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Anexperimental assessment of the potential impacts of longline mussel farming on the infauna in an open coastal embayment.

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AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS
Aquatic Conserv: Mar. Freshw. Ecosyst. 16: 289–300 (2006)
Published online in Wiley InterScience
(www.interscience.wiley.com). DOI: 10.1002/aqc.710
An experimental assessment of the potential impacts
of longline mussel farming on the infauna in an
open coastal embayment
T.A. LASIAKa,*, A.J. UNDERWOODa and M. HOSKINa,b
a
Centre for Research on Ecological Impacts of Coastal Cities, Marine Ecology Laboratories A11,
University of Sydney, NSW 2006, Australia
b
Marine Environmental Research Consultants, c/o 2 Raleigh Place, Falmouth,
Cornwall TR 11 3Q5, England
ABSTRACT
1. The existence of perceived ecological impacts and development of adaptive management
solutions to mitigate these problems are important issues in sustainable aquaculture. This paper
examines the general impacts of two newly established trial longline installations on the infauna in
Twofold Bay, a large, coastal embayment in south-east Australia.
2. We hypothesized that the physical presence of these longline installations and the biological
activities of the mussels they supported would result in temporal changes in densities of infaunal taxa
below installations differing from those at undisturbed control sites. We also predicted different
patterns of variability in infauna between longline and control sites from before to after the longlines
were stocked. These hypotheses were tested by using a beyond-BACI sampling design and
asymmetrical analyses of variance to compare changes in densities of taxa at several different spatial
scales below the proposed longline sites with those at two adjacent control sites, before and after the
longlines were stocked.
3. After 18 months of longline operations, there was no evidence of any impact on total number of
taxa, nor densities of individual taxa. Short-term temporal trends in densities in plots at control sites
from April to May 2001 were often as different from each other as from those at the longline site.
This indicates that densities of taxa at the farm site were within the range typically found at
undisturbed sites, so there were no ecological impacts from the farm.
4. These results do not concur with previous studies on impact of mussel farming in semi-enclosed
coastal waters. Differences in location, scale of production, duration of operation and assimilative
capacity of the environment probably contributed to this discrepancy. The infauna in Twofold Bay
either do not respond to this form of disturbance or have not yet been exposed to disturbance of a
sufficient magnitude, or for a sufficient period of time, to elicit a detectable response. Better
definition of the potential ecological impacts associated with aquaculture, plus their scale and
magnitude in different environments is needed to design experiments and monitoring programmes to
*Correspondence to: Dr Theresa Lasiak, 8/35-41 Mallett Street, Camperdown, NSW 2050, Australia. E-mail: tlasiak@hotmail.com
Copyright # 2006 John Wiley & Sons, Ltd.
Received 15 July 2004
Accepted 27 February 2005
290
T.A. LASIAK ET AL.
detect specific impacts. This activity can only be considered sustainable once we know that these
impacts are localized, reversible and short-term.
Copyright # 2006 John Wiley & Sons, Ltd.
KEY WORDS: longlines; mussel farming; ecological impacts; infauna; beyond-BACI; coastal embayment;
south-east Australia
INTRODUCTION
Ecosystem-based management of the marine environment requires attention to components of the
natural system needed to sustain activities such as fisheries, aquaculture and waste disposal
and components likely to be adversely affected by such activities. Implicit in this approach are
some definition of desired ‘ecosystem quality’ and the identification of quantitative indicators sensitive
to various forms of anthropogenic disturbance and the managerial actions necessary to ameliorate
them. The existence of perceived ecological impacts and the development of adaptive management
solutions to mitigate these problems are thus important issues in sustainable aquaculture. In many
countries, further expansion of aquaculture is now dependent on assessment of potential environmental
impacts.
