ICES Journal of Marine Science, 58: 404–410. 2001
doi:10.1006/jmsc.2000.1043, available online at http://www.idealibrary.com on
Characterization of suspended particulate matter surrounding a
salmonid net-pen in the Broughton Archipelago, British Columbia
T. F. Sutherland, A. J. Martin, and C. D. Levings
Sutherland, T. F., Martin, A. J., and Levings, C. D. 2001. Characterization of
suspended particulate matter surrounding a salmonid net-pen in the Broughton
Archipelago, British Columbia. – ICES Journal of Marine Science, 58: 404–410.
A field study was carried out on the central coast of British Columbia in March 1999
to determine particle fluxes arising from a salmonid net-pen during feeding. Water
samples were collected within, beside, and at two depths relative to a net-pen and
analysed for suspended particulate matter (SPM), major and minor elemental abundance, carbon/nitrogen content, and stable carbon isotopes. Sediment traps were also
deployed immediately beside the bottom of the net-pen. The highest mean concentration of SPM (0.6 mg l 1) during the feeding cycle was observed within the central
region of the net-pen. Approximately 87% and 30% of the mean SPM were observed
at depth and beside the net-pen, respectively, suggesting that transport of suspended
particulates was predominantly in the vertical direction. Sediment trap deployments
revealed that sedimentation fluxes of total SPM, carbon, and nitrogen were higher
below the farm than at the control site located 500 m away. Major and minor
elemental analyses of feed pellets and sediment trap contents showed that calcium,
phosphorus, sulphur, and strontium were removed within the net-pen system. The
feed-specific carbon isotope signature (13C= 21.4 to 22.0‰) was not evident in
the trap samples deployed beside the bottom of the net-pen (13C= 23.4‰),
suggesting the relative absence of feed pellet particles, isotopic alteration through fish
assimilation and/or dilution of the isotope signature with other carbon sources.
However, a feed-signature was evident in samples collected in the upper water column
(depth 5 m), suggesting that 13C might serve as a useful tracer of feed particles.
Key words: aquaculture, carbon isotope signature, sedimentation flux, trace metals.
Received 16 October 1999; accepted 15 March 2000.
T. F. Sutherland and C. D. Levings: Department of Fisheries and Oceans, West
Vancouver Laboratory, 4160 Marine Drive, West Vancouver, BC, Canada V7V 1N6 A.
J. Martin: Lorax Environmental Services, 1108 Mainland St., Vancouver, BC, Canada
V6B 5L1. Correspondence to T. F. Sutherland; e-mail: [email protected]
Introduction
The impact of aquaculture on surrounding biota has
been a growing concern because of the rapid expansion
of salmonid farming in the past few decades. Uneaten
feed and faeces contribute significantly to the overall
solid waste production from cage systems (Penczak
et al., 1982). Attention has recently been focused on the
effects of farm discharges on neighbouring shellfish and
eelgrass beds. Antibiotics and metals may accumulate at
these sites through the ingestion of treated feed particulates by shellfish, and through the potential deposition
of these particulates within low energy habitats such as
eelgrass and kelp beds. The Salmon Aquaculture Review
(SAR, 1998) has recommended that a programme be
developed to accurately determine the fate of farm1054–3139/01/020404+07 $35.00/0
derived suspended solids. Knowledge of the dispersal
boundaries of farm discharges will aid in the evaluation
of the existing siting criteria for net cages. For example,
the minimum distance between a net cage and shellfish
bed is 125 m, while the minimum distance between a net
cage and eelgrass bed is 50 m, according to Fisheries and
Oceans Canada guidelines (Levings et al., 1995). However, these standards have been set without a detailed
environmental assessment study. Another recommendation was to review the federal and provincial policy
stating the prohibition of polyculture (combined
shellfish and fish farming systems). Polyculture could
maximize energy flow arising from feed loss and
minimize the environmental impacts of salmonid
farming through the ingestion of feed particulates by
shellfish.
Characterization of suspended particulate matter surrounding a salmonid net-pen
The effects of solid wastes on the environment surrounding net-pen systems have caused shifts in sediment
texture and benthic biodiversity. The accumulation of
uneaten feed and faeces below net-pens may result in: (1)
organic enrichment (Merican and Phillips, 1985; Gowan
and Bradbury, 1987); (2) enhanced bacterial numbers
(Korzeniewski and Korzeniewska, 1982); (3) increased
sediment oxygen consumption (Hargrave et al., 1993);
(4) alterations in biochemical sediment properties
(Troell and Berg, 1997); (5) the production and release
of methane and H2S (Hargrave et al., 1993); and (6)
shifts in benthic infaunal communities (Weston, 1990;
Henderson and Ross, 1995). In addition, antibacterial
agents and drug-resistant pathogenic bacteria have been
reported to persist in the bottom deposits of impacted
sediments (Bjorklund et al., 1991).
