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  • 15 mins
  • Level 1: Introductory

Ever Wondered About... Fish?

Updated Wednesday 17th February 2016

Chef Paul Merrett and scientist David Shuker explore the science of fish - and why some advisors want us to eat more, but others want us to lay off.

Fish and chips served on 'newspaper' Creative commons image Icon sstrieu under Creative Commons BY-ND 4.0 license Copyright The Open University


Paul Merrett: Hi, I'm Paul Merrett, chef and presenter of the BBC Open University TV series Ever Wondered About Food? where we explore the science of food and cooking whilst preparing some fantastic recipes. Throughout the series I've had the huge support of David Shuker of the Open University, making sure that we get the science right, and we've just finished doing the Fish programme.

And I think it's a really, really interesting programme, with a sort of strange paradox lying behind it because, whilst we're told to eat far more fresh fish because it's so good for us, there are those who would have us stop eating it altogether because the supply of fresh fish is really under threat from over fishing. So I think it's a subject that we've approached with responsibility, I hope.

David Shuker: Yes, Paul, I must admit I was fascinated by your comment in the programme about your preference for fish that are line-caught, and I guess the image came up in my mind of us all going out with our rod and line to catch fish, but I don't think that's what you had in mind, and perhaps you could tell us a little bit about what that means to you and…

Paul Merrett: Well, where I come at fish from is, my dad's a marine biologist, and I'm completely and utterly immersed in fish, all my life, even to the extent of being born on the first day of the fishing season - which I'm sure he contrived -but one thing that I've become very aware of, through talking to him, is that the stocks of fish left in the oceans are seriously under threat due to over fishing and, were it up to him, he would stop fishing altogether tomorrow. Now from a chef's point of view that would be a tragedy because cooking fish is probably the pinnacle of any young chef's progress through a kitchen; that's the last thing you get to cook, because it's the easiest thing to mess up, and it's also the most expensive protein that we deal with.

But when it comes to line catching fish, in kitchen terms what we really mean by that is a fish that's been pulled out of the sea on a rod and line; is having no further ecological impact on the ocean than that one fish being pulled out of the sea on a rod and line. Whereas using trawler nets, or the other methods of fishing, there's often a by-product and there's often a huge damage done to the seabed, so you're actually pulling up a fish to sell but you're also destroying a lot of the marine ecology at the same time.

David Shuker: Yes, no, absolutely. And of course in some senses, more generally, that carries with it the notion of a sort of fairer relationship - that is, you go out there to catch the food that you want and you don't end up, as you say, pulling out lots of other fish which, in the process of catching them, you'll probably kill them, in which case you throw them back, but then you've absolutely disturbed the ecology. So in the way people think, and like to think, about the better relationship with food, then that makes sense.

I just wonder how practical it is because, when you think about all of the advice to eat more fish - and I think there's very good evidence that fish is part of a healthy balanced diet and there's all this recent interest in certain fish oils being important for development of the brain in growing children and all these kind of things, so I guess we've got to find a middle road which enables us to eat a balanced diet, which is good for our health, but at the same time is sustainable so that obviously if we decide it's a good thing, it can still be a good thing for our children and their grandchildren, and it's getting that right.

Paul Merrett: Absolutely. Responsibility - I think that's the key word. And maybe really decent organic fish farms is a good way forward as well; you know the sea bass that we've actually used on the programme is a very well-farmed sea bass, it's not a wild fish at all, and it's very good quality - very good quality. But there are impacts, even there. I think it takes ten grams of protein to convert to one gram of sea bass protein, so you're sweeping up all the sand eels and feeding that to your fish. Well there's nothing there for the sea birds.
So it is a balance and I do think that, in this day and age, we've got to a point now where we do know the facts and we mustn't be blind to them; but I agree with you that I couldn't bear to never cook fish again, it's a really interesting thing to cook. And I think it's something that people in this country don't cook as much as they could… in Britain, don't cook as much as they could.

