52 Scientific American, May 2022
them. Today more than 95 percent of Atlantic salmon
sold in the U.S. is foreign-grown, most in net pens
anchored just below the surface in coastal waters.
Open fish pens have been used in Norway since the
1960s and are still in use in Canada and Chile, but envi-
ronmental concerns have led to bans in most U.S. coastal
states. A ban in Washington, which goes into effect in
2025, will leave Maine as the only state that still sanc-
tions their use. Despite rigorous regulations, every year
net-pen operations are cited for environmental and
labor-related malfeasance, infestation by sea lice and
other parasites, and infectious disease. Scientists say an
equally vexing challenge is posed by renegade fish that
escape the pens and mate with wild salmon to produce
offspring that are genetically ill-equipped to survive. This
last threat is especially concerning in Maine, home of the
nation’s last remaining wild Atlantic salmon population.
Feed is another major problem. Salmon farmers
sometimes inadvertently overfeed their stock, and theuneaten feed promotes the growth of algal blooms that
deoxygenate the water as they decompose. This can
lead to “dead zones,” wide swaths of ocean that can no
longer sustain life. Another often cited concern is that
until quite recently, a high proportion of fish feed con-
sisted of small wild-caught “forage fish” such as ancho-
vies, herring and sardines, all rich in the long-chain
omega-3 fatty acids necessary for the growth of larger
fish. From 1950 to 2010 roughly 27 percent of all wild-
caught fish was used in fish meal. Many observers saw
this as a terrible waste: they argued that feeding edible
fish to edible fish is unsustainable and ultimately makes
little economic or ecological sense.
Food scientists have sought alternatives, but it has
not been easy. Carnivorous fish have difficulty digest-
ing carbohydrates, so researchers tried various combi-
nations of plant fats and proteins, including those
found in soybeans. “It’s like Impossible Burger for fish,”
Townsend says. Unfortunately, like humans, not allSALMON NET
PENS off Black
Island, Maine,
run by Cooke
Aquaculture,
were the scene
of a massive
sal mon die-off
in 2021. More
than 100,000
fish perished.
The company
placed the
blame on
unusually low
oxygen levels
in the pens.