Tarang Mehta: super fish for fish suppers

Tarang Mehta, image courtesy of Tarang Mehta

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We’ve just heard how geneticists are working to make wheat a more reliable crop for the future. Now we’re going to be moving from bread to the fishes, and the future of fish farming.

Across the world, over 3 billion people, or nearly half the global population, rely on fish as a significant source of animal protein, and since 2012, more of our fish has come from aquaculture or fish farms rather than catching wild fish. As the global population continues to grow, farmed fish are going to become an increasingly important protein source, but fish farming is not without its own issues.

The amount of freshwater fish eaten each year continues to increase, but fresh water is already a scarce resource and one that is only going to become more precarious as climate change intensifies. Farmed fish need to be fed, and so in the case of predatory species like salmon, we currently have to catch fish from the wild to feed the farmed salmon.

Dr Tarang Mehta is a molecular evolution scientist at the Earlham Institute who has been looking at future-proofing one group of fishes in particular, tilapia, which is already a hugely important fish for people around the world, as Sally Le Page found out…

Tarang: So we primarily look at tilapia, which is the second most farmed fish in the whole world, just behind carp, and provides really, really important resource for nutrition in a lot of developing countries. And so a lot of our research is aimed at tilapia within Africa, of which there are the majority of the species of which they are native.

Sally: So tilapia is a group of species of fish.

Tarang: Yeah, that's right. So tilapia is a cichlid. We've always been quite interested in cichlids. Not even just us, but a lot of scientists, just because they're a prime example of an adaptive radiation. And really the most speciose modern example of an adaptive radiation.

Sally: By radiation, what are we talking about? We're not talking about like superman fish, fish giving off energy.

Tarang: No, when we talk about radiation... So an adaptive radiation is basically, the rapid diversification of a single lineage from a common ancestor into loads of species that can inhabit a variety of environments or use a variety of resources; and therefore their then differ traits required to exploit those resources.

Sally: So they're really good at breaking off into separate species.

Tarang: Yeah. And so within the last kind of 10 million years, one few ancestral lineages of cichlid fish have independently radiated into well over 2000 species.

Sally: 10 million years, and you get 2000 species?!

Tarang: It is off the scale! And so to kind of put that into perspective, in basically the same 2 to 3 million years it took for only 14 species of finch, bird species, to evolve in the Galapagos, around a thousand cichlid species evolved in Lake Malawi alone.

Sally: That's bonkers! Darwin should have gone to Lake Malawi. He would've had a field day!

Tarang: I mean, yeah, indeed. If Darwin did dabble and go a little bit deep water, then he would've explored something quite incredible.

“Standard grey-looking fish”

One of the tilapia species, Astatotilapia burtoni, ready for sampling. Image courtesy of Tarang Mehta.

Sally: So we're talking about tilapia. What do tilapia look like? Because I think when I visited The States, I may have eaten tilapia as just a generic white fish. But what does it look like when it's not on a plate?

Tarang: Yeah. So you're likely to have eaten tilapia. You can get tilapia in the UK as well. Tilapia is quite a… it's quite a large fish. It grows to something like 50 - 60 centimetres kind of in size. It's a bit sad really because it doesn't look like the really nice kind of cichlids, which are really fanciful and colourful, and have really funky kind of mouths and eyes and stuff like that. It's just a kind of standard grey looking fish.

Sally: And if they've come from, or if a lot of these cichlids are in the Great Lakes, does that mean they're all fresh water fish?

Tarang: No, not at all. So tilapias are quite different and there's roughly 50 or so species of tilapia and they're mainly freshwater. But then you get some - and what's quite cool then about tilapia is you get some which are highly adapted as well.

Tarang: So you can find some offshoots almost of tilapias, which are found in quite salty waters, murky waters, quite alkaline waters. You get a really cool offshoot of tilapia called Alcolapia grahami, which is a small fish. We're talking about a few centimetres here.

Sally: Is it called Alcolapia Tilapia?

Tarang: Yeah

Sally: That's a great name. Alcolapia Tilapia? That sounds like a wizard spell.

Tarang: It does indeed. I mean, they're almost wizardry in the way that they've adapted to their environment. So they're found in quite crazy lakes, some which are in the north of Tanzania and then the south of Kenya. And we're talking about temperatures which are above 40 degrees, super salty, super alkaline. And essentially, they go off into the kind of the middle of the lake to quickly feed during the day, but then quickly have to swim back into kind of slightly cooler waters. Otherwise they will just cook.

Sally: Yeah. Salty, about 40 degrees. They come pre boiled, pre-seasoned.

