the cable length.
In the case of a pure reciprocal current, the reduction
in deposition at the ebb and flood positions is at least
halved, but increases with the duration of slack tide and
the amount of time the array is out of these two
positions. If the pen array takes a certain amount of time
to adjust to its alternate position, the accumulation at
the two principal positions is proportionally reduced.
The following relation approximates that reduction for
reasonably small amounts of wander time [Figure 2(b)]:
qf/qspm 2/(1P)
(2)
where P is the percentage of time the cage is wandering.
Mitigating the environmental effects of mariculture
501
Table 2. Comparison of conventional multi-anchor mooring
(MAM) and single point mooring (SPM) of a 12-cage array (see
Figure 1) with associated costs.
Number of anchors
Anchors holding against current
Number of anchor lines
System load (kg)
Design safety factor
Load/anchor (kg)
Required GMBL*/anchor line (kg)
Rope type
Diameter (mm) for GMBL*
Published GMBL* (kg)
Water depth (m)
Minimum scope ratio
Single anchor line length (m)
Total rope length used (m)
Nominal selling price/m
Total price of anchor line
Cost/anchor-installed
Cost/anchor base
Total anchor costs
Riser floats
Cost/float
Total cost of floats
Total anchoring costs
MAM
SPM
18
4
18
14 000
4
3 500
14 000
PES12
25
16 420
28
4
112
2 016
$6.49
$13 083.84
$3 000.00
$0.00
$54 000.00
18
$400.00
$7 200.00
$74 283.84
8
1
1
14 000
5
14 000
70 000
Plazma
32
74 800
28
4
112
112
$61.72
$6912.64
$3 000.00
$4 000.00
$28 000.00
1
$4 000.00
$4 000.00
$38 912.64
*GMBL=guaranteed minimum breaking load.
Of course, neither of these simplified examples
adequately characterizes a specific situation. Specific
environmental factors, subtleties of the SPM components, and stochastic effects will result in a cage trajectory that defies precise prediction, but the implications
are clear. Neither do these simplifications elucidate the
importance of currents in dispersing the solids, something cage operators and regulatory agencies are fully
aware of. However, through the adoption of SPM, the
local amount of deposition obviously could be reduced
and consequently lead to an increase in the acceptable
biomass that may be cultured in a pen volume.
SPMs offer additional benefits compared to conventional multiple anchor systems. The single anchor and
mooring components, though larger and more expensive
than conventional individual anchors, allow considerable savings in total mooring costs (Table 2). Also, the
cage array typically aligns itself with the current providing a consistent direction of water flow (and therefore
oxygenation) through the system. Stocking densities can
be based on that knowledge. In contrast, leading and
trailing pens alternate with the ebb and flood tides in a
fixed pen array if currents reverse. The predictable
orientation of the cage array to the current also facilitates the designation of specific areas for loading or
other specialized activities, unhampered by contrary
currents.
Figure 3. A 64 000 m3 self-propelled ocean drifter cage (after
Goudey, 1998).
Implications: drifting cages
If the relationships presented in Figure 2 are taken to an
extreme, i.e. an infinitely long mooring cable, we begin
to recognize the advantages of a drifting cage as a way of
eliminating deposition as a siting factor. Indeed, a
drifting cage is a fish farm without a site and is only
feasible in larger bodies of water and where predictable
currents exist. This novel concept is being studied as a
way of exploiting the vast areas of ocean that are
currently inappropriate for conventional moored
operations (Goudey, 1998, 1999). Such systems (Figure
3) must be large and have the capability of operating
independently of day-to-day vessel support. Their operation in a combination of drifting and moored conditions may represent a more realistic scenario. For
example, semi-protected sites might be pre-determined
for extreme weather or seasons while the drifting mode
would be favoured at other times.
Engineering aspects
The design, fabrication, and installation procedures of
SPMs are not very different from standard mooring
procedures. For example, the requirements can be
directly compared for a 12-cage array (Table 2). The
conventional fixed-pen array requires 18 anchors with a
minimum of four holding the cage array against a
current at any given time. Using estimates of worstcondition loading, the likely maximum load per anchor
can be quantified. The implications on anchoring costs
are clear from the table.
Less clear and not reflected are the simplifications
associated with SPM. The optimum system may be
selected based on bottom characteristics to provide
favourable holding power, thus eliminating the common
502
C. A. Goudey et al.
dilemma of needing different types of anchors because of
varying bottom types within the same site. The operational advantages associated with diver inspections are
similarly considerable.
Some cage types may seem inappropriate for use with
SPM, because the arrays require tensioning to maintain
their shape and to prevent abnormal chafe and damaging collisions between cage collars. However, most situations severe enough to generate damaging interactions
are typically associated with a sum of forces on the cage
array sufficient to keep the cages aligned and separated.
Conclusions
The analyses indicate a potential twofold to 70-fold
reduction in benthic accumulation of waste products
under a fish cage array if a SPM is used, depending on
SPM geometry and the nature of the tidal currents.
The use of a SPM could reduce the anchoring costs
of a cage operation by 50% compared to current
multi-anchor methods owing to reduced hardware,
installation, and maintenance costs.
Drifting or self-propelled cages essentially eliminate
the common concerns associated with benthic
impacts associated with intensive cage production
of finfish because of the roaming nature of such
operations.
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