It is important to be able to predict the dimension and
characteristics of the sedimentation field under fish pens
in order to determine threshold distances for farm siting
criteria. Further, knowledge of the sedimentation and
transport fluxes of feed particulates will aid in the
determination of potential contamination of sensitive
fish habitats by feed organics, metals, and antibiotics.
Our objectives were to: (1) determine particle concentrations and fluxes surrounding a fish farm and (2)
characterize the composition of farm-derived particulate
matter.
Materials and methods
The study was carried out in the Broughton Archipelago
at a fish farm located on the central coast of British
Columbia between 16 and 18 March 1999. The area
currently contains 29 fish farms (approximately 37% of
the total number of BC fish farms). The net-pen system
consisted of six circular pens (dimensions: 20 m in
diameter and 20 m in height) arranged in two rows.
Sampling was coordinated with fish feeding cycles to
compare suspended particulate concentrations during
and in between feeding events. Fish were fed twice a
day, with each feeding period lasting 30–60 min. Water
samples and sediment trap samples were analysed for
suspended particulate matter (SPM), major and minor
elemental abundance, carbon/nitrogen content, and
stable carbon isotopes. Current speed was determined
using an Acoustic Doppler Current Profiler (Aanderaa
RCM9) at various locations coinciding with the water
sample locations.
SPM samples were collected within and beside a
central net-pen (5 m depth), as well as immediately
beside the bottom of the net-pen (20 m depth), during a
feeding cycle (source and near-field measurements).
Background samples were collected beside the net-pen
during a non-feeding period. A depth of 5 m was chosen
for the upper water column sampling because the fish
405
occupied this depth during feeding periods. The pump
hose (2.54-cm inner diameter) was attached to a Jabsco
bilge pump (Model 37202) that was mounted to the deck
of a small boat moored beside the net-pen. The pump
flow rate was approximately 18 l min 1, and the water
contained within the hose was allowed to flush before
each sample (approximately 500 ml) was collected. The
current meter and pump hose intake were deployed
simultaneously and water samples were collected every
3 min during each feeding cycle. Another series of
samples was collected at varying distances from the
net-pen (1, 2, 5, 10, 15, 20, 25, and 30 m), also at a depth
of 5 m, to determine far-field SPM concentrations; a
rope marked in metres was tied to the fish farm to
measure the distance.
A vertical flux was determined by deploying a current
meter and sediment trap at a water depth of 20 m
immediately beside the central, leeward pen during a
single feeding cycle (approximately 40 min). A sediment
trap and a current meter were also deployed at a control
site located at a similar depth approximately 500 m from
the pen system. The sediment trap design and subsampling were as described in Timothy and Pond (1997).
Sediment traps were deployed in tandem, each with a
height of 0.48 m and an inner diameter of 0.127 m.
SPM was determined gravimetrically through filtration. Approximately 250 ml of seawater was filtered
onto pre-weighed 1-m Nucleopore filters and frozen
immediately. These samples were dried at 55C for 24 h
and subsequently desiccated for 2 h (until a constant
weight was achieved). Blank filters were also analysed by
running filtered seawater through ten replicate filters and
a salt correction applied. Total particulate C, N, and
13C analyses were made at the Department of Earth
and Ocean Sciences, University of British Columbia.
Total C and N were analysed using a Carlo-Erba CHN
analyser (precision 1.2%). The stable-C isotopic
composition was analysed using a Carlo-Erba CHN,
coupled to a VG prism mass spectrometer (analytical
precision 0.2%). The concentrations of major (Al, Ca,
Fe, Mg, K, Na, P, Si, and S) and minor (Sb, As, Ba, Be,
Bi, Cd, Cr, Co, Cu, Pb, Li, Mn, Mo, Ni, Se, Ag, Sr, Th,
Sn, Ti, U, V, Zn, and Zr) elements in suspended
particulates were determined via acid digestion of
Nuclepore filters, followed by multi-element analysis
via inductively coupled plasma optical emission
spectroscopy.
Results and discussion
Suspended particulate material
The highest mean concentration of SPM (0.61 mg l 1)
was found inside the central, leeward net-pen (Figure 1).
