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Food 101: Farmed Salmon (bioaccumulation)

August 3, 2003

On Friday afternoon I had a bit of a headache. A Thursday night for a friend, who announced a baby was on the way, was celebrated by six guys drinking and eating steamers (little neck clams), steak, and lobster. All of the cooking came courtesy of the father-to-be who pulled off a miracle with a borrowed grill, and culinary tools more suited to stir frying over an electric stove than a flaming half drum. My night lasted until nearly 3am. I woke up with a face sore from laughing, 15 minutes to make it to a 7:30 am haircut, and a instructions for making steamers written on my hand.

The headache came around 1pm, not a real headache, but a furrowing of the brow when I read yet another food had been added to the danger list. A July 30, 2003, New York Times article* reported that farmed salmon contains dangerous levels of PCBs--substances that cause cancer, play havoc with the immune system, etc, etc.
(* I read the Thursday paper on Friday.)

Why salmon? Was this just another paranoid scare statistic? I re-read the article today and it turns out that it's quite valid but also predictable. First, it's farmed salmon, not wild salmon, that has the higher level of PCBs. Secondly, it's not that the salmon are farmed, but how they are farmed. Specifically, the feed given to the salmon makes them toxic to consume.

Most farm raised salmon are fed fishmeal. Fishmeal comes in two basic types: low fat fish meal created from lean fish and industrial grade fishmeal, made from high oil fish. The second type, the industrial fishmeal, accounts for "90% of world fishmeal production" and is used for farm raised salmon. The intended effect of the fishmeal is to raise the bodyweight of the salmon. The oils from the fishmeal pack fat onto farm raised salmon--52% more fat than wild salmon. But the weight gain from additional fat also results in higher levels of PCBs and other contaminants in the farmed salmon.

Farm raised salmon is dangerous because of the process of bioaccumulation1. When you eat salmon, your digestive processes break down the contaminated salmon fat, releasing the PCBs into your system. Your body then stores the PCBs in its own fat. So if you eat several salmon, the PCBs of each one become stored in your body. The PCBs eventually leave your body but over the course of years. The danger comes when you eat salmon often--the PCBs and other contaminants bioaccumulate inside of you.

This article from Thursday was not new news--the knowledge first came to light in a 1987 study indicating that farm raised salmon feed contained high PCB levels. However, the article being displayed in the New York Times has the power of changing the buying behavior of consumers and ultimately influencing the type of feed given to salmon. Some salmon farms do raise their fish on better feed. With more consumer knowledge this trend will continue.

Related links:

What's Behind That Farmed Salmon Steak? (with cartoons)

EPA fact sheet on PCBs and Fish (PDF file) (September 1999)

Two Studies Compare Levels of Contaminants in Farmed versus Wild Salmon (Early Fall 2002, Cornell University)

Potential Ecological Risks of Aquaculture (Harvard Medical School)

Biomagnification: how DDT becomes concentrated as it passes through a food chain (good graphic)

1In the case of the salmon, the process of bioaccumulation begins with PCBs in the water. For example, plankton in polluted waters begins to accumulate PCBs. When a fish eats plankton, the PCBs are ingested and stored in the fat of the fish. As the fish eats more plankton with PCBs, the amount of contaminants in the fish's fat accumulates. If a bigger fish eats several of these smaller fish, the same process occurs--the PCBs are passed on and stored in the fat of the bigger fish. The process continues up the food chain with the contaminants being passed in increasing concentrations to each "consumer".