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A SCIENTIFIC REVIEW OF THE POTENTIAL ENVIRONMENTAL EFFECTS OF AQUACULTURE
IN AQUATIC ECOSYSTEMS - VOLUME 1
Table of Contents
FAR-FIELD ENVIRONMENTAL EFFECTS OF MARINE FINFISH
AQUACULTURE
B.T. Hargrave
Marine Environmental Sciences, Fisheries and Oceans Canada
Bedford Institute of Oceanography, Dartmouth, Nova Scotia
EXECUTIVE SUMMARY
This review evaluates the existing knowledge and research needs required
to determine the ability of coastal waters to support a sustainable marine
finfish aquaculture industry. A central question is what methods, environmental
observations and models exist, or are required, to determine the capacity
of coastal areas to assimilate additional sources of dissolved and particulate
matter released by cultured finfish.
Pillay (1992) provided an early review of major environmental effects
of all types of aquaculture on a worldwide basis. Over the past decade,
several international groups have considered various environmental issues
surrounding the development of marine finfish aquaculture (Rosenthal 1988,
1994; GESAMP 1991, 1996; Buerkly 1993; Stewart et al. 1993; Ervik et al.
1994a, 1997; Stewart 1994, 2001; Rosenthal et al. 1995; Silvert and Hargrave
1995; Burd 1997; Goldberg and Triplett 1997; Milewski et al. 1997; Fernandes
et al. 2000; Harvey 2000; Milewski 2000; EVS 2001; Holmer et al. 2001).
Much of the information on environment-finfish aquaculture interactions
in publications cited above is focused on measurable near-field changes
in water and sediment variables sensitive to organic matter and nutrient
additions.
Despite the difficulties of observing far-field effects, published
literature shows that in some locations, measurable effects attributable
to finfish aquaculture development have been observed at the ecosystem
level. The impacts may be categorized into three types of broad-scale
changes distant from farm sites: eutrophication, sedimentation and effects
on the food web.
It is a common observation that the amount of suspended particulate
matter increases in the immediate vicinity of finfish net-pens. When feed
pellets are distributed by hand or automatic mechanical feeders, a fine
dust may potentially be transported in the air or trapped in the water
surface film and spread over a broad area. Unconsumed feed pellets and
fish feces usually contribute to increased local concentrations of suspended
and sedimented particulate matter. While much of the released material
is assumed to settle rapidly at or near cage sites (Gowen et al. 1994;
Silvert 1994e; Findlay et al. 1995; Findlay and Watling 1997), there is
potential for horizontal transport and widespread dispersion, particularly
in areas with high currents (Sutherland et al. 2001; Cromey et al. 2002).
Holmer (1991) collected material, directly attributable to a finfish aquaculture
source, at distances up to 1.2 km from a farm site in Danish coastal water.
The extent to which resuspension and lateral transport increase sedimentation
at locations remote from farm sites depends on both physical and sedimentological
processes. Tidal flow, residual circulation, patterns of turbulence, wind
and wave energy, and flocculation (aggregation) will determine large-scale
patterns of particle dispersion. The distances and locations of accumulation
are highly site-specific and depend on bottom topography, currents, erosion
and flocculation processes that affect the residence time of material
both in the column (Sutherland et al. 2001) and on the bottom (Milligan
and Loring 1997).
Specific compounds associated with organic matter, such as fatty acids,
digestible proteins, sterols, elemental sulfur, pristane and stable carbon/nitrogen
isotopes (Li-Xun et al. 1991; Johnsen et al. 1993; Findlay et al. 1995;
McGhie et al. 2000) and trace elements such as zinc that might be used
as tracers of fish feed pellets, have been measured in surface sediments
to determine far-field dispersion patterns (Ye et al. 1991; McGhie et
al. 2000; Sutherland et al. 2002; Yeats 2002). Alteration of bottom type
to more fine-grained sediments through enhanced deposition of flocculated,
fine-grained material may also account for the speculation that a population
of lobsters was displaced from their historic spawning ground after a
salmon farm was located at the site (Lawton and Robichaud 1991). However,
an opposite effect of salmon farm operations causing aggregations of lobster
may also occur. Salmon farm sites may be a refuge for lobsters from harvesting.