One component of aquaculture that has undergone dramatic expansion is mussel farming, with
global production having increased from 164 000 tonnes in 1950 to 1.6 million tonnes in 2000 (Vannuccinni,
2002). The culture installations and the unusually large densities of mussels grown on them each have
the potential to modify the environment. The major concerns associated with mussel rafts and longline
systems are their modification of the velocity and direction of water-movements which can, in turn,
alter natural patterns of erosion and sedimentation, shading of the underlying seabed, provision of a
new substratum for settlement and growth of epibiota and coverage of large areas of coastal waters
(Barg, 1992). Concerns about the large densities of mussels relate to their alteration of the composition,
biomass and productivity of phytoplankton and seston in the water-column (Figueras, 1989; Hickman,
1989), respiratory demand for oxygen (Barg, 1992) and production of large quantities of organic-rich
faeces and pseudofaeces (Kautsky and Evans, 1987; Barg, 1992). Deposition of the latter can alter the
particle-size, organic content, nitrogen-cycling and redox potential of the underlying sediments (Dahlbeck
and Gunnarsson, 1981; Mattson and Linden, 1983; Smaal, 1991; Grant et al., 1995). These, in turn,
can affect the associated benthic fauna (Grant et al., 1995; Stenton-Dozey et al., 2001). The fallout of
mussels and fouling organisms dislodged from culture installations alters the topography of the seabed and
can also influence the structure of benthic assemblages (Tenore et al., 1982; Jaramillo et al., 1992; Grant
et al., 1995).
The extent and magnitude of environmental impacts are known to vary with the method of
culture, location, scale of production, duration of operation, management and assimilative capacity of
the surrounding environment (Fernandes et al., 2001). All studies published so far on environmental
impacts of mussel culture have been in semi-enclosed coastal waters (Mattson and Linden, 1983; Kaspar
et al., 1985; Hatcher et al., 1994; Grant et al., 1995; Stenton-Dozey et al., 1999, 2001; Christensen et al.,
2003; Crawford et al., 2003). It is important to establish whether mussel culture under very different
environmental conditions (e.g. inside well-flushed open coastal embayments) is less intrusive or causes no
ecological impacts, rather than to generalize from results in semi-enclosed habitats. If production of
mussels causes little or no impacts in some habitats, further expansion of mussel culturing can be
sustainable in these areas.
Previous studies of the effects of mussel farming have been based on post-impact assessment of
differences in environmental variables at farm and adjacent control/reference sites. Although some
studies have been done on impacts associated with newly established farms (Mattson and Linden, 1983;
Copyright # 2006 John Wiley & Sons, Ltd.
Aquatic Conserv: Mar. Freshw. Ecosyst. 16: 289–300 (2006)
IMPACTS OF LONGLINE MUSSEL FARMING
291
Stenton-Dozey et al., 1999), most workers have focused on differences that have arisen after >5 yr of farm
operation (Kaspar et al., 1985; Grant et al., 1995; Stenton-Dozey et al., 1999, 2001; Christensen et al., 2003;
Crawford et al., 2003). The lack of pre-impact data means that the possibility of intrinsic differences in
variables among sites prior to the establishment of the farm cannot be disproved and that changes
coincident with the commencement of farming cannot be identified (Green, 1979). Differences that are
observed cannot therefore be attributed solely to some form(s) of disturbance associated with mussel
farming. Studies that contrast environmental variables at a putatively disturbed farm site with those at a
single undisturbed control/reference site (Kaspar et al., 1985; Grant et al., 1995) are also subject to
criticism, because observed differences may likewise be due to some other intrinsic factor, besides the
presence of the farm, that differs between the two sites (Hurlbert , 1984). Underwood (1992, 1993, 1994) has
argued that differences and changes detected by means of simple spatial or temporal comparisons,
respectively, are inadequate for identifying impacts, because of the likelihood of confounding by the natural
temporal and spatial variability in environmental variables. Instead, he proposed that assessments should
be based on the detection of temporal changes at the disturbed site(s) that differ from those at the control
sites and on spatial differences between disturbed and control sites that differ from before to after the
disturbance.
