The vegetarian carnivore: Can we really feed plants to salmon?


On the face of it, the idea of feeding plants to salmon and other carnivorous fish might seem ludicrous, but it’s something that has been developing for a number of years. 

Fish feed is a whole industry in itself – one that generates substantial research to create the ‘optimal’ feed for securing fish health and growth, and of course be cost-effective. Thanks to innovative recipes, the amount of fishmeal used in feed has been declining in developed nations. In 1995, for example, 45% of salmon compound feed was fishmeal but by 2008, this had dropped to 20%, and is predicted to drop to around 12% by 2020.  Some of this fishmeal has been replaced with agriculture and fishery waste. Some of it has come from plants.

Whilst adding plants to feed is possible, the difficulty in creating a 100% plant-based feed for carnivorous fish largely comes down to physiology and nutritional needs. Corn gluten meal, for example, is widely used in aquaculture feeds but it contains a high level of non-soluble carbohydrates which offer few nutrients, and none of the omega-3 that fish require. Marine algae does include omega-3, but adding it to feed doesn’t seem to do much for carnivorous fish. Camelina, one of the few terrestrial plants that naturally produce omega-3, has also attracted attention, but unfortunately it isn’t the perfect solution either. Carnivorous fish need two types of ‘long-chain’ omega 3 – eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are, amongst other things, essential for a healthy brain and heart. With EPA and DHA only found in the marine environment, Camelina’s omega-3 is the ‘wrong’ type. Genetically engineered camelina plants use a gene found in marine algae that produces EPA and DHA at the levels needed for the fish. Young Atlantic salmon at least seem to do well with this new type of feed. But it is arguable whether genetically engineering plants like camelina to produce the nutrients carnivorous fish need, results in an environmentally ‘sustainable’ plant-based feed.

The fact that EPA and DHA are not naturally found in land-based plants presents new ecological issues. When plants grow outside, they become food to myriad land-based animals who would never have been exposed to EPA and DHA. Laboratory tests with cabbage white butterflies fed on omega-3 derived from marine algae saw adults substantially heavier than they would normally be and a range of wing deformities. The omega-3 acids that the engineered camelina produces are different to the marine algae given to the butterflies, but the tests do highlight the potential risk of introducing novel genes into the environment. At the present time, producing engineered camelina in closed containment indoor systems is probably not commercially viable.

Plants are not the only unusual nutrient source being looked at. In Europe, the PROteINSECT project is looking at options to develop insect base feeds for fish and other livestock in the European Union. Insects are packed with all sorts of nutrients, and can be reared on waste foodstuff and manure, though they provide the greatest benefits as feed if they themselves have a good food source – like grain-based chicken feed. A lot of work still needs to be done to fully understand the benefits of insects as feed, like which species are the most nutritious and how best to rear them. As it stands, insects may be able to replace 50% of the fishmeal used in feed – and still give the fish the vital nutrients they need. Insects are, of course, not the perfect answer either. First there is the issue of those marine-based omega-3 acids that salmon need, but terrestrial insects lack. Then there is the question of efficiency in the food chain. We would be using crops to feed to insects to feed to salmon to feed us.

Rather than focusing on the feed itself, some scientists are looking towards the fish. Fish do have some limited ability to convert ‘short-chain’ omega-3 to the EPA and DHA they need. Just how efficient they are at doing so is down to its genes. This means that, just as fish are selectively bred for traits like fast growth rates, they could be selectively bred for their ability to maximize production of their own long-chain omega-3. The fewer long-chain omega-3 they need to get from the environment, the less these will need to be put into feed. Proponents of genetic modification argue that the technology could offer another avenue for reducing aquaculture dependency on dwindling fish populations. The developers of the world’s only genetically modified salmon indicate that their fish require up to 25% less feed – and thus less fishmeal/oil – over their lifespan than ‘conventional’ salmon. Farming genetically modified fish, however, is highly controversial.

Some of these alternative feed developments have likely been spurred on by a desire to increase the sustainability of aquaculture, though perhaps the biggest driver has been declines in fishmeal and fish oil production since 2005 – and of course the increasing price-tag that goes along with a dwindling resource that is unlikely to increase in the future. Reducing reliance on marine life, it seems, could be good for the industry as well as the environment.

More details here with links to sources: Samantha Andrews, Sustainable Food Trust