Eutrophication is the process of natural or anthropogenic enrichment
of aquatic systems with inorganic nutrient elements (Jørgensen and Richardson
1996; Strain and Yeats 1999; Cloern 2001). Long-term eutrophication
of coastal and estuarine waters results from the additions of both dissolved
inorganic and organic nutrients and increased biological oxygen demand
(BOD) from oxygen-consuming material from all sources (Rosenberg 1985;
Costa-Pierce 1996; Johannessen and Dahl 1996; Cloern 2001). Dissolved
inorganic nutrients released by finfish culture and regenerated from sediments
enriched with sedimented organic matter under fish pens may stimulate
phytoplankton production and increase oxygen demand. It is often difficult
to accurately estimate the magnitude of additions of nutrients and organic
matter from finfish aquaculture when many environmental factors and possible
sources of addition occur (Einen et al. 1995; Strain et al. 1995). Models
can help determine the relative amounts of organic loading from aquaculture
from all natural sources (river discharge, tidal exchange, rainfall, phytoplankton
and macroalgal production) and human inputs (Valiela et al. 1997). The
degree of nutrient enrichment is influenced by the scale of aquaculture,
local hydrographic characteristics, the magnitude of other sources relative
to aquaculture and internal processes, such as uptake by phytoplankton,
algae, internal recycling, resuspension of fine material, and uptake by
biofouling communities that colonize net- pens.
The effects of eutrophication may extend into shallow water littoral
and intertidal zones. Intertidal areas, subject to daily movements of
water and sediment, are locations influenced by broad-scale processes
affecting chemical fluxes of mass and dissolved material throughout an
inlet system. Nutrient enrichment can stimulate the extensive development
of macroalgal beds (Soulsby et al. 1982; Petrell et al. 1993; Campbell
2001), which have a large capacity for nutrient uptake (Chopin and Yarish
1999; Chopin et al. 2000) and may affect benthic fauna through changes
in the rates and nature of deposition of particulate organic matter (Bourget
et al. 1994). However, few studies have unequivocally linked the establishment
of aquaculture farm sites to environmental or ecological changes in intertidal
areas.
Eutrophication can alter the ratio between essential nutrients (carbon:
nitrogen:phosphorus), as well as absolute concentrations by causing a
shift in phytoplankton species assemblages. It has proven difficult to
directly relate the occurrence of harmful algal blooms (HAB) to finfish
farms. As with other types of plankton blooms, many environmental factors
appear to control the formation of HABs. Water column mixing and stratification
that maintain cells in the photic zone with an adequate nutrient supply
are critical variables. In contrast to numerous studies of localized benthic
effects of finfish aquaculture at farm sites, there have been very few
observations of effects on plankton communities (Burd 1997). Reductions
in zooplankton standing stock with oxygen depletion could allow standing
stocks of phytoplankton to increase. With sufficient nutrient and light
supplies, higher rates of primary production and increased sedimentation
would result in even further oxygen depletion in deep water.
There is an extensive literature documenting changes in benthic infauna
community structure associated with high levels of nutrient and organic
matter additions (Burd 1997). Only fauna (e.g. nematodes and polychaetes)
tolerant of low oxygen conditions and reduced sulfides are able to survive
under conditions of high organic sedimentation (Hargrave et al. 1993,
1997; Duplisea and Hargrave 1996). The presence/absence of these 'indicator'
species or faunal groups may show transitions from lower (background)
levels of organic matter supply to high deposition rates caused by unconsumed
feed pellets and fish feces in areas subject to low transport (Weston
1990; Pocklington et al. 1994; Burd 1997). Moderate increases in organic
matter supply may stimulate macrofauna production and increase species
diversity; however, with increasingly higher rates of organic input, diversity
and biomass decrease.
Widespread changes in species community composition of benthic macrofauna
distant from farm sites are more difficult to detect and have been less
studied. Temporal and spatial scales of changes in benthic macrofauna
species composition and biomass have been measured over the past decade
in some areas as part of long-term monitoring programs near net-pens to
determine if organic enrichment effects from aquaculture can be detected
(Burd 1997; Brooks 2001). Most studies have shown that the local extent
of altered benthic community structure and biomass is limited to less
than 50 m. Water depth and current velocity are critical factors determining
patterns of sedimentation around cage sites (Weston 1990; Pohle et al.
