The many sides of sustainability in aquaculture

Following recent claims that culturing salmon in open net pens could become the most sustainable form of healthy protein production on the planet, Dr Bill McGraw assesses the relative merits of different aquaculture species and systems. As a specialist in the culture of shrimp in RASs, the conclusion he draws may not come as a surprise.

The sustainability of aquaculture has been debated for decades. Potential issues include habitat destruction, the use of marine ingredients in feeds, freshwater usage, using wild juveniles for farm stocking, influencing wild gene pools through farm escapees, and the excessive loss of stock through disease and associated overuse of antibiotics. 

In this article I will briefly review these challenges, and outline how the impact of different types of aquaculture can differ on each count.  

Environmental degradation

Many opponents of aquaculture claim that environmental degradation is a primary reason why aquaculture is not sustainable. Traditional shrimp farming, for example, has a long history of habitat destruction by converting environmentally valuable and sensitive mangrove areas to ponds. Further to this, water exchange is conducted by at least 90% of shrimp farms to maintain water quality. Over time this can result in eutrophication, resulting in low dissolved oxygen in estuaries. Waste solids from shrimp farms cover estuary bottoms, creating anaerobic conditions. These inhospitable environments contain very low plant and animal diversity and can become biological wastelands.  

Yet water exchange is an aquaculture procedure that is ubiquitous, from the tightly regulated, half billion dollar-a-year catfish industry of the southeastern US to the coastal abalone farming of South Africa, to the land-based tilapia fish farms all around the world. All pen-raised finfish, such as salmon, release all food waste into the water, which can and has degraded aquatic environments the same way as effluents do if estuary and ocean currents do not disperse solids before they are allowed to settle. Some companies have decreased salmon biomass at harvest from >25 to 12 kg/m3 to counteract this.

At 30 million tons per year, seaweeds are the most cultured aquatic organism. They are the most sustainable aquaculture product as they actually improve the environment by absorbing waste nitrogen and phosphorous, without any feed input. Bivalves such as oysters, mussels and clams, which are produced at about half the weight per year compared to seaweeds, also clean the water by consuming algae and solids without any feed input. However, at high densities, bivalves may contribute to habitat destruction by depositing large amounts of waste on estuary bottoms in the form of feces, ejected pseudofeces, especially when solids and algae concentrations are high.

Feed

Another reported problem is the issue of using large amounts of fishmeal in aquaculture feeds. Salmon production needs 4 kg of pelagic fish (such as anchovies) to create one kg of whole fish, with an overall FCR of an incredibly low 1.3. Shrimp aquaculture uses less fishmeal, with similar FCR. Carp and tilapia (1st and 8th most produced finfish species by weight) feed even lower down the food chain, with no fishmeal required for growth and similar low FCRs. It needs to be stated here, however, that not all terrestrial ingredients – such as soybean, corn and wheat – may be easily digested (especially GMO ingredients) by fish and shrimp, which may result in lower nutritional value. An example of this would be the lower concentration of omega-3s in tilapia grown on a diet rich in corn and soybeans, which is not part of their standard diet. These examples are in stark contrast to tuna where it takes 20lb of whole fish to make one pound of ranched tuna grown in pens.

Yet traditional terrestrial livestock farming has much higher FCRs and can be viewed in this context as being less sustainable. For example, it takes 13lb of grain to create one pound of beef, 6lb per pound of pork, and 2.3lb per pound of chickens. One third of all wild caught fish ends up in fishmeal or oil, with 81% of that total going to farmed fish. Most of the remaining fishmeal ends up feeding terrestrial livestock.

Freshwater resources

Another issue of sustainability is the use of valuable freshwater for aquaculture, when water is needed for human consumption and domestic needs. This would obviously not include seaweeds, bivalves and marine finfish, which are all grown in seawater. While recirculating aquaculture systems typically use as little as 100 liters per kilo of product produced, traditional outdoor shrimp ponds use about 15,000 liters of water to produce 1 kg of product, with tilapia at 21,000 and catfish at a low of 4,000. This is roughly equivalent to land animal production, as it takes 13,000 litres of water to produce one pound of pork, with poultry at 7,500 and beef production at 9, 500 liters per pound.

Collection of wild juveniles

A lesser complaint posted by organizations such as Greenpeace is the use of wild juveniles as stock, which can contribute to wild population decreases. This is not an issue with species such as salmon, catfish, seaweeds, bivalves and more recently less of an issue with shrimp. However, milkfish – which are produced at the rate of half a million tons per year – are a different story. Unfortunately, due to the difficulty in producing milkfish fingerlings, almost all juveniles are still obtained from the wild. Overall, however, this is not a major issue for modern aquaculture.

Mortality rates

Although exact figures for all species are hard to come by, it seems approximately 25% of all intensively produced (not including zero water exchange and biosecure) aquaculture production is lost due to disease. The shrimp industry alone has had reported losses of $20 billion during the last decade, and half that the decade before – which is hard to view as sustainable. Salmon farms have been accused of transferring parasites from fish in cages to wild migrating counterparts swimming nearby, making them more vulnerable to bacterial and viral infection. Thus mortalities of wild salmon are claimed to be linked to cultured salmon placed near migration routes.

The overuse of antibiotics to mitigate loss of stock through disease is a serious issue in sustainability in all areas of intensive aquaculture. Antibiotic residues in fish may negatively affect the immunity of the consumer potentially contributing to the creation of “super bacteria” resistant to all known antibiotics. This is more of an issue in countries which do not have tight regulations in place to prevent their indiscriminate use. The issue of the overuse of antibiotics is relevant for all forms of animal husbandry – for example, its use in feed lots to increase feed consumption for beef production.

Diluting the wild gene pool

The disruption of the genetics of wild animals due to escapees from aquaculture farms is a constant concern. Examples of this would include shrimp escaping from the farms off the Pacific coast of Mexico around 2005 and interbreeding with wild populations. Salmon escaping from net pens in the ocean, such as recently occurred both off the coast of Scotland and Washington state in the US, have also been blamed for contributing to changing the genetics of wild stocks. This is a serious issue for cage farming in open waters, and for irresponsible, non-contained pond farming.

Conclusions

Is there aquaculture available that has all positive attributes? In other words:

• Uses lower amounts of fishmeal.
• Uses a very small amount of freshwater per weight of product produced.
• Has a positive impact on the environment by incorporating all waste from the system into other forms of agriculture.
• Has a small footprint with large production per unit area.
• Does not promote controversial GMOs.
• Has a low FCR.
• Does not use juveniles from the wild, or affect wild genetics.
• Loses minimal biomass to disease without the use of antibiotics
• Is able to provide products 365 days a year.
• No contamination from pesticides, herbicides, heavy metals or harmful chemicals