Several physical, chemical and biological variables are routinely used to assess the environmental
effects of aquaculture. Regulatory agencies often require biological variables to be assessed because
maintenance of environmental quality and protection of aquatic life are their stated managerial objectives.
GESAMP (1996) has pointed out that easily measured surrogate measures of overall environmental change, such as those based on chemical variables, can be misleading. It is also well-known
that biota do not necessarily respond to the presence of contaminants (e.g. Underwood and Peterson,
1988). The variables selected for use in a particular study also depend on the size and operation of
the farm and on the characteristics and sensitivity of the receiving environment (GESAMP, 1996). In
enclosed coastal waters with weak currents and restricted exchange of water, waste materials from
mussel farms are likely to settle on the seabed close to their source and accumulate over time. In these
environments, assessments of impacts have focused on physico-chemical indicators of nutrient and
organic enrichment, such as rates of organic sedimentation, nitrogen dynamics, oxygen demand
and sulphate reduction, in addition to ecological attributes of the associated benthic macrofauna
(Dahlback and Gunnarson, 1981; Kaspar et al., 1985; Baudinet et al., 1990; Hatcher et al., 1994; Grant
et al., 1995; Stenton-Dozey et al., 2001; Christensen et al., 2003; Crawford et al., 2003). In large open
coastal embayments with turbulent mixing and moderate to fast rates of exchange of water, organic and
nutrient enrichment is likely to be much less severe owing to the more rapid and wider dispersal of
aquaculture wastes.
In Australia, mussel farming is a relatively small venture undertaken in embayments along the southern
coast. In 1998, New South Wales Fisheries issued permits for a 2-yr, trial expansion of mussel culture in
Twofold Bay on the south-east coast of Australia. One of the conditions of this permit was that the mussel
growers commission an independent scientific assessment of potential environmental impacts of this
activity. This was done so that the regulatory agency could make an informed decision about future
expansion of mussel farming in the bay. This paper describes the results of the beyond-BACI (before/aftercontrol/impact) sampling programme (Underwood, 1992, 1993, 1994) used to examine the general impacts
of two newly established longline installations and the mussels they support on the infauna in the bay. We
focused on general impacts because experimental analyses of each source of disturbance in isolation are
needed to distinguish impacts associated with the large densities of mussels from those due to the physical
presence of the culture installations. Infauna were selected for study, in preference to mobile organisms,
because their sedentary habit continually exposes them to local environmental conditions (Jones and Kaly,
1996). This is, as far as we can ascertain, the first study of the potential ecological impact of mussel-farms in
a large, open coastal embayment and the first to incorporate pre-impact data.
Copyright # 2006 John Wiley & Sons, Ltd.
Aquatic Conserv: Mar. Freshw. Ecosyst. 16: 289–300 (2006)
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T.A. LASIAK ET AL.
METHODS
Study area
Twofold Bay is a large, open coastal embayment near Eden (378 040 S, 1498 550 E), on the south-east coast
of Australia (Figure 1). The bay has a total water area of 30 km2, perimeter of 31.5 km, maximal length of
7.4 km and a 5.1 km wide entrance into the Tasman Sea (www.ozestuaries.org). The general pattern of flow
is clockwise, but is modified by irregular areas of the shoreline and submerged reefs. Currents are mostly
wind-induced or wave-generated. The tidal exchange rate of 12–20% of the bay per day is considered
moderate to large relative to other open coastal embayments in which mussels are cultured (Pacific Seafood
149˚ 55′ E
37˚ 4′ S
Figure 1. Location of sampling sites in and around mussel farms at Torarago and Oman Point in Twofold Bay, Eden.
Copyright # 2006 John Wiley & Sons, Ltd.