Tarang: Oh indeed! And then they've got... what's quite cool is they've got a specialised oesophagus to deal with being in alkaline waters. Because obviously if the alkaline water then mixes with the acid within their stomach, then they will be no more basically. So they've got incredible adaptation in that way.

Tarang: And so they're quite cool in a way. So not in terms of food sustainability and eating in general, but if we can learn how fish, which are really closely related to the tilapias, which do grow to such a large size and quite quickly, if we can adapt them to non fresh water environments, then we obviously breed them in non fresh water environments. I mean, fresh water is becoming quite scarce now and already doesn't really make up that much of the water in the world. Sort of like 2.5% only.

Sally: So this is where your work as a geneticist and an evolution scientist comes in. So how domesticated are the tilapia that people eat? Because when we think of, say, farming cattle or farming chickens, these are genetically now very different from their wild ancestors.

Tarang: Of course. Yeah.

Sally: And you get a few strains that are common all around the world. Is everyone farming... when people farm tilapia, are they farming the same tilapia? Is there a lot of diversity? How different is it from a wild fish?

Tarang: It's a really good question. And there's a big problem related to that as well at the same time. So primarily everybody's always focused on the Nile Tilapia because it was the one which was wild caught kind of at first, and it was grown and it grew to such a large size and to a decent weight as well, and everybody liked it.

Tarang: And so what everybody started to do was to take the Nile Tilapia, and then dump it in water bodies in and around parts of East Africa.

Sally: What decade are we talking about?

Tarang: I mean this is not so, so long ago, so we're talking only just a few decades ago.

Sally: Have people not learned that you can't just dump a whole bunch of fish and expect there not to be problems?

Tarang: Yeah, exactly. I mean, when they hybridise so readily - and tilapia do hybridise very, very readily - you then have a massive, massive problem. So essentially then renders them vulnerable to genetic swamping.

Sally: What is genetic swamping?

Tarang: Well, you, you then start to lose the natural genetic diversity that exists within a population because they hybridise. So you then essentially swamp the natural genetic diversity that exists within, say, the Nile Tilapia population. One because then if they start mixing with another tilapia species, which is not so adapted and doesn't grow to a great size, you create horrible hybrids, which will then take over the whole water body. And then you'll lose the natural population then as well at the same time.

Don’t play with your food!

Tarang Mehta dissecting a tilapia as part of his field work, image courtesy of Tarang Mehta.

Tarang: And so that's been a major problem within East Africa as well. And so some of our efforts have looked at trying to sample what exists within different lakes in East Africa. So what species already exist there, whether we see any evidence of hybrids and hybridisation as well.

Tarang: And almost using that to then inform farmers of what exists there and what really should not be going on. So we shouldn't be introducing, saying, Nile Tilapia into a particular lake where we could have Mozambique. We want to keep those two fish separate.

Sally: You said tilapia was the second most important commercial fish?

Tarang: Second most farmed fish in the world just behind carp. Yes.

Sally: That's bonkers.

Tarang: So yeah, global tilapia production, I think, reached something like 6 million tons for the first time in 2020. And so it really highlights the kind of huge growth in freshwater aquaculture, but more so in tilapia as well.

Tarang: So tilapia is a fantastic fish in the developing world. It's heavily farmed in Africa, Asia and also South America.

Sally: Well, the two things I know about fish farming that are of common issues... when I'm thinking of fish farming, by the way, I'm thinking of like Scottish salmon, so it might, it might be quite different from tilapia.

Sally: But it's one, you've gotta collect all the food for these fish. So you've gotta catch a load of fish to feed the fish that you then eat. And two is that then you get these diseases, these parasites, that leave these highly dense populations of fish in these tunnels of nets and go into the world. Is that the same thing with tilapia as it is with like salmon farming?

Tarang: No. No at all. It's another allure I think of tilapia as well. They don't require kind of high protein fish meal, like what salmon does. Tilapia could happily survive off what's kind of pre-existing and almost in a way some of the kind of waste, which is in the water as well. And it's so much cheaper to then farm this fish.

Tarang: And then, if you then think beyond the kind of Nile Tilapia and ones which are adapted to environments which are a little bit harsher, it then also becomes a lot cheaper to have this fish where it doesn't need fresh water. And so kind of in that context, our actually wider research, not only looks at the analysis of wild tilapia, natural tilapia, which is found in Africa.

Tarang: But we also look at improved strains of tilapia; the genetically improved farmed tilapia, otherwise known as GIFT, that was actually developed by World Fish, and they also developed another strain, which was called the Genetically Improved Abbassa Nile Tilapia, known as GIANT...

