A Scientific Review of the Potential Environmental Effects of Aquaculture in Aquatic Ecosystems - Volume 5

Overview of the Environmental Impacts of Canadian Freshwater Aquaculture

C.L. Podemski
Freshwater Institute, Environmental Science Division, Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Manitoba, R3T 2N6, Canada

P.J. Blanchfield
Freshwater Institute, Environmental Science Division, Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Manitoba, R3T 2N6, Canada

Executive Summary

Worldwide, aquaculture operations have been linked to a number of environmental effects that include nutrient enrichment, habitat alteration, and damage to wild fish populations (Gross 1998). A sound scientific understanding of potential effects on the freshwater environment is required if the industry is to grow in an environmentally sustainable fashion. This document provides an overview of the current state of scientific knowledge of the environmental effects of Canadian freshwater aquaculture activities, and identifies areas for future research. The use and potential environmental influence of chemotherapeutants is outside the scope of this review. World literature that is relevant to aquaculture practices occurring in Canada has been included because the scientific literature dealing expressly with Canadian freshwater aquaculture is extremely limited. Substantial changes in husbandry techniques have occurred in the aquaculture industry and these changes have rendered older publications less relevant to the current experience. Wherever possible, we have limited review to peer-reviewed scientific information published within the last decade.

The effects of aquaculture are complex and related to the production and release of organic waste materials as well as the interactions between cultured species and wild species. The bulk of aquaculture waste constitutes fish metabolic wastes and uneaten feed. Factors affecting waste production include fish size, water temperature, and husbandry practices (i.e., feed composition, ration, and feeding methods). The primary environmental concerns associated with waste generation are the potential for nutrient-induced stimulation of local algal blooms and the creation of hypoxic waters and sediments underlying net pens. The primary mechanism through which escaped fish affect native freshwater fish species is competition for limited resources and predation.

The primary constituent of solid wastes is faecal material with waste feed a secondary and much smaller component (Ackefors and Enell 1990). Faecal production, which is difficult to estimate accurately, ranges from 15% to 30% of applied feed (Costello et al. 1996; Cho and Bureau 2001; Bureau et al. 2003). Waste feed estimates, which are rarely reported, constitute between 3–40% of feed (Weston et al. 1996), and anecdotal reports and modeled predictions suggest that waste feed at Canadian farms is currently approximately 5%. There is a gap in data regarding feed waste. Solid wastes settle to the lake bottom where they are consumed by biota (Johansson et al. 1998) or decompose. The greatest accumulation occurs directly under cages (Enell and Lof 1983), suggesting that direct effects on sediments may be geographically restricted. Sediments beneath fish cages generally show enrichment in phosphorus, nitrogen, organic carbon, and zinc (Cornel and Whoriskey 1993; Kelly 1993; MacIsaac and Stockner 1995; and Troell and Berg 1997). Although there is extensive literature on the benthic effects of marine aquaculture, few recent publications document the benthic effects of freshwater aquaculture. Few peer-reviewed Canadian studies have been published within the last decade. Effects of fish farm wastes may be similar to those associated with other forms of organic enrichment, including decreased taxa richness and diversity, and increased abundance and dominance of organisms resistant to sedimentation and low oxygen availability (Hynes 1963; Johnson et al. 1993). Generally, effects on the sediments and benthic community are restricted to areas directly under the pens and a small distance away. There are no published studies of the recovery of sediments and sediment-associated communities at former Canadian farm sites. In Scottish freshwater lakes, significant alterations of benthic communities below cage sites were still apparent more than 3 years after cessation of farming (Doughty and McPhail 1995). Recovery of lotic systems from fish farm emissions is generally more rapid than in lentic systems, due to the increased dispersion of wastes by water flow and the relatively swift re-colonization by invertebrate drift (Doughty and McPhail 1995).

