s previously mentioned, according to Cermaq, the typical impact zone from one of its fish farms is 80 ha (198 acres). This means that if there were four active farms in a bay, the impact area would be spread over 320 ha (800 acres). Cermaq estimates that one of their farms would produce 42 metric tonnes of waste per month, and when spread out over the impact area, would amount to 17 kg per hectare per day.
“For context, an average cow in the field creates about 30 kg of solid waste per day and there’s usually more than one cow per hectare.”
Fortunately, Hardy is still at the University of Idaho and now teaches in the Aquaculture Research Institute. I contacted him to see if he thought it was correct to compare the waste from salmon farms with cow manure. Hardy replied:
[A]bsolutely not. The rate of passage of food through a salmon or trout is around 24 hours, although in cold temperatures it could be up to 48 hours. There is little fermentation or microbial degradation of food in salmon guts. Nutrients are removed from food and indigestible material, mainly fiber, connective tissue and bones, plus some odd ball sugars found in soy, is excreted.“Is it more accurate to compare salmon feces to that of a human,” I asked. “No, still not a great comparison,” he said.
Contrast that with a cow where fermentation of fiber and other plant material takes place in the rumen. A cow is essentially a living fermenter. That’s where the methane comes from. The bacteria in cows converts plant material to bacterial protein, etc., and bacterial protein is what provides nutrients to the cow. Cows don’t actually digest hay and grass.
So, comparing cow to salmon feces is totally inappropriate and misleading.
Hardy explained that humans are monogastric animals, with a single-chambered stomach, like dogs, cats, and horses. There is still too much fermentation in monogastric animals. “Maybe poultry” would be a better comparison, he offered.
Applying Hardy’s more conservative 2001 estimate to what Cermaq is proposing for Nova Scotia, where each active farm would be stocked with roughly 900,000 5-kg fish, one farm would produce the fecal equivalent (in terms of weight) of 12,000 people; five farms, 60,000 people. 4
Hardy said that today with “advances in feed formulation and efficiency of nutrient retention, the current values are at least 20% better than before, and maybe even higher.” So, five farms could produce the fecal equivalent of 48,000 people.
- According to NSDFA data, total finfish production in NS in 2018 (the most recent year data were available) was 8.2 million kg. Federal data is from National Aquaculture Public Reporting Data. ↩
- Data from Stats Canada, Fishing-Related Employment by Industry and Province, 2016-2018. ↩
- This is Hardy’s reply in full: “Digestibility of dry matter in salmon feeds is no less than 75%; protein is 85% and fat is 95%. Feed conversion ratios for salmon are about 1.1. If you raise 1,000 mt of salmon, you will require about 1,100 mt of feed (ignoring the freshwater phase before stocking in marine pens). At least 75% of this feed is digested and absorbed by the fish, leaving 275 mt of undigested material as fecal excretion. Of this, most is starch or other non-digestible carbohydrates and some is non-digestible protein, mainly scales, skin, connective tissue and bony material. This is not the same as human fecal waste. Multiply 275 mt by 20 to get the 20,000 mt farm amount and you have 5,550 mt of fairly benign undigested nutrients of which a small amount is protein (nitrogen) and a smaller amount is phosphorus. The remainder is mainly plant material and, as I said, some bony fish tissue. ↩
- An example of this closer to home occurred in early 2012, when Federal charges were filed against Cooke Aquaculture of New Brunswick, alleging illegal use of pesticides to fight a stubborn case of sea lice — a move that ended up killing hundreds of lobsters in the Bay of Fundy. The company eventually pleaded guilty to the charges and was ordered to pay $500,000. ↩
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