Sally: Geneticists do love their acronyms.

Tarang: I know, I know. Yeah, and so, this is developed by an organisation called World Fish. So they're a nonprofit research and innovation organisation. And so to put it into perspective, around 50% of tilapia, which is farmed globally, comes from GIFT.

Sally: So these are genetically improved fish.

Tarang: Yeah, that's correct.

Sally: What are we improving them with? Are we giving them like Steve Rogers super serum

Tarang: [Laughter] Do you know what I kind of, I wish it was something like that and it would be a great story. But it's... I wouldn't say it's boring. The way they created it is actually quite cool. And so, GIFT is a faster growing strain of Nile Tilapia, which is basically derived from a breeding program comprising wild Nile Tilapia from. So essentially they were selected for growth - faster growing - and higher survival rates.

Sally: Is this all conventional breeding methods?

Tarang: This is all conventional, yes. So don't really be duped by when it says "genetically improved". There's no GM or anything like that. This is straight-up breeding program. And so what they found was, in the eighth and ninth generation of the GIANT program, they grew 28% faster and heavier than the commercial strain.

Sally: So that's like the difference between a normal chicken and a broiler chicken is that they grow super big, super fast.

Tarang: Indeed.

Sally: And now we've got this for tilapia. Does this mean though, that we are now getting, if they're all coming from one breeding program, does that mean we're kind of getting a monoculture and making them more susceptible to disease? Particularly if we're shipping them all around the world?

Tarang: Yeah, we can do, yeah. I mean, there was cases of Tilapia Lake Virus, and potentially having an impact on GIFT. But again, they've been selected for higher survival rates as well. So yeah, potentially over time this could be a problem. But again, with our research, what we can do is we can compare... so we can sequence the genomes as we've done of GIFT and now GIANT, but then also wild tilapia - and compare to see where certain regions are for, or markers for, let's say disease like the Tilapia Lake Virus. And so we know the region associated with the Tilapia Lake Virus, so we can monitor and track that.

Tarang: And in one way, what's fantastic is we can then compare the genomes of these improved strains with wild strains to then see how we can improve breeding programs. So can we then leverage traits from wild populations such as salinity tolerance - so the ability to hang around and survive and salty waters - and then bring that over into things like GIFT and improve it further.

Sally: And when you say bring over, is that again normal breeding or is that...

Tarang: Breeding yeah. Normal breeding.

Sally: And are you able to do that because you say that, all of these tilapia and cichlids in general, they're very good at branching off into different species, finding their own niche. So you've kind of already got a natural library. I'm a fly researcher formally, so we, as fruit flies, we literally would just bombard them with things that cause mutants and now we have a library of these weird mutated flies and we can order them online. It was fantastic. But nature's kind of done that for you. You've already got your huge assortment of different traits you can pick and mix from.

Tarang: It has!

Sally: Is it just because they can all still, they're still so closely related that they can still hybridise, that allows you to just rely on selective breeding to pick and choose.

Tarang: Indeed yeah, we can rely on selective breeding. And the way, yes, that you describe it as nature's already done the job for us, is the perfect way of talking about cichlids in general. As in, it's already been screened.

Sally: Yeah. You just got this natural library of mutants.

Tarang: Yeah!

Sally: And final question, what's getting you super excited at the moment for kind of the future of cichlids with your work in genetics? What's the cool new stuff that's happening?

Tarang: I think it's the push now towards the food sustainability work. And really trying to get down to the nitty gritty kind of regulatory networks and genetics behind adapting to something which is relevant towards food sustainability.

Tarang: So looking at salinity tolerance and temperature tolerance, and how some like tilapias, which are a cichlid, have adapted to such an extreme environment. And some just trundle along in their nice, fresh water, but grow to super-sizes. Ones that remain super small but manage to survive and super temperate waters. And then ones that want a kind of lavish lifestyle and nice fresh water and that everybody - well, not everybody but quite a few people - love to eat.

Tarang: And it'll be cool to get down to the genetics and identify the markers for some of these adaptations. And essentially what gets me dead excited about it is just finding just these fine-scale differences that drive such major phenotypes for what are pretty cool fish. Because everyone's always like, well we've got such massive problems in the world with human health and disease, why don't you work on human genetics? But we can learn so much from looking down the tree and looking at fish. And you know, we can't test - we shouldn't be testing much - in humans, whereas we can test stuff in fish. And they're such tractable system for testing some of these things. And we can learn so much in the non-coding space by looking in fish that is translatable to humans, not only a level of health and disease, but then stuff which is important for developing countries and providing food and nutrition on the plate for a lot of these places.