Concentrations immediately beside the bottom of the
net-pen were approximately 87% (0.51 mg l 1) of those
406
T. F. Sutherland et al.
The lateral flux, based on a time-averaged concentration
of SPM as well as current speed, was 0.04 g m 2 h 1.
0.9
0.8
Vertical fluxes
0.4
0.3
0.2
0.1
Beside net-pen
non-feeding (5 m)
Figure 1. Mean concentrations of suspended particulate matter
(SPM) in water samples collected within the central net-pen
(5 m depth), beside the bottom of the net-pen (20 m depth),
and beside the net-pen system (5 m depth) during feeding
and non-feeding periods (error bars represent one standard
deviation).
Major and minor elemental abundance
With respect to major elements, the composition of fish
feed pellets was dominated by Ca, P, K, Na, and S
[Figure 3(a)]. Mean (n=3) concentrations of these elements were 16 900, 12 270, 6030, 4900, and 4420 g g 1,
Nitrogen flux (mg m–2 h–1)
250
200
150
100
0
Control
50
Fish farm
Carbon flux (mg m–2 h–1)
30
(b)
12
(c)
(d)
25
10
Carbon:Nitrogen
300
(a)
Control
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Fish farm
Sedimentation flux (mg m–2 h–1)
observed within the net, suggesting that a large percentage of the particulate material was transported vertically. Thirty percent of the SPM concentration in the
net-pen was observed immediately beside the net-pen at
a depth of 5 m (near-field). Time-averaged current
speeds observed inside and outside the net-pen were 6.0
and 5.9 cm s 1, respectively, while current speeds
observed immediately beside the bottom of the net-pen
were 10.8 cm s 1. The net-pens appeared to baffle
ambient current flows, thereby reducing current speeds.
20
15
10
8
6
4
5
2
0
0
Control
Centre of
net-pen feeding (5 m)
Beside bottom
net-pen feeding (20 m)
Beside net-pen
feeding (5 m)
0.0
Sedimentation fluxes were higher immediately beside the
bottom of the net-pen (0.77 g m 2 h 1) than at the
control site (0.48 g m 2 h 1) located approximately
500 m offshore [Figure 2(a)]. The sedimentation rate
associated with the farm site would give rise to an
accumulation rate of approximately 6 mm per year,
given a bulk density of 1150 kg m 3 of deposited
material. Gel-mud deposits typically found in association with aquaculture systems range in bulk density
values up to 1150 kg m 3 (Sutherland et al., 1998). This
accumulation rate does not incorporate deposition and
erosion factors that take place as a result of sediment
transport processes. Sedimentation fluxes of carbon and
nitrogen were also found to be higher immediately
beside the bottom of the net-pen relative to the control
site [Figure 2(b),(c)]. Particulate matter in the sediment
trap was visually determined to consist primarily of
faecal material, while that observed in the control consisted largely of wood particulates. Large wood fibres
observed within all sediment trap samples were removed
to minimize sample contamination with respect to carbon and nitrogen. The results demonstrate the need for
careful selection of control sites when assessing the
environmental impact of fish farms to ensure that influences from other carbon-rich sources are minimized.
Control
0.5
Fish farm
0.6
Fish farm
SPM (mg l–1)
0.7
Figure 2. Sedimentation fluxes of (a) particulate matter, (b) carbon, and (c) nitrogen, and (d) carbon:nitrogen ratio observed beside
the bottom of a net-pen system and at an offshore control site (n=2; error bars represent one standard deviation).
Characterization of suspended particulate matter surrounding a salmonid net-pen
30 000
407
200
(a)
(a)
20 000
100
30 000
Element concentration (µg g–1)
0
(b)
20 000
10 000
0
10 000
(c)
0
200
(b)
100
0
Figure 3. Major element concentrations (error bars represent
one standard deviation) observed in (a) fish feed pellets (n=3)
and (b) sediment traps (n=2), and (c) the difference between
these concentrations (positive values: influx into the sediment
traps; negative values: removal by the net-pen system).
respectively. By contrast, net-pen sediment trap material
was characterized by more uniform concentrations
[Figure 3(b)]. With the exception of potassium and
sodium, the mean values (n=2) for all elements were
<5000 g g 1. Such values are consistent with the concentrations of major elements measured in sediment
trap samples collected in BC coastal inlets (Francois,
1987).