Aquatic Conserv: Mar. Freshw. Ecosyst. 16: 289–300 (2006)
IMPACTS OF LONGLINE MUSSEL FARMING
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Management Consulting Group Pty Ltd, 1997). The temperature of the water varies annually from 138C to
218C, the tidal range is 1.84 m and waves have an average height of 1.6 m (Alavi et al., 1994). The musselfarms are restricted to well-mixed saline areas, with great primary production and depths of 6–10 m. They
normally experience moderate current velocities (>10 cm s1), waves less than 1 m in height and swells less
than 2–3 m (Pacific Seafood Management Consulting Group Pty Ltd, 1997).
The first experimental farm in Twofold Bay was established in 1979. This system, based on locally
collected spat of Mytilus edulis grown on ropes suspended below floating rafts, currently covers an area of
7.5 ha between Oman Point and Cocora Point (Figure 1) and produces approximately 45 tonnes of mussels
per annum. In 1998, New South Wales Fisheries issued a permit to Eden Shellfish P/L allowing them to use
6 ha of seabed adjacent to their existing raft for a longline operation and another lease to NSW Cultured
Mussel Growers Association enabling them to establish a 2-ha prototype longline operation at the northwest end of Torarago Point (Figure 1). The longline systems consist of 100–200 m long horizontal headlines
set 30 m apart and securely anchored to the seabed at each end. Each headline is fitted with large floats;
together they support several hundred 5 m long droppers. The latter are initially stocked with around 300
mussels (20–25 mm length) per metre and harvesting takes place around 10.5 months later by which time
the biomass of mussels has reached 10–40 kg m1 of rope (Mike Bamford of Eden Shellfish Pty Ltd,
pers. comm.).
Sampling design
Two sets of control sites, one on either side of each of the longline sites, were established. Each control site
was located 250 m away from a longline site (Figure 1). To ensure appropriate spatial replication within
sites, two areas approximately 35 m apart were selected and, in each area, four randomly located plots were
established (Figure 2). This spatial design was guided by the findings of an earlier study on the spatial
variability of infauna below mussel rafts, at control sites tens of metres away and reference sites hundreds of
metres away in Twofold Bay (Underwood and Hoskin, unpublished report). After 10 years of growing
mussels on rafts in the bay, this study found that impacts were restricted to within 100 m of the rafts. In
November 1999, two sets of ‘before’ samples were collected from the two sites in the bay where the longlines
were due to be established and from the two control sites on either side of the two leases. These represent
two replicated times of sampling before cultivation started. Divers collected five, 10 cm diameter 10 cm
deep cores of sediment from each of the four plots in each area, over a 2-day period. Seventeen months later
(April/May 2001), two further sets of samples were collected from the Torarago plots; these constitute the
‘after’ samples. The selection of appropriate scales of temporal replication was based on the findings of a
previous study on the infauna in another tide-dominated coastal embayment on the New South Wales coast
(Morrisey et al., 1992a). Sampling was done at multiple spatial and temporal scales because previous
studies have shown that the density of the infauna varies markedly at scales ranging from centimetres to
kilometres and from days to years (Morrisey et al., 1992a, 1992b). The overall duration of this study
was determined by the need to report back to New South Wales Fisheries before the end of the 2-yr
trial expansion.
Movement of the longlines at Oman Point 80–100 m closer to shore prior to the second phase of sampling
meant that the ‘before’ and ‘after’ samples from the putatively impacted site were not collected from the
same general area. The new site was assumed to be similar to the original.
Sorting macrofauna
The cores were preserved in 7% formalin in sea water. The macrofauna (animals 50.5 mm) in each core
were separated from the sediment by elutriation onto a 0.5 mm mesh sieve. Elutriation was continued until
no more small animals rose to the surface. The remaining sediment was then transferred to an enamel tray
and checked with a magnifier. Larger animals, such as bivalves and gastropods, were removed. The animals
Copyright # 2006 John Wiley & Sons, Ltd.
Aquatic Conserv: Mar. Freshw. Ecosyst. 16: 289–300 (2006)
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T.A. LASIAK ET AL.