I'm not sure why. We're an island. We should be eating lots of fish, simply because we're surrounded by sea, but I meet lots of people who love it in restaurants but never cook it at home; whether it's the smell of the thing or the entire animal thing, I don't know, but… So if we can get people to go out and buy the right fish and cook them in the right way, then I think that's a great thing.

David Shuker: Yes, I mean it is interesting, of course. I mean, as you're saying this I'm thinking that actually fish is the last wild food - at least predominately wild food - that we eat, because, after all, a lot of the vegetables we eat, it's farmed in an organised way - we don't rely on going and eating wild fruit or foraging. All the other meat that we eat, the cattle and all the rest of it, are also farmed consciously - we don't go out and hunt wild buffalo or whatever, so… But fish is out there as that one resource, that one major food resource that we go out and we rely on its natural occurrence. And so maybe, because all of the other things became part of agriculture centuries, perhaps thousands, of years ago, the idea of farming fish seems a radical thing; but actually it will be more consistent to do it that way.

I think the other thing about fish farming is, there have been a lot of concerns recently that, because of the very constrained way the fish are penned and all the rest of it, they end up accumulating environmental toxins and all the rest of it. And my view on this is interesting because I think the health benefit of eating fish vastly outweighs the chance that you might pick up a small amount… I mean anything which is living in the environment is going to accumulate some residues of metals and pesticides.

Paul Merrett: So is it what's been washed into the sea that they're picking up or…?
David Shuker: Well, remember of course that fish are living in the water. Anything that's in the water, very diluted; because fish effectively process so much water and end up retaining things, then of course they do tend to accumulate these things. But even the levels that are there are very, very low, and you're not going to have a major health problem from eating fish because of that; you're more likely to benefit from eating fish.

Paul Merrett: Should we be taking omega-3 supplements in our diet, or how much fish would you have to eat to get the right level of benefit?

David Shuker: Well, I think there's been quite a lot of work on this and certainly the Food Standards Agency have recommended recently that two portions of fish per week will provide the dietary requirements of things like the omega-3 fatty acids. I mean there's another issue there about supplements. I mean there are lots of other supplements available. I think the interesting thing about supplements is, it suggests that, if a little bit of something is good for you, then a lot more of it is even better and that's not often the case; and you can actually overdo it.

I mean, with some of the vitamins and things like that you can overdo it. And there's also some evidence that it's eating something which is good for you in the context of the real food which is actually beneficial because, as well as the individual chemicals which are good for you, there's probably a lot of other things in food and it's the way that that food delivers the natural ingredient to you which is probably important.

So my view on this is, it's probably better to try and eat a balanced diet which delivers the omega-3, the vitamin C, or whatever it might be, rather than specific - as it were - taking a pill.

Paul Merrett: I can remember my granny pouring cod liver oil…

David Shuker: Oh, well that's the classic, of course, absolutely.

Paul Merrett: …in liquid form. I would rather eat a fish any day of the week!

David Shuker: Now, interestingly, for several reasons, over the past few years I've consciously tried to incorporate more fish into my family's diet, so we just ring the changes. And of course trying to persuade kids to eat fish is…

Paul Merrett: What about tins, because I'm sure I've read somewhere that a tin of sardines, that's good?

David Shuker: That's good, an oily fish, and of course that's the other thing is that…

Paul Merrett: So these nutrients, these fatty acids, they are retained during the canning process. There's no loss of nutrients from fish because of that, the process it goes through in order to end up in a tin.

David Shuker: As far as I'm aware… - and fatty acids anyway are not the kind of nutrients which are so delicate that if you heat it a bit they go away; they're quite robust kind of molecules. And as far as I know that tinned fish, fish which has been caught and then processed probably on the boat that catches it, it preserves well the nutritional benefit. And mainly it is things like the sardines and that, what are called the oily fish, which contain higher levels of those omega-3 fatty acids. But the fish like cod and all the rest of it, which are eaten in far greater quantities, are probably not in themselves as beneficial. Like everything else, the variety of fish that's available means you can prepare interesting fish dishes which makes it easier to integrate it into a more varied diet anyway.