The relative differences between mean values of feed
pellet and net-pen sediment trap data were used to assess
the fate of feed-derived materials in the pen system
[Figure 3(c)]. The negative values observed for Ca, P,
and S indicate that at least 76%, 84%, and 48% of the
respective feed-derived inputs of Ca, P, and S were
retained within the net-pen. In addition to uptake and
assimilation by fish, other possible removal mechanisms
include algal uptake, scavenging by invertebrates growing on the net and/or particle removal as part of a
fouling process. Positive relative differences between
elemental mean abundances (e.g. Al, Fe, Mg, Si, and
Na) reflect the minimum magnitude of input to the
receiving environment from non-pellet sources, and may
represent one or a combination of natural inputs, fish
faeces, or losses from net-pen surfaces.
(c)
–100
Antimony
Arsenic
Barium
Berylium
Cadmium
Chromium
Cobalt
Copper
Lead
Lithium
Manganese
Molybdenum
Nickel
Strontium
Titanium
Zinc
Zirconium
Sulphur
Sodium
Silicon
Potassium
Phosphorus
0
Magnesium
–10 000
Iron
100
Aluminum
0
Calcium
Element concentration (µg g–1)
10 000
Figure 4. Minor element concentrations observed in (a) fish
feed pellets and (b) sediment traps, and (c) the difference
between these concentrations (see Figure 3 for further details).
Contrasts between the trace metal signatures of the
feed pellet and sediment trap data are evident upon
comparison of their respective minor elemental abundances [Figure 4(a),(b)]. The concentrations of the
majority of these elements fell below their respective
analytical detection limits. With the exception of Sr,
trace element concentrations were higher in sediment
trap material than in feed pellets [Figure 4(c)]. In
particular, trap values for Ba, Cr, Cu, Mn, and Ti were
considerably greater than the pellet signatures. The
mean concentrations of the latter of 54, 155, 84, 67, and
128 g g 1, respectively, fall within the ranges observed
in sediment trap material collected in another BC inlet
(Francois, 1987). The feed pellets themselves were relatively trace metal depleted; mean concentrations of Ba,
Cr, Cu, Mn, and Ti were 4, 0.8, 11, 25, and 2.5 g g 1,
respectively. Examination of the relative differences
between mean values of sediment trap and pellet data
implies that non-pellet sources of Ba, Cr, Cu, Mn,
and Ti contributed to the sediment trap trace metal
signature.
The absence of detectable Sr in the sediment trap
samples (detection limit=1 g g 1) suggests that pellet-
408
T. F. Sutherland et al.
Table 1. Mean carbon (C) and nitrogen (N) concentrations (g
mg 1) measured in fish feed pellets and sediment trap samples
moored at a fish farm location and a control site. Standard
deviations are given in parentheses.
Fish feed
pellets
(n=3)
Farm
sediment trap
(n=2)
Control
sediment trap
(n=2)
571 (12)
61 (5)
9.4 (0.9)
637 (269)
67 (21)
9.4 (1.1)
442 (118)
51 (13)
8.7 (0.1)
Sediment trap
(channel)
Sediment trap
(farm)
(a)
Background
Far-field
Near-field
C
N
C:N
Net pen (Trial 2)
Net pen (Trial 1)
Fish feed pellets
derived Sr was quantitatively removed within the netpen system. The concomitant removal of Sr and Ca in
the pen is consistent with their similar biogeochemical
behaviour. Strontium substitutes for Ca in mineral lattices such as calcium carbonate (CaCO3) and apatite
(Ca5(PO4)3), and as a result their distributions are often
well correlated in marine sediments (Calvert, 1993).
Compared to the other trace elements, zinc was present
in relatively high concentrations. Zinc is an essential
element in fish diets and is added to prevent the formation of cataracts in juvenile fish (Richardson et al.,
1986). The observed range in particulate-Zn concentrations in the farm sediment traps (13740 g g 1) fell
within natural background concentrations reported in
another BC inlet (10827 g g 1; Francois, 1987). This
comparison suggests that the majority of pellet-bound
Zn was removed by the net-pen system, even though
an influx was observed within the sediment traps
[Figure 4(c)].
Carbon and nitrogen concentrations
Feed pellet and farm sediment trap material exhibited
similar C and N contents (Table 1). Few inferences can
be made with respect to the fate of pellet-derived C and
N because of the small differences between the two
concentrations. In general, all samples, including the
control, exhibited C:N weight ratios in the range
observed for marine organic matter of 6 to 10 (Andrews
et al., 1998). However, the control ratio was lower than
that observed in the feed pellet and farm sediment trap
samples. The inclusion of the woody debris found within
the sediment traps would have resulted in enhanced C:N
ratios.
