Early failure in fish farming highlighted the importance of omega-3s
by Susan Allport
With all that has been written about the health benefits of wild versus farmed fish, little mention is ever made of the esoteric, but interesting fact that farmed fish were the first animals to be diagnosed with an omega-3 deficiency.
Or to put it another way: it was the aquaculture practices of the 1960s, the beginning of the aquaculture boom that created the first omega-3 deficient fish. These poorly-nourished fish were rainbow trout, and they manifested their deficiency by going into shock—fainting and falling to the bottom of their tanks—when they were being moved from the hatchery. Many of them subsequently died.
Fish farmers called this behavior transportation shock, but they didn’t know it had anything to do with what they were feeding their fish. In the 1960s, farmers everywhere were experimenting with less expensive feeds for their livestock (whether that livestock be terrestrial or aquatic) and those included corn and corn oil, commodities that postwar surpluses had made abundant and cheap. It took a young PhD student at Oregon State University to put two and two together and connect these feeding practices to transportation shock.
John Castell arrived in Oregon in 1965 when the only fats thought to be essential for any animals, including fish, were fats of the omega-6 family, fats derived from the 18-carbon linoleic acid (LA), the principal fatty acid in corn oil. This was accepted truth at the time, but it didn’t make sense to Castell, who had done his master’s thesis with the famous marine chemist Robert Ackman in Nova Scotia and knew that the tissues of fish were full of fats of the omega-3 family, fats that were known to compete with those of the omega-6 family for positions in cell membranes.
So when Castell arrived in Oregon, he and his advisers, Ross Sinnhuber and Don Lee, decided that he should take a careful look at the nutritional requirements of fish. Castell started feeding groups of rainbow trout on diets in which the primary source of fat was either fish oil (high in omega-3s) or corn oil (high in omega-6s), and after about two months, he noticed that the fish raised on corn oil were behaving strangely, especially at night. When Castell turned on the light in his lab, these fish would race about their tank, then fall, in a faint, to the bottom. Many of them would later go belly up. Autopsies of the dead fish revealed heart lesions and mitochondrial swelling, indications of a general metabolic disorder.
Castell searched the literature to see if anyone else had reported this unusual fainting behavior and soon came across references to transportation shock. Fish farmers were creating this problem of transportation shock, he realized, by feeding their fish large amounts of omega-6s. Later, he confirmed this by raising groups of trout on carefully purified diets in which the only variable was omega-6 LA and omega-3 alpha linolenic acid (ALA), the parents of the two families of essential fats. Trout with omega-3 ALA in their diet grew well and were able to handle stress. Those trout with omega-6 LA as their only source of fat were very prone to going into shock.
An immediate result of Castell’s work was that farmed- raised fish were no longer given corn oil as their sole source of fat, and transportation shock became a thing of the past, a curiosity in the annals of aquaculture. But the full implications of Castell’s work—and the full requirements of most fish for essential fats—are still far from understood.
At first Castell didn’t realize that fish, like land animals, also require omega-6s in their diet. Their requirement for omega-6s is only about a tenth that of land animals—0.1% versus 1% of calories—which is why he had a hard time making his trout deficient in omega-6s. By contrast, fish require more omega-3s than land animals: 1% as compared to 0.5% of calories.
And the requirement for omega-6s varies with the fishes’ environment. Fish like sea bass, which live in turbulent in shore waters require more omega-6s than fish like halibut and turbot, which occupy more constant, less turbulent, environments. Scientists think this is because fish living in stressful environments require more omega-6 arachidonic acid (AA)-derived eicosanoids to cope with that stress, an idea that probably has some interesting implications for us humans.
It also took a while for Castell and other scientists to understand that marine fish have different requirements for the different essential fats than freshwater fish, including fish like salmon that begin their lives in fresh water. This became apparent when halibut raised on the same vegetable oils that could sustain salmon had poorer vision and were less able to capture prey than halibut raised on fish oils.
Freshwater fish, it is now known, have the enzymes that enable them to turn 18-carbon fatty acids into the longer and more desaturated EPA, DHA, and AA, so they do well on a diet in which most of their essential fats come in the form of the parent fats. In fact, salmon given too much fish oil in their diet are not as able to osmoregulate their body fluids in sea water, a fact that scientists attribute to lower levels of AA in their tissues. Marine fish, such as turbot and halibut, do not have these enzymes and require more fish oil in their diet, as well as preformed AA.
The fatty acid requirements of fish is a much more complex story than Castell, now a scientist emeritus at St. Andrew’s Biological Station in New Brunswick, first imagined it to be. And aquaculturists are increasingly looking to the fats in the natural diets of wild fish for clues as to the best ratios of essential fats in their captive animals’ diets.
In the meantime, we should remember that fish are what they eat too and that wild fish not only taste best; in terms of what's essential, including these important fats, they also know best.