Paul Merrett: Yes absolutely. And we're really lucky in this country, I hold fast that fish from colder water is a better quality than from warmer, and I think that we have got some of the best - brill and halibut and turbot - and there are some fantastic fish that are swimming all around our shores. I mean we are very, very lucky in our access to good fish. If we can support fishmongers and get out there and use them rather than going to supermarkets and finding, I think, really poor quality fish, which is I think a bit of a shame.

Now I've got two young children, so their health benefits are quite big in our family. We talk about them quite a lot, and when my wife saw the sea bass recipe - on the programme I cooked a bass and I buried it in three kilo of salt and baked it in the oven - she was a bit distraught, and she said, "Well, surely that's not very good for you?" But, amazingly, we cooked this bass in the salt and there was no penetration - it wasn't salty at all. In fact, if anything, you probably needed to season the finished fish, which is an amazing thing. And can you explain what's gone on there?

David Shuker: Well it is interesting, when I looked at that recipe I thought to myself, "Well, yes, it's bound to taste salty," but actually I'm thinking about it and looking a little bit into the science; it is interesting because what's happening is that, effectively, as you heat this dish up, the moisture that's in the fish reaches the salt and you get the little bit of salt solution form but then, as the heat increases, that water evaporates and you end up forming a solid crust of salt over the fish.

Paul Merrett: And it was a whole fish I should say.

David Shuker: It's a whole fish, so you've got the skin on as well, so the skin is another little barrier. Once that crust forms, interestingly, solid salt is enormously, mechanically strong. I mean, historically, there are two types of salt that you get, of course: there's rock salt and sea salt. Well rock salt, there's actually a whole mountain in Spain which was mined for salt for many centuries, and that's rock salt.

And if you try and mine rock salt it's like mining solid rock. I mean it's absolutely… It's enormous. It doesn't come out as crystals; it's like a big piece of rock. In fact, there's an Island in Louisiana, for example, in the swamps of Louisiana, which is actually a mound of solid salt, and that where they got the salt for the Tabasco sauce that's famous for New Orleans. And again one of the difficulties was actually getting enough of it out to use it.
So solid salt is actually quite mechanically strong, so I think what you're doing in this recipe is, you're effectively, inadvertently… (or probably whoever developed the recipe knew what they were doing, perhaps, or discovered it by accident) …is you're actually making a sort of container of salt around it. And then, as the fish is cooked, then it's sealed in its moisture and then that salt doesn't then leech in, so you're covering it in a mound - well of course that's just to make sure you've got the whole thing covered.

Paul Merrett: And to cook, that's a great thing because all those flavours are kept inside the fish; I mean there's no loss of flavour at all.

David Shuker: No, that's right because, I don't know, I mean when you're cooking it are you aware of a sort of fishy smell or you're cooking fish?

Paul Merrett: No, you are, as soon as you crack it.

David Shuker: Absolutely.

Paul Merrett: What happens is you take the fish out, you get a big sort of salt-covered fish-shaped thing, and you just crack the salt and pull off the bits of salt, then you get the full flavour.
David Shuker: That's right.

Paul Merrett: Full smell, but until that point, no it could just be an old sock buried in a bit of salt, you really don't know.

David Shuker: You would be very surprised that ten grams of salt is a very small pile of salt and so you can… unless you're very careful you can do it, and salt is really a very strong flavour, so actually you don't need too much. And what's interesting is that, if you try and wean yourself off salt - it's very hard if you do it straightaway; it's like, for example, sugar in coffee. I mean I did that; I used to take two spoons of sugar in my coffee. If I didn't have sugar, and I noticed it was a big difference in the flavour, but I weaned myself off it to the extent that I find now that if any sugar's in coffee it's too sweet. Same thing with salt - over the years I've weaned myself off salt. I often now sit down to the table, I only put the pepper pot on the table and in fact you can use pepper. Now there's far less of a problem with eating too much pepper than there is with eating too much salt.

So why is salt a problem? Well, the trouble is that our body requires an intake of salt, because we get rid of salt through our sweat, for example. And in fact it is possible to have too little salt in your diet and that will give you a problem, because it's the balance of irons and the sodium and the chloride which make up salt are actually important to the body. And we take in that salt and we also get rid of it, we excrete it in the form of urine, and that's the way it goes, and we need to have a balance.