Stable carbon isotopes
The stable carbon isotopic signature (13C) was determined in feed pellets, suspended particles, and sediment
trap material to assess the utility of 13C as a tracer for
farm-derived suspended particles (Figure 5). The 13C
signature of feed pellets will reflect the isotopic composition of the protein, carbohydrate, and lipid sources used
–25
Sediment trap
(channel)
Sediment trap
(farm)
–24
–23
δ 13Carbon
–22
–21
–24
–23
δ 13Carbon
–22
–21
(b)
Background
Far-field
Near-field
Net pen (Trial 2)
Net pen (Trial 1)
Fish feed pellets
–25
Figure 5. Mean (a; error bars represent one standard deviation)
and raw (b) values of 13C signatures in feed pellets and also in
particulate matter determined from water samples and sediment trap material. Water samples were collected within the
net-pen during two feeding trials, beside the net-pen (near-field)
and offshore (far-field). Sediment traps were deployed beside
the bottom of a net-pen system and approximately 500 m
offshore (control).
in feed manufacturing. Examination of mean 13C values and the associated precision (1 rsd) demonstrate that
the feed pellets (21.4 to 22.0‰) are comparatively
enriched in 13C (isotopically heavier) in comparison to
suspended particulates [Figure 5(a)]. The observed feed
values are similar to those reported in Ye et al. (1991)
from Australia (13C= 21.53‰) and slightly higher
than the range measured by Hansen et al. (1991) in
Norway (13C= 23 to 24‰). Pellet values in suspended particulates exhibited some temporal variability
associated with the feeding schedule. Near-field values,
for example, ranged from 21.6 to 24.4‰ [Figure
5(b)]. The natural background value is expected to lie in
the range observed between terrestrial organic matter
(23 to 30‰) and marine phytoplankton (18 to
Characterization of suspended particulate matter surrounding a salmonid net-pen
24‰); minor contributions may also be realized from
seagrasses (3 to 15‰) and macroalgae (8 to
27‰) (Fry and Sherr, 1984). In coastal zones, variations in 13C generally reflect the relative proportions
of terrestrial organic matter and marine phytoplankton,
which are governed by primary productivity as well as
hydrographic variations in tidal cycles and seasonal
shifts in local circulation.
A feed pellet 13C signal became apparent within the
water column particulates as the feeding process progressed. Specifically, values increased from 23.5 to
21.91‰ over the 40-min feeding cycle, suggesting a
commensurate increase in feed particle concentration
over this period. The overlap in the signatures in the feed
pellet, Net Pen (Trial 2) and near-field signatures may
indicate the presence of pellet-derived material. Indeed,
the far-field and background (in between feeding cycles)
signatures are comparatively lower. The observation
that the feed pellet and farm sediment trap 13C signals
are isotopically distinct implies that uneaten feed represented a minor component of the total vertical flux
during the feeding cycle at this farm site. In contrast,
Hansen et al. (1991) found that stable carbon isotopes
served as a useful tracer for identifying sediments
affected by fish farms in a Norwegian fjord. The higher
sedimentation flux measured immediately beside the
bottom of the net-pen as compared to the control does
suggest that other farm-derived particulates (e.g. faeces)
contributed in part to the 13C signature in the sediment
trap. Indeed, the presence of fecal material was verified
visually during sample processing. The absence of a
feed-specific 13C signature in the sediment trap material
suggests that (1) isotopic fractionation occurred by fish
digestion, (2) a lack of critical mass of feed pellets
allowed for detection of pellets, and/or (3) dilution of the
feed signature by additional sources of organic matter
occurred.
The study identified the particulate loading surrounding a fish farm during single feeding events. Monitoring
the various types of waste transport caused by different
farming and hydrographic processes may explain the
discrepancies previously found between theoretical calculations and direct measurements of carbon loading.
Future considerations will involve examining variations
in particulate loading that occur over several feeding
schedules and over the outer dimensions of the entire
net-pen system.
Acknowledgements
The work was supported by the DFO Science Branch,
with supplemental funding from the DFO Oceans
Directorate. We appreciate the logistical support and
accommodation on the site from Stolts Seafarm Ltd. We
thank Karen Perry, Wayne Goslin, Todd Casey, and
James Allard for their help during the field programme
409
and Gary Robinson for his help in coordinating field
logistics. Shane Peterson and Sasha Badr were responsible for data input and worksheet management.
2001 Crown copyright
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