Figure 2. Diagram showing the spatial and temporal components of the experiment to assess potential ecological impacts of longline
cultivation of mussels in Twofold Bay, Eden. In association with each farm, there were two control sites. In each site, there were four
randomly chosen plots in each of two randomly chosen areas. Five replicate cores were sampled two times before and two after the
farms were stocked.
Copyright # 2006 John Wiley & Sons, Ltd.
Aquatic Conserv: Mar. Freshw. Ecosyst. 16: 289–300 (2006)
IMPACTS OF LONGLINE MUSSEL FARMING
295
retained on the sieve were sorted under a dissecting microscope. Polychaete worms were identified to
Family and other animals to Order or Class. Previous studies have shown that identification of animals to
species is not necessary for assessing ecological impacts (Warwick, 1988; Gray et al., 1990; Chapman, 1998).
Statistical analyses and their interpretation
Asymmetrical analyses of variance were used to test hypotheses about changes in mean number of faunal
taxa and mean densities of each of the numerically dominant taxa found at the farm site differing from
changes averaged across the two control sites before and after the longlines were stocked (Underwood,
1992, 1994). Data from the two farms were treated separately. Statistical interactions due to changes in
variables through time being different at the farm as opposed to control locations are indicative of impacts
(Green, 1979; Underwood, 1992). Such interactions may be short-term (e.g. take place between April and
May 2001 after the longlines were stocked) or longer term (from before to after stocking). Impacts that
affect the entire farm (i.e. an interaction involving the farm versus controls) or smaller spatial scales
(interactions among areas or plots, respectively) can also occur. The sequence of procedures described in
Underwood (1992, 1993, 1994) is needed to distinguish between these impacts.
RESULTS
The samples from Oman and Torarago contained a total of 72 and 69 different taxa, respectively.
Cumaceans, amphipods, capitellid polychaetes, ostracods and bivalves dominated the fauna at Oman; these
five taxa accounted for 64% of the total number of animals. At Torarago, ostracods, amphipods, cumacea,
bivalves and syllid polychaetes were the most abundant taxa, comprising 82% of all animals.
After the longlines at Oman were stocked, significant short-term temporal interactions in all taxonomic
groups, except bivalves and oligochaetes, were evident in the plots at the control sites (tests b/a in Table 1).
The short-term temporal variation in number of taxa and densities of polychaetes, amphipods, cumaceans,
ostracods, capitellids and spionids was not greater in the plots at the farm versus control sites than among
the plots in the control sites (tests c/b in Table 1). There was thus no evidence of any short-term impact on
these variables. There was no short-term interaction in the difference in density of oligochaetes between the
plots at the farm and the plots at control sites (test c/a in Table 1) and hence no short-term impact. Shortterm temporal variability in bivalves, however, was evident (test c/a in Table 1), but the change in density
was not coincident with stocking of the longlines (tests c:e and b:d in Table 1).
No significant short-term temporal interactions in the densities of oligochaetes were evident among areas
in control sites, nor among areas at the farm versus control sites at Oman (tests b/a and c/a respectively in
Table 2). At the scale of sites, there was no temporal interaction in the numbers of oligochaetes between the
controls (test d/a), but there was a different pattern of temporal interaction in the differences between sites
at the farm and those in the controls (test e/a in Table 2). The change in density of oligochaetes, however,
was not coincident with stocking of the longlines (tests d:f and e:g in Table 2).
At Torarago, significant short-term temporal interactions in the densities of bivalves, polychaetes and
cumaceans were evident in the plots at the control sites after the longlines were stocked (tests b/a in
Table 1). The short-term temporal variations in the difference in densities of these three taxa in the plots at
the farm versus plots at control sites were not greater than the differences in the plots from one control site
to the other (tests c/b in Table 1). There was, consequently, no evidence of a short-term impact on these
taxa. There was no short-term temporal interaction in the number of taxa or densities of amphipods,
ostracods, oligochaetes, capitellids and spionids in the plots at the control sites (tests b/a in Table 1). There
were also no short-term temporal interactions in the differences between the numbers of taxa or densities of
the three annelid taxa between plots at the farm and plots at control sites (tests c/a in Table 1). Hence, there
Copyright # 2006 John Wiley & Sons, Ltd.