And the problem is, if you take in too much salt it causes a real problem for your kidneys, because the kidneys have to get rid of that excess salt, and in order to get rid of that excess salt then they have to get rid of more water, so it affects the fluid balance in the body.

Paul Merrett: Ah, right, okay.

David Shuker: Okay, but the other problem is, that's a sort of, what you might call an acute effect; that is, if you take in too much salt then your body has to deal with there and then. The more subtle and perhaps more insidious effect is the effect of salt in relation to heart disease. And why is that a problem? Well the muscles that make your heart work - a part of the way they work is, they use a thing called the sodium pump and that's the way that partly muscles work. And if you have too much sodium in your body, then it can start disrupting those mechanisms.

The problem is, of course, that people tend to eat salt with foods that are fairly fatty, and therefore it's a double whammy. So that's why the very traditional English breakfast, which has a lot of… if it's cooked a certain way can have a lot of fat in it - put a lot of salt on it, then you've got problems with that. And the problem with the saturated fats, particularly, is that they're very bad, they're high risk, they increase your risk of getting heart disease. So there is not usually one thing in isolation, the worse thing that happens is when you get two things which are bad for you working together on the same target.

Paul Merrett: Well, I think that most people would be like me that you don't start considering your diet really until you have children, or perhaps it's in your thirties type thing - you know, that's when you start considering what you're eating and you become slightly more sensible. But also you've got these little people that you want to give a healthy start to in life, and certainly I've started cutting back on salt at home and I think I season things considerably less. But also with fish, I've - like you - started trying to get all of us, as a family, to eat more fish, and I expect we eat it two of three times a week.

But if I was honestly asked by them why are we having to eat so much fish, I'm not sure that I…. Oh I could tell them it's good for them, and I could quote omega-3s and fatty acids and all sorts of things, but I'd probably tell them that it will stop their hair curling rather than give them a very medical scientific explanation!
David Shuker: Yes, I mean, I agree with you and I think it is very difficult. And I've got three children and I'm trying to persuade them to eat more healthily and you talk to them about omega-3 amino acids and they don't get it. Although I have to say, interestingly though, I think the schools are doing a much better job. I think for various reasons my children are aware, and they will start asking me about "Are we eating five portions of fruit and veg a day?" and they're aware of this now, when they're confronted with the reality of what that actually means, because eating five portions of fruit and veg a day is quite difficult.

Paul Merrett: Yes it is for my kids.

David Shuker: You know, that's right, and "Do three cups of orange juice count as three portions?" well no, they don't because… but the interesting thing about it is, I think the other way to try and explain to people is a sort of historical, evolutionary way, and that is that we evolved as a species, particularly, people evolve living close to the sea, and therefore our bodies evolved based on eating a lot of fish in our diet.
So that the reason why, for example, fish oils are particularly important to us is those oils are not just, as it were, fuel for the body, we actually use some of those components, some of those molecules, in fish and all other food as well, to actually make bits of our own body - cell walls for example - and that's why certain fish oils are very important.

They're particularly important, for example, for some of the cells that make up the nervous system, and those cells are highly specialised, they require very particular fatty acids to help make the cell membranes, and that's why they're important.

And the other thing is that the body can't always make everything it needs, it needs some raw materials in the form that they're used and so, perhaps, the way to say to kids - and I think they're quite aware of this kind of thing - is that it's more natural for us to eat a balanced diet which contains fish than anything else and it's actually unnatural for us to live, consume a diet that doesn't contain fish at all, you know, so…

Well I don't know if that would convince them. Maybe I should try it on my kids and see.

Paul Merrett: I'm going to give them the option. I'll take that info home. Now to find out more about the science of food, to get the recipes from the series and more of David and me, chewing the fat, visit our website at I'm Paul Merrett. I've been talking to David Shuker of the Open University. The Producer is Michael Brodbin. This is a BBC Worldwide production for the Open University.





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