Dissolved carbon, nitrogen and phosphorus are released into the water column by solubilization from feed and faeces, and through the gill and urinary excretions of fish (Bureau and Cho 1999). Approximately 3 to 10 kg of phosphorus and 39 to 55 kg of nitrogen are released to the environment for every metric ton of fish that is produced (Ackefors and Enell 1994; Cho et al. 1994; Bureau et al. 2003). The majority of phosphorus in farm wastes is lost to sediments as solids (Enell and Ackefors 1991; Phillips, et al. 1993). Nitrogenous wastes, particularly ammonia and urea, form the largest component of the dissolved waste fraction. In general, detectable increases in water column ammonium or ammonia concentrations are reported in the vicinity of cages (NCC 1990) and in receiving waters downstream of land-based facilities (Selong and Helfrich 1998). There are no published reports of concentrations exceeding local water quality guidelines or causing toxicity, and concentrations downstream of land-based facilities are reported to return to background levels 400 m to 12 km from cages (Selong and Helfrich 1998). Cage farms that are located in shallow basins or basins with poor flushing have often reported detectable increases in total phosphorus, while farms located over deep water and with adequate flushing have generally reported no detectable change. Several studies have reported elevated phosphate in waters receiving effluent from land-based farms (Munro et al. 1985; Trojanowski 1990). The decomposition of solid waste accumulations results in the release of labile P to the water column (Kelly 1992; Kelly 1993). During periods of stratification, phosphorus released from sediments into hypolimnetic water will not be available for primary production. There has been little research into the cycling of P between farm waste accumulations and the water column, and the proportion of this P that is eventually available for primary production is unknown. This knowledge would be of significant value to the sustainable management of the industry.

Decomposition of wastes may result in hypoxia in sediments and the water column (Axler et al. 1998) but these outcomes have been rarely reported. Respiration by cultured fish may produce localized reductions in dissolved oxygen concentrations. Reports of reductions in dissolved oxygen concentrations in the vicinity of net pens are variable, but for the most part reductions are minor and of short duration at sites with adequate water exchange (Weston et al. 1996; Demir et al. 2001; Veenstra et al. 2003). A single study in the primary literature has provided limited data about dissolved oxygen profiles at Canadian cage farms in the last decade (Hamblin and Gale 2002), suggesting that the collection and compilation of these data from Canada is required. The biological and chemical oxygen demand of wastes discharged from land-based aquaculture facilities can reduce dissolved oxygen concentrations in lotic waters for short distances downstream, however there are no recent Canadian data.

Stimulation of pelagic bacterial populations may result from nitrogen, phosphorus, and organic carbon in dissolved metabolic wastes and leaching from faeces and feed. A single study investigating effects on pelagic microbial communities reported no increase in the abundance of bacteria near net pens in British Columbia, but significantly higher production (MacIsaac and Stockner 1995). Microbal stimulation has been observed in lotic waters receiving fish farm effluents. For example, river water and sediments downstream of fish farm effluent outfalls in New England showed a significant increase in bacteria abundance and heterotrophic activity when compared to control sites (Carr and Goulder 1990a).

Studies in Canadian lakes have thus far found no differences in chlorophyll a concentrations between control and farm sites (Cornel and Whoriskey 1993) and only localized effects on periphytic algae (MacIsaac and Stockner 1995). In addition to stimulating production in bacterial populations, the release of nutrients from aquaculture facilities can enhance primary production (Kelly 1993). In Finland, fish farm emissions into a lake resulted in significant increases in chlorophyll a and primary productivity and changes in species composition of phytoplankton (Eloranta and Palomaki 1986). Primary productivity in rivers can be stimulated by discharges from land-based facilities (Carr and Goulder 1990b). For example, Munro et al. (1985) reported a significant increase in epilithic algal biomass, chlorophyll a, and changes in algal species composition downstream of hatcheries in several British Columbia streams.