Aquatic Conserv: Mar. Freshw. Ecosyst. 16: 289–300 (2006)
Copyright # 2006 John Wiley & Sons, Ltd.
Taxa
Bivalves
Polychaetes1
Amphipods
Cumaceans1
Ostracods
Oligochaetes
Capitellids1
Spionids
Torarago
7.52
133.54
0.47
884.96
0.46
374.46
12.08
0.24
1.39
12.52
0.47
483.17
0.49
0.46
0.46
25.30
0.84
0.42
a
Residual
2.36
505.82
1.04
54.09
1.15
583.00
14.62
0.26
0.75
33.64
0.64
2775.65
1.01
1.57
1.10
12.55
3.14
3.60
b
P(A(C)) T(After)
13.86
135.06
0.23
5425.48
0.32
1884.44
10.13
0.28
0.60
14.99
2.19
2137.63
0.69
0.58
0.31
36.11
0.85
0.31
c
P(A(M)) T(After)
Mean squares
13.06
0.72
1.05
0.51
0.99
0.81
3.39
1.67
2.22
10.15
380.25
1.05
1683.23
2.68
1271.10
21.62
0.21
4.47
d
P(A(C)) T(Before)
10.83
0.77
1.22
0.30
1.08
0.41
51.01
2.53
1.96
14.90
108.29
1.22
2954.13
0.54
447.80
6.80
0.23
1.34
e
P(A(M)) T(Before)
1.64
***3.79
*2.20
0.06
**2.52
1.56
1.21
1.05
0.54
**2.69
1.38
***5.74
*2.07
*3.42
**2.39
0.50
***3.72
***8.62
b/a
1-tailed
***5.03
0.84
1.16
0.43
***6.13
1.84
1.43
***4.67
c/a
0.25
0.27
0.22
0.29
0.08
0.77
0.70
0.38
0.27
0.45
c/b
F-tests
4.21
1.84
2.75
c:e
2-tailed
1.17
2.18
***31.11
b:d
a
25.30
Variable
Oligochaetes
Taxa
Oligochaetes
Capitellids
Spionids
Oman
Torarago
34.73
3.06
0.11
0.05
63.98
b
A(C) T(After)
7.81
12.80
0.24
0.80
78.01
c
A(M) T(After)
45.16
0.03
0.12
4.23
5.26
d
C
T(After)
0.35
6.53
2.32
3.01
115.05
e
M
T(After)
Mean squares
0.63
5.74
0.02
4.90
28.06
f
C
T(Before)
4.80
60.42
2.15
1.30
46.25
g
M
T(Before)
*4.62
0.25
0.45
0.04
2.53
b/a
1-tailed
0.64
1.06
1.00
0.58
3.08
c/a
50.01
0.48
3.04
0.21
d/a
0.54
*9.60
2.16
*4.55
e/a
2-tailed
F-tests
1.08
0.19
d:f
6.52
2.49
e:g
Note. C=control, M=mussel farm, T=times of sampling; *, P50.05; **, P50.01; ***, P50.001; F-tests are 1- or 2-tailed ratios of the sources of variation labelled a, b,
c, d, e, f and g (see Underwood, 1992).
7.52
12.08
0.24
1.39
Residual
Source of
variation
Location
Table 2. Results from the sequence of tests used to detect environmental impacts in asymmetrical designs at spatial scales larger than plots
Note. C=control, M= mussel farm, A=area, P=plot, T=times of sampling; *, P50.05; **, P50.01; ***, P50.001;1 data log(x+1) transformed to remove
heterogeneous variances; F-tests are 1- or 2-tailed ratios of the sources of variation labelled a, b, c and d (see Underwood, 1992).
