Comparative Risks with Fish Farming

From reading David Niewart, I learn that the Bush administration proposes to massively expand the practice of fish farming to waters as far as 200 miles offshore. David catalogs the impacts associated with fish farming: accumulation of chemical contaminants into the human foodchain; parasite infestations, water quality impacts, and the most significant problem, accidental escapes (or deliberate releases) of farmed fish which represent introductions of exotic species potentially affecting wild populations. Incidentally, the Alaska Department of Fish and Game (ADFG) has a web page addressing the invasive threat of Atlantic salmon. A recent overview of the impacts of fish farming is summarized in this ADFG white paper. According to this white paper, the state of Alaska banned finfish farming in 1990 in order to protect wild stocks from disease, pollution and escaped farm fish displacing wild fish.

This is a tough balancing act. Depletion of wild fisheries is viewed as one of the outstanding problems for nature and ecosystem services, according to the Millennium Ecosystem Assessment. At the same time, The Institute of Medicine and the American Heart Association recommend consumption of fish at least twice a week to attain the benefits associated with intake of the omega-3 fatty acids, which are important for cardiac health. Perhaps fish farming, if it is done in a sustainable manner (to the extent that a feedlot can be sustainable) with best management practices, could reduce pressures on ocean fisheries while giving people the health benefits of fish consumption. That’s admittedly a big if.

With regard to the human foodchain issue, recent studies reported that concentrations of contaminants including dioxins, PCBs, polybrominated diphenyl ethers, and pesticides such as toxaphene and dieldrin are significantly higher in farm-raised salmon than in wild Pacific salmon. A risk assessment published last month in Environmental Health Perspectives concluded that most farmed salmon should be consumed at rates of < 10 meals/month to limit exposure of dioxin-like compounds (DLC) to the Tolerable Daily Intake (TDI) of 1 pg/kg-day (normalized to TCDD - TEQ) published by the World Health Organization (WHO).

Consumption by an adult of farmed salmon at rates that limit DLC intake to 1 pg /kg-day results in a 100% incremental increase over background DLC exposure – where background is considered to be around 65 pg/day, normalized to TCDD. So, the conclusions from this study were that modest consumption of farmed salmon contaminated with DLCs potentially raises human exposure levels above the lower end of the WHO TDI, and considerably above background intake levels for adults in the United States. EPA’s draft dioxin reassessment (currently under review by the National Academy of Sciences) suggests that the upper bound cancer risk associated with 1 pg/kg-day DLC intake could be 1 in one thousand. This is just the estimated risk occurring from fish ingestion alone - not the only source of dioxin intake in humans, and not the only contaminant found in farmed salmon. While the EPA’s dioxin reassessment is controversial and not yet finalized, it was apparently sufficient as a basis for the NAS to look in to approaches to reduce dietary exposures to DLCs.

Those approaches were: reducing introduction of DLCs into the animal production system; reducing DLCs in human foods and modifying food-consumption patterns to reduce intake of foods higher in DLCs (particularly animal fats). The range of options explored included traditional regulatory mechanisms; collaborative and voluntary mechanisms; subsidies and economic incentives; and information and education interventions. Most of these would have institutional hurdles to overcome (food producers will not want to be regulated), but reducing exposure through changing dietary preferences is something that, for many people, could happen right now (healthier diets, particularly ones low in animal fat also could reduce DLC exposure). With regard to changing dietary preferences, the authors of the salmon risk assessment conclude:

Our data provide opportunities to reduce DLC intake and still gain the benefits of ingestion of omega-3 fatty acids by choosing fish, including most wild salmon, that have lower concentrations of DLCs or by eating other foods, such as various nuts, oils, and vegetables that are high in these healthy fats.

Of course the problem there is that wild salmon populations are declining, so while eating wild salmon may be precautionary for reducing DLC exposure, it wouldn’t be precautionary for preserving biodiversity. If it’s important to have salmon in the diet, it may still be worth exploring the possibility of being able to farm salmon in a sustainable manner.