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

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.

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