Overfishing 2: Aquaculture - The Struggle for Balance

This is a continuation of the Overfishing series. While Part 1 is not needed to understand this essay, all parts will work together to paint the bigger picture of overfishing and the impact our civilization is having on the oceans.

To continue where we left off, we now turn to aquaculture, or fish farming.  Due to the ever-increasing world population and the overfishing of wild fisheries, aquaculture has been an incredibly fast-growing market and has now surpassed wild catches as the most prolific form of fish harvesting.  If you take a step back and think about it, it’s astonishing that it took this long for aquaculture to grow larger than wild catches, given that all our other food comes from domesticated sources.  Can you imagine if all the meat came from wild game? If all the berries were picked on the edge of football fields and the side of roads?  If all the fruit came from wild trees?  The rise of civilization is so tightly associated with agriculture that we take it for granted.  This temporal difference in aquaculture and agriculture speaks volumes to the vast bounty of the oceans.  It took us thousands of years, billions of people, diesel engines, state of the art electronics and sonar, and a wonton care for the environment, to finally start buckling the wild fish supply.

More than 50% of all seafood is now produced by aquaculture.  This is a dramatic increase that occurred in recent decades.  In 1950 there were around 1 million tons of fish farmed; today that is over 55 million.  The majority of aquaculture is performed by China.  While China has a long history of fish farming that dates to antiquity, things really sped up in the 1990s and have only increased since.  Between 1971 and 1990 China farmed around 3 million tons of fish per year.  In 2022, the number is around 45 million tons.  In general, Asia provides most of the world’s farmed fish.  Aquaculture is one of the great pillars of the food web and gives humans a different lever to pull in regard to providing protein to the ever growing (for now) population.  Just like in every other facet of life, nothing is free and fish farming has some downsides.  One of the problems with aquaculture is that we envision it like farming animals on land.  For most livestock, we feed them food that we cannot eat, such as grass, to create food that we can eat, such as beef.  We turn the inedible edible.  While this is true for the most popularly farmed fish in China, like the carp, this is not the case for some of the fish we, at least in America, love the most.  These include salmon, sea bass, and tuna.  These fish are all carnivores and we have to feed them perfectly edible fish such as sardines, shrimp, and krill.  As fish farming grows in scale, the use of smaller fish to feed larger fish that we prefer to eat has some unintended consequences.  The most obvious being trophic waste, that is waste that occurs when something ‘higher up’ on the food chain eats something lower down.  If we were to just eat the perfect edible smaller fish, or farm fish that are not primarily carnivores, this could alleviate some of this trophic waste.  Due to the major financial pressures in the aquaculture industry to limit the amount of smaller fish used in farming fish there has been success in increasing the amount of vegetable feed these carnivores get; however, omega 3 fatty acid, which is crucial to the diet of a salmon, is still only found in fish.  There has been progress in this area; in the year 2000, for every pound of salmon produced, it needed to be fed a pound of fish (in addition to vegetable feed), that 1:1 ratio is now down to 0.69:1.

Farmed fish also often suffer from density issues, such as disease and parasites, from the high population in close proximity.  To combat this, some farms continuously treat their fish with antibiotics, pesticides, and fungicide.  As the pens are mostly in the sea (at least for salmon), this can lead to contamination of these medications to the surrounding ocean.  The same is true for the waste from fish farming, which often collects in the ocean floor directly underneath the farming cages.  As these cages are often in shallow areas and many cages are in close to one another, this can lead to a large amount of waste pollution, which can contribute to localized dead zones.  Spoiler alert: I plan to cover dead zones, waste runoff, and nutrient pollution and depletion in a following essay.

Another issue for salmon farms is that they are often in the path of spawning salmon.  In the wild, it is more difficult for juvenile salmon to get parasites from adult salmon as they don’t encounter adult salmon due to the nature of the distances traveled to spawn; but when juvenile salmon must pass several giant fish pens on their way to the ocean, they can get parasites, such as sea lice, which can be deadly.  These sea lice can then be transferred to the wild population and wreak havoc.  Another disease spread through fish farming is salmon anemia, which is characterized by pale gills, pinpoint areas of bleeding, and bulging eyes.  This virus can be carried by salmon who have themselves recovered or even the previously mentioned sea lice.  Chile in 2007 halved the local Atlantic salmon population by culling after this virus was detected.  Aside from the diseases and waste, there is an issue of salmon escaping and joining the wild population.  Salmon that are used in farming are not the same as wild salmon and are not subject to the same selective pressures.  The farmed salmon are often genetically modified to grow faster and have a higher fat content. Wild salmon need to be able to swim up stream in strong currents and jump waterfalls.  If farmed salmon get out, it is unlikely that they will be able to meet these demands; however, the danger comes with the potential for the escapees breeding with the wild population.  This can cause the proliferation of different genes into the local gene pool and negatively impact generations of wild salmon.   

