Invertebrates could provide a sustainable protein source for fish food

Nearly one third of the global fish catch is currently processed into animal feed. Much of this is then used in the growing aquaculture industry, which means we are effectively recycling fish to produce fewer than we started with. As marine stocks become ever more depleted, using fish in this manner is becoming increasingly unsustainable. In some instances, marine species are so heavily fished that they are facing extinction. A new approach is urgently needed.

Fishmeal is a ready source of protein, however, and that will be challenging to replace. Scientists have identified many potential alternative protein sources in the invertebrate kingdom, but limited research has been done to see if bugs and worms can provide long term adequate nutrition for a range of captive fish.

So a team from WALTHAM, together with the National University of Ireland in Galway and Harper Adams University UK, have been investigating whether fish food incorporating protein from ragworms and silkworms could fill the gap.

The initial results have just been published in the journal Aquaculture Research and progress looks promising.

Choosing carp

Carp were chosen for the feeding trials. They are an important freshwater farmed fish in Eastern Europe, India and China where demand is high. In 2010, for instance, China has produced over 15 million tonnes of farmed carp. Furthermore, Koi carp are highly prized in the ornamental pet fish trade.

The team replaced a quarter of the fishmeal in mirror carp food for a period of eleven weeks with either rag worm meal or silkworm pupae. These fish were then compared with a control group solely fed with fishmeal protein.

All the diets balanced to ensure that the amounts of nitrogen, lipids and calorific content they offered were the same. That meant that the researchers could be sure that any variation in weight gain would be due to differences in the protein content.

The carp that grew most were on the fishmeal and rag worm diet. They gained 50% more weight than those on the fishmeal and silk worm pupae diet. The least weight gain was seen in the control group and those fish fed on a combination of all three proteins.

Cleaner water?

The researchers also looked at concentrations of ammonia in the fish. Ammonia is a major end product of nitrogen metabolism in fish. The efficiency with which individuals metabolise protein is linked to how easily utilised the dietary protein source is, which is then reflected in plasma ammonia concentrations. In fact, the ragworm/fishmeal group had significantly lower amounts of ammonia compared to the others. This suggests that the nitrogen is being effectively metabolised from the protein in the diet.

This also means that there is likely to be less ammonia in the fish tank water. Given that this can be a serious pollutant in aquaculture systems, ragworm may be a more environmentally friendly ingredient than fishmeal for mirror carp and could improve fish health and welfare. This would be is equally important for ornamental fish kept in tanks.

Overall, the decrease in plasma ammonia and increase in protein efficiency ratio suggest that the ragworm inclusion diet has an amino acid profile that is a better fit for the dietary requirements of fast-growing mirror carp.

“Invertebrate meals look as though they can offer a sustainable feed material for farmed and ornamental carp,” says WALTHAM’s Donna Snellgrove, one of the paper’s authors. “What’s more they can be sourced from waste, such as silk production in Asia. However there is still much that we do not know. The aquaculture community needs to broaden research to look at novel protein sources for a wider range of fish species within different production systems and phases of development.”