1994; Silvert 1994e; Henderson and Ross 1995; Burd 1997; Pohle and Frost
1997; Brooks 2001; Cromey et al. 2002), and therefore impacts of benthic
fauna differ at different farm sites. In southwest New Brunswick, organic
enrichment effects at newly established farm sites were localized to within
30 m of cages. After approximately five years, changes were measurable
over greater (>200 m) distances. Macrofaunal community diversity was
most reduced close to a farm site that had been in operation for 12 years,
but significant declines in diversity also occurred throughout the inlet
system. Benthic epifauna and infauna at two intertidal sites at varying
distances from aquaculture sites showed that the diversity of infauna
was significantly higher away (>500 m) than near (<500 m) farm sites
(Wong et al. 1999). Loss of diversity at distances less than 500 m may
indicate that benthic infauna are more sensitive to organic matter additions
than epifauna (Warwick 1986, 1987), possibly reflecting changes in sediment
physical structure (grain size), oxygen supply and sulfide accumulation
associated with increased organic matter supply.
Another far-field effect of local sources of organic matter produced
by finfish farm sites involves the use of chemotherapeutants. Antibiotics
in medicated fish feed have the potential to induce drug resistance in
natural microbial populations on an inlet-wide scale. Concentrations of
a commonly used antibiotic, oxytetracycline (OTC), largely disappeared
within a few weeks, but traces of the antibiotic were detectable for up
to 18 months (Samuelsen et al. 1992). In Puget Sound, the highest numbers
of bacteria (as colony-forming units) in sediments generally occurred
at farm sites (Herwig et al. 1997), but the proportion of OTC resistant
bacteria declined exponentially with increasing distance from a farm.
Ervik et al. (1994b) also observed antibiotics in fish and wild mussels
near a farm site after medicated food had been administered, and OTC resistance
has been observed in bacteria cultured from sediments up to 100 m away
from salmon farm sites in inlets in the Bay of Fundy where salmon farms
are concentrated (Friars and Armstrong 2002).
GAPS IN KNOWLEDGE
1. There is a need to determine sustainable
levels of salmon production within coastal regions, inlets or embayments
where marine finfish aquaculture is currently practiced in Canada.
2. Mass balance models of nutrient loading
(inorganic and organic) from all sources (natural and anthropogenic) may
be used to assess potential additions from finfish aquaculture. Budgets
must take into account internal nutrient recycling as well as external
sources.
3. General circulation models can be developed
and improved to resolve combined effects of tidal and wind-driven forcing
and that reflect complex topography and intertidal drying zones.
4. New methods are required to quantify
processes of resuspension that redistribute fine material produced locally
by finfish aquaculture sites over large areas.
5. New methods are required to quantify
processes, such as flocculation and aggregation, that affect dispersion
of particulate matter from finfish farm sites.
6. Studies are required to determine
if the frequency and location of HAB or plankton blooms are related to
the expansion of finfish aquaculture.
7. New studies are required to determine
changes in water column variables in areas of intensive finfish aquaculture.
In comparison to benthic studies, there have been very few investigations
of changes in planktonic communities around finfish aquaculture sites.
8. Further studies are required to document
environmental or ecological changes in intertidal areas and to determine
if these can be linked unequivocally to the establishment of aquaculture
sites.
9. Mass balance and numerical models are
required to link production and external loading with aerobic and anaerobic
oxidation of organic matter (pelagic and benthic), sedimentation and sulfide
accumulation in sediment.
10. Further studies are required to determine the extent of
far-field effects on ecological and biological impacts of antibiotic resistance
induced in microbial and other wild populations in areas of intensive
finfish aquaculture.
The complete papers can be found in the following document:
Fisheries and Oceans Canada. 2003. A scientific review of the
potential environmental effects of aquaculture in aquatic ecosystems.
Volume 1. Far-field environmental effects of marine finfish aquaculture
(B.T. Hargrave); Ecosystem level effects of marine bivalve aquaculture
(P. Cranford, M. Dowd, J. Grant, B. Hargrave and S. McGladdery);
Chemical use in marine finfish aquaculture in Canada: a review of
current practices and possible environmental effects (L.E. Burridge).
Can. Tech. Rep. Fish. Aquat. Sci. 2450: ix + 131 p.
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