Sticking to disease but moving on from salmon, another example of high-density ocean farming causing widespread disease is white spot syndrome in shrimp.  This virus appeared in China in the 1990s and has the ability to decimate a shrimp farm within days.  By the mid-90s, this virus caused the virtual collapse of the Chinese shrimp market and had made it to the Americas.  As a demonstration of the dual nature of human resourcefulness and technology, some good has come of this virus.  Before the prevalence of this disease, shrimp were often caught as wild juveniles to stock pens but now farms increasingly rely on certified disease-free brood stock, alleviating some pressure on wild shrimp and the horrible waste that comes with catching them.  Pens also previously exchanged the highly polluted enclosure water with open ocean but there has been a shift toward greater separation between pens and the sea to minimize the possible spread of infection.  This isolation is a good thing as when farms are faced with impending disease, they will often be treated heavily with antibiotics which can then lead to the rise of antibiotic-resistant bacteria.  In Chile, where over 100 tons of antibiotics are used on fish farms, some of which are important in human medicine, bacteria tested from sediment and water near fish farms, not just inside them, have found strains of bacteria that are resistant to several antibiotics.  While obviously bad for fish farms, it can also be bad for everyone else as bacteria have the ability to swap genetic material and share resistance.  (Spoiler alert 2: I plan to cover antibiotic resistant bacteria and the potential impacts in another essay.) This happened in South America in the 1990s during an outbreak of cholera.  A strain of cholera picked up antibiotic resistance from Ecuadorian shrimp farms.  Some E. coli strains have also picked up antibiotic resistance from aquaculture. 

I would be remiss if I talked about fish farming and didn’t talk about the impact the industry (as well as other industries) have on mangrove forests.  It is estimated that 35-40% of worldwide mangrove forests have been cleared, in large part, due to shrimp farming.  Mangroves are an incredibly important part of ecosystem, not only serving as habitats for many species, but they are also incredible carbon sinks, barriers for erosion, and provide a great deal of protection from tropical storms, hurricanes and cyclones, and tsunamis.  We will talk in more depth about coastal erosion in a separate essay.

In the images below, you can see the impact shrimp farms can have on mangrove forests. The bottom picture is prior to mass mangrove deforestation and insertion of shrimp farms and the top is after many shrimp farms were added. The barrier between ocean and land not as clearly defined.

It does seem like there is a little room for hope in this regard as environmental biologists have developed a method of farming fish that yields more product (that is, money) for the farmer while minimizing size.  The idea put forth is that if farmers can produce just as much, or more shrimp, in a smaller area, they can let the other areas they traditionally farmed on return to mangrove forest.  The largest increases in production are caused by more intensive farming, supplying food to the shrimp, aerating the water, and controlling the brood stock.  This obviously will result in an increased cost of farming that will need to be offset; however, if governments can regulate the industry and require restoration of mangroves in certain areas, this could be fruitful. If governments take a hands-off approach, it’s difficult to imagine why the farmers wouldn’t just use the intensive farming methods over their entire area and not restore mangrove forests. 

Even with all these issues, aquaculture is necessary and isn’t going anywhere but improvements should be made sooner than later.  While necessity is the mother of invention, we should try and curb the damage as soon as possible.  Some improvements would be to either decrease or slow down on the farming of predators such as salmon, increase the amount of herbivores and lower trophic fish, like carp, anchovies and sardines that we eat, and we should favor smarter techniques of farming, such as carp polyculture that is practiced in freshwater ponds and rice paddies in Asia.  We need to find species that grow well together.  An ideal scenario would be if the waste of the fish in the farm could be cleaned up by another fish that feeds on it or used for some other productive purpose.  As with many of these topics, a little regulation in the right place can have an oversized impact. 

That’s where I’m going to leave this one.  Next time we’ll focus on some of the environmental impacts our civilization is having on ocean ecosystems, including the terrifying topic of ocean acidification.  This one will likely be the longest and most depressing article of the overfishing series, so buckle up.

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