There are no published studies on the effects of freshwater cage-culture operations on native fish communities in Canada. The potential influences of cage operations on native fish communities include trophic alterations and interactions between native and farmed fish. In Canadian freshwaters, cage-culture generally occurs in oligotrophic systems. Nutrient enrichment of oligotrophic systems can lead to greater in-lake growth of native and stocked fish species (Stockner and MacIsaac 1996). Further trophic changes in native fish species can occur through the consumption of waste feed and faeces. Consumption of wastes by biota may reduce the localized effects of waste build-up under pens but there have been no attempts to quantify this mechanism in Canadian ecosystems.

Cage farming inevitably results in a small number of escaped fish, even in the absence of any catastrophic containment failure. The causes of escape include storm damage, collisions, predator attacks, vandalism and accidental losses associated with fish handling. There are no published estimates of the numbers of farmed fish that escape freshwater net pens in Canada. Studies in other countries have estimated that escaped fish within given freshwaters represent approximately 3% to 5% of total cage production (Phillips et al. 1985). Predation and the competition for limited resources are the principal ways that escaped fish can alter the native fish community. The introduction of a new species, or greater numbers of a species already present, into an ecosystem results in some redistribution of resources among the fish community. The characteristics that favor certain species for aquaculture are the same ones that may allow these species to flourish when introduced into foreign water bodies (i.e., generalists with broad environmental tolerances). However, there are no published studies that provide information on the survival of escaped fish in North American freshwater ecosystems.

Species interactions, especially those from the establishment of self-sustaining introduced species or the alteration of indigenous gene pools, are potentially damaging consequences of aquaculture. The escape of farmed salmonids is not necessarily equivalent to the intentional introduction of the same species for management purposes. Agencies responsible for stocking programs may have different selection criteria and thus prefer different broodstock than that selected for farmed fish. The traits selected for aquaculture programs differ significantly from those required for survival in the wild (Bridger and Garber 2002) and divergence in behavior between native and domesticated fish increases with time in captivity. Interactions between escaped farmed fish and wild fish may be very different than interactions between native fish and stocked hatchery fish that have established self-sustaining populations, depending upon how much selection has occurred in the broodstock. The extent of any permanent effect of escaped farmed species depends on successful reproduction in the wild with other farmed, hatchery, or native fish of the same species, or through hybridization with closely related species. Interbreeding of farmed and native fish or farmed and naturalized stocked fish can produce long-term genetic changes in these populations that can be detrimental (McGinnity et al. 1997; Fleming et al. 2000).

Knowledge Gaps and Research Priorities

The environmental effects of marine aquaculture are fairly well documented, but little research has been done on the environmental effects of freshwater aquaculture on Canadian ecosystems, and such studies have been extremely limited elsewhere. Research is needed in the areas listed below.

• There is need for knowledge about the factors that determine the amount of accumulation or the fate of bottom deposits in freshwater ecosystems. Information regarding current waste feed levels and feed conversion ratios at commercial freshwater finfish facilities in Canada is required to support this research.
• Research is needed to determine what effects freshwater aquaculture activities have on benthic habitats and communities. Research is also required to elucidate the relationship between amount of waste deposition and the severity of effects on benthic communities.
• There is a need to research the recovery of sediments and sediment-associated biota after fish farming has ceased.
• Research is needed to develop an understanding of how phosphorus, nitrogen and carbon from aquaculture facilities cycle in the freshwater environment to determine the ecological consequences of these subsidies.
• There is a need for knowledge about the effects of aquaculture wastes on the species composition, biomass, and productivity of primary producers, microbial communities, zooplankton, and native fish populations.
• Research is needed to determine the role of native (and escaped) fish in the removal and dispersion of phosphorus through consumption of waste feed and faeces. This information will improve the ability to estimate the proportion of waste phosphorus that becomes available to affect primary production.
• There is need for knowledge about the effects of all life-history stages of escaped fish in Canadian aquatic ecosystems. Accurate documentation on the number of escaped fish entering freshwater systems and a determination of the degree and outcome of wild, stocked and farmed fish ecological and genetic interactions is needed to support this research.

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