Taxa
Bivalves1
Polychaetes
Amphipods1
Cumaceans1
Ostracods1
Oligochaetes
Capitellids1
Spionids1
Variable
Source of
variation
Oman
Location
Table 1. Results from the sequence of tests used to detect environmental impacts in asymmetrical designs at the scale of plots
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IMPACTS OF LONGLINE MUSSEL FARMING
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was no evidence of any short-term impacts. Significant short-term temporal interactions in densities of
amphipods and ostracods in the plots at the farm and control sites were evident, but did not coincide with
the stocking of the longlines (tests c/a and c:e in Table 1). The change in density of amphipods was not
associated with the farm site (test b:d in Table 1).
A temporal interaction in number of taxa was evident among the areas in control sites, but not in the
difference between areas at the farm and areas in the controls (tests b/a and c/a in Table 2). This difference is
not indicative of a short-term impact. No significant short-term temporal interactions in densities of
oligochaetes, capitellids or spionids were detected among areas in control sites, nor in differences between
areas at the farm and those at control sites (tests b/a and c/a in Table 2). At the scale of sites, there was no
temporal interaction in oligochaetes or spionids either between the controls, or between the farm and controls
(tests d/a and e/a in Table 2). No temporal interaction was evident in densities of capitellids between control
sites, but the difference between the farm and the controls did alter through time (tests d/a and e/a in Table 2).
This change was not coincident with stocking of the longlines (tests d:f and e:g in Table 2).
DISCUSSION
After 18 months of longline operations in Twofold Bay, there was no evidence of any ecological impact on
the total number of taxa or densities of major taxa in the underlying sediment. This was despite the use of a
rigorous sampling design enabling changes in the biota at several different spatial scales in the potentially
impacted farm sites to be compared with those observed at multiple reference sites before and after the
farms began operating. After the longlines were stocked, short-term temporal changes (from April to May
2001) in densities of most taxa in plots at the control sites were, in fact, as different from each other as they
were from those at the farm. This indicates that the densities of taxa at the farm sites were within the range
of natural undisturbed populations in the bay. These results are not consistent with most of the previous
studies of the impact of mussel farming on infauna done in semi-enclosed waters where there is significant
build-up of wastes. In these areas, the taxonomic composition, overall abundance and biomass, trophic
structure, diversity and structure of assemblages of infauna below mussel culture installations differ
significantly from those at adjacent control sites (Kaspar et al., 1985; Grant et al., 1995; Stenton-Dozey
et al., 1999, 2001; Christensen et al., 2003). It should, however, also be noted that a recent study of the
effects of three long-established combined oyster and mussel farms in sheltered coastal areas characterized
by slow currents (520 cm s1) showed that physical and chemical sediment variables, univariate and
multivariate measures of infauna were not significantly different inside and outside the farms (Crawford
et al., 2003). In this case, the lack of impact was attributed to the relatively small stocking densities on
Tasmanian farms.
The extent to which mussel farming alters benthic fauna depends not only on stocking densities, but also
on the strength of water-flow. In Twofold Bay, the small densities of mussels stocked on the culture ropes,
presence of a 2-m gap between the ropes and seabed, spacing and free-swinging movement of the longlines
prevent significant build-up of biodeposits on the seafloor (Pacific Seafood Management Consulting Group
Pty Ltd, 1997). The prevailing wind patterns and direct exposure of the farms to ocean swells also
contribute to rapid dispersal of biodeposits. Although the possibility of a gradual build-up of biodeposits
over the medium to long term cannot be discounted, it is worth noting that, after 10 years of raft culture in
the bay, samples taken by NSW Fisheries indicated that the proportion of organic carbon in the sediment
below the rafts was not significantly different from that in other areas of Twofold Bay (Pacific Seafood
Management Consulting Group Pty Ltd, 1997). Comparisons of the infauna below these rafts with that at
adjacent control sites revealed minor differences in densities of only four of the 32 groups of animals
examined and only one of these groups, oligochaeta, showed differences indicative of an impact
(Underwood and Hoskins, unpublished report).
