The fish that grows all year: genetically modified salmon and gene-edited fish

Part 1: A fish that grows all year

1Salmon is one of the most popular fish in the world, and most of the salmon that people eat is now farmed rather than caught in the wild. Farming salmon takes time, food and space. In 2015, a genetically modified Atlantic salmon became the first genetically modified animal to be approved for food in the United States. Its supporters said it could produce more fish using less food. Its critics worried about the effects on wild fish and on the people who eat it.

2Atlantic salmon do not grow at a steady rate. Like many fish in cold water, they grow quickly in the warmer months of the year and much more slowly in winter. This pattern is controlled partly by growth hormone, which is released from the pituitary gland mainly when the water is warm and the days are long. A farmed Atlantic salmon normally takes about three years to grow from an egg to the size at which it is sold. In the wild, young Atlantic salmon spend their first years in rivers before migrating to the sea to feed and grow.

3The genetically modified salmon contains an extra gene. The gene codes for growth hormone and was taken from the Chinook salmon, a large species from the Pacific Ocean. It is joined to a promoter taken from a different fish, the ocean pout, which lives in very cold water. The promoter normally controls a gene for an antifreeze protein that protects the ocean pout’s tissues from freezing, and it is active throughout the year.

4A promoter is a section of DNA close to the start of a gene. RNA polymerase and transcription factors bind to the promoter before transcription can begin, so the promoter helps to determine when, and in which cells, a gene is transcribed. By joining the growth hormone gene to the antifreeze promoter, the scientists created a gene that is transcribed in winter as well as in summer.

5To make this recombinant DNA, the scientists cut the DNA containing the promoter and the DNA containing the growth hormone gene with restriction enzymes. Each restriction enzyme cuts DNA at a particular sequence of bases, called its recognition site. Many restriction enzymes make a staggered cut, leaving short single-stranded sections called sticky ends. Fragments cut with the same enzyme have complementary sticky ends, so their bases can pair. The enzyme DNA ligase then joins the sugar–phosphate backbones together.

6The recombinant DNA was copied many times by inserting it into plasmids and growing the plasmids in bacteria. It was then injected through a very fine glass needle into fertilised salmon eggs, a method called microinjection. In a small proportion of the eggs, the DNA became inserted into one of the chromosomes. These fish were identified by testing their DNA, and they were bred to produce a line of salmon in which the gene was passed on to every generation.

7Because growth hormone is now produced all year round, the modified salmon grow quickly even in winter. They reach the size at which they are sold in about 18 months instead of three years. In an illustrative comparison, they also needed about 20 per cent less food to produce each kilogram of growth, because less of their food was used for maintenance during the longer life of a normal salmon.

8In an illustrative trial, modified and unmodified salmon that had hatched on the same day were grown in separate tanks of the same size, at the same temperature and on the same food. After 16 months, the mean mass of the modified salmon was 2.8 kg and the mean mass of the unmodified salmon was 1.1 kg. The researchers also measured the length of every fish and checked their fins, gills and skeletons for abnormalities, finding no significant differences between the two groups apart from size.

9Growth hormone is a peptide hormone. It is carried in the blood to target cells in organs such as the liver and the muscles, where it binds to receptors in the cell surface membrane. Binding activates a chain of reactions inside the cell, which changes the activity of transcription factors in the nucleus. The liver responds by releasing another hormone, insulin-like growth factor 1, which stimulates cell division and protein synthesis in muscle and bone.

Part 2: Keeping modified fish contained

10The main concern about genetically modified salmon is what might happen if they escaped. Hundreds of thousands of ordinary farmed salmon escape from sea cages every year when cages are damaged by storms or seals. If modified salmon escaped and bred with wild salmon, the growth hormone gene could spread into wild populations. Faster-growing fish might also compete with wild fish for food. Escaped farmed fish can also spread diseases and parasites, such as sea lice, to wild salmon.

11Several safeguards are used. The modified salmon are all female, and they are triploid, which means that their cells contain three sets of chromosomes instead of two. Triploid fish are produced by exposing the eggs to high pressure shortly after fertilisation, so that the eggs keep a set of chromosomes that would normally be lost. Triploid salmon grow normally but are sterile. The fish are also grown in tanks on land, far from the sea, rather than in cages in coastal waters.

12Triploid fish are sterile because of what happens during meiosis. Normally, the two chromosomes of each homologous pair line up next to each other and then separate into different cells. With three copies of each chromosome, the chromosomes cannot pair and separate evenly, so the cells produced have unbalanced sets of chromosomes and almost no working gametes are made. The method is not perfect, though, and a very small proportion of the fish may still be fertile.

13Experiments have tried to predict how modified fish would behave in the wild. In an illustrative study, modified and wild young salmon were kept together in artificial streams. When food was plentiful, the modified fish grew faster. When food was scarce, they fed more aggressively but also took more risks, and fewer of them survived to the end of the experiment than wild fish. The results suggest that the effect of an escape would depend greatly on local conditions.

14"The biology of this fish is impressive, but the hard questions are not really about biology," says Dr Ingrid Solberg, a fisheries geneticist. "They are about trust, labelling, who controls the technology and what happens if something goes wrong. A fish that grows twice as fast is only useful if people are willing to buy it and if we are confident that it will stay where it is supposed to be." She argues that independent monitoring, not just company data, is needed to build that confidence.

15The modified salmon was finally sold to consumers in 2021, after more than 25 years of development and safety testing. Many shops and restaurants refused to sell it, and some countries did not allow it to be imported. Farming in land-based tanks also turned out to be expensive, and in 2024 the company that produced the salmon announced that it would stop farming the fish. Small amounts had already been sold in Canada from 2017.

Part 3: Editing instead of adding

16A newer approach changes an animal’s own genes rather than adding a gene from another species. In CRISPR-Cas9 gene editing, scientists make a short guide RNA molecule with a base sequence complementary to the target sequence in the gene. The guide RNA binds to the Cas9 enzyme and leads it to the target, where Cas9 cuts both strands of the DNA. When the cell repairs the cut, a few bases are often added or deleted by mistake. This can change the reading frame, so the gene no longer codes for a working protein.

17In 2021, Japan approved a gene-edited red sea bream for sale. The fish have an inactivated gene for myostatin, a protein that normally limits the growth of muscle. The edited fish produce about 1.2 times as much muscle as unedited fish on the same amount of food. A gene-edited pufferfish, which lacks a working receptor for a hormone that reduces appetite, eats more and grows faster than normal pufferfish.

18Because no DNA from another species is added, some countries regulate gene-edited animals in the same way as animals produced by selective breeding, as long as the change could also have happened naturally. Other countries treat them in the same way as genetically modified organisms, which means much longer and more expensive testing before they can be sold. In some places, foods made from gene-edited organisms must be labelled, while in others there is no requirement to tell consumers how the animal was produced.

19Some biologists are concerned about the welfare of edited animals. Cattle breeds that naturally carry a mutation in the myostatin gene have very large muscles, but the calves are often so large that they must be delivered by caesarean section. Fish with extra muscle may have problems with their skeletons or their ability to swim. Monitoring the health of edited animals over several generations is therefore an important part of testing. Breeders also have to check that an edit has not changed other, similar sequences elsewhere in the genome by mistake.

20"Whether we add a gene or edit one, we are making choices about what animals are for," says Dr Solberg. "The tools are now so precise that the science is rarely the limiting factor. The real work is in deciding, openly and carefully, which changes are worth making and which are not. That conversation has to include farmers, consumers and the people who live near the farms." "Gene editing does not remove the need for that conversation. It makes it more urgent."