Copyright # 2006 John Wiley & Sons, Ltd.
Aquatic Conserv: Mar. Freshw. Ecosyst. 16: 289–300 (2006)
298
T.A. LASIAK ET AL.
A number of factors may therefore have contributed to the contrasting results obtained in Twofold Bay
and previous studies. These include differences in location (open coastal embayment vs semi-enclosed
waters), scale of production (40–50 vs 200–300 tonnes p.a.), duration of operation (18 months vs > 5 years)
and assimilative capacity of the surrounding environment (i.e. well- vs poorly-flushed).
There are several possible explanations for the apparent lack of impact of the mussel longlines on the
benthic macrofauna in Twofold Bay. First, if the fauna are inert to the types of disturbance associated with
longline operations, the latter may not actually cause any impact (Underwood, 1989). Second, the animals
below the longlines may not have been exposed to disturbance of sufficient magnitude yet to elicit a
response in populations. The time required to detect such responses depends on the relative sensitivity of
different organisms, their patchiness in space, recruitment, longevity, etc., the biomass of mussels attached
to the longlines and their rate of production. Given the limited operating period of the two longline farms,
relatively small stocking densities and rate of production of mussels in Twofold Bay, this possibility cannot
be discounted. Third, impacts resulting from mussel farming may be trivial relative to those caused by
periodic natural disturbances such as the lateral surge associated with a 1-m south-east swell which disturbs
the sediment and results in a turbidity layer ca. 2 m deep (Hoskin, pers. obs.). Fourth, impacts associated
with other anthropogenic activities in the bay, such as the discharge from a nearby tuna cannery, cleaning
and discarding of catch by commercial fishermen and mussel rafts, may have masked the effects of the
longlines or, alternatively, resulted in cumulative effects. Such larger-scale impacts would affect the sites
with farms and the control sites, making it difficult to detect an additive effect due to the farms. Since there
is no evidence of gross organic contamination in the area or any indication that the fauna is in any way
affected, the latter are not at all probable.
Our understanding of the functioning of coastal ecosystems and their response to disturbances related to
aquaculture is still woefully lacking. The major issues that need to be resolved with respect to sustainable
aquaculture and conservation are better definition of the potential impacts associated with this industry,
their scale and magnitude in different environments. These would not only facilitate the design of better
experiments and/or monitoring programmes to detect specific impacts, but may also provide some indication
of how the associated problems could be solved. Regulatory agencies need to ensure that these studies are
done over appropriate timescales, particularly in large open coastal embayments where certain types of
impacts (e.g. those associated with enrichment) may take longer to manifest. In countries such as Australia,
where the regulatory agencies charged with conservation of coastal organisms also have a commitment to
ecologically sustainable development, potential impacts associated with aquaculture also need to be assessed
in relation to the use of the surrounding environment by other groups. This activity can only be considered
sustainable once we know that potential environmental impacts are localized, reversible and short-term
(Fernandes et al., 2001). One of the keys to sustainable mussel farming is to ensure that stocking density and
biomass remain in accordance with the assimilative and dispersive capacity of the surrounding environment.
ACKNOWLEDGEMENTS
The authors thank Eden Shellfish P/L and the NSW Cultured Mussel Growers association for logistical support. We
are also grateful to D. Blockley, S. Diller, S. Gartenstein, A. Grigaliunas, W. Green, R. Hunt, G. Kaplan, E.
Lazzarotto, K. Mills, C. Myers, V. Padula, A. Palmer, R. Reinfrank, K. Thorne and M. Worth for assistance.
Preparation of the paper was supported by the Australian Research Council through the Special Research Centre on
Ecological Impacts of Coastal Cities. We also acknowledge helpful comments from anonymous referees.
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