Why Do Bigger Fish Carry More Mercury?
― The Answer Was Hiding in Old Eggshells in a Museum
Sardines swim in the same sea as tuna, yet you can eat sardines freely, while tuna comes with a guideline of "once a week during pregnancy". The reason is that mercury in the sea becomes more concentrated each time one animal eats another. The first person to show this to the world was not a chemist. It was a conservation researcher who toured bird nests and weighed eggshells in museums, one by one.
The booklet you got at a prenatal check-up has advice on eating fish. For golden redfish, swordfish and bluefin tuna, the guideline is "no more than once a week, at about 80 grams a serving".
For sardines, mackerel and horse mackerel, though, there is no limit on how often you can eat them. Canned tuna is also considered fine to eat normally.
They all come from the same sea, so what makes the difference? It isn't that some places in the ocean have a lot of mercury and others have little. The difference is what the fish ate as it grew up.
Mercury builds up for just two reasons
For a fish to gain 1 kilogram, it is said to need about 10 kilograms of food. The other 9 kilograms are burned as energy for swimming and keeping warm, and are gone.
Some of the mercury in the sea is turned into a form called "methylmercury" by microbes. It sticks to the proteins in muscle and hardly ever leaves the body.
The food is burned away, but the mercury stays. So the mercury from 10 kilograms of food ends up packed into 1 kilogram of body. This repeats at every step of the food chain.
How many times stronger at each step up?
The ocean food chain starts with plant plankton too small to see. Animal plankton eat them, small fish like sardines eat the animal plankton, medium-sized fish like mackerel eat the sardines, and finally tuna eat the mackerel.
Look at the left side of Figure 1. The lower the step, the more living things there are, and the higher the step, the fewer. Mercury, on the other hand, gets more concentrated the higher you go. The pyramid of quantity and the pyramid of concentration point in opposite directions.
If we assume each step multiplies the concentration by about 8, tuna four steps up would have about 4,000 times as much. In reality some mercury leaves the body along the way, so it doesn't get this extreme. Even so, tuna are said to carry dozens of times more mercury than sardines.
The one who found it out was a man who weighed eggshells
In 1950s Britain, peregrine falcons suddenly began to decline. A researcher named Derek Ratcliffe, who toured the nests, noticed something odd. Eggs that the parent birds should have been sitting on were broken in the nest.
Perhaps the shells had become thinner. With that idea, he borrowed old eggs kept by museums and private collectors, and weighed the shells one at a time. The records he gathered ran from around 1900 to the 1960s.
The result is shown on the right of Figure 1. Shell weight had hardly changed for nearly half a century, then suddenly became about 20% lighter around 1947. That was just when the pesticide DDT began to be widely used on farmland. In 1967, the finding was published in a scientific journal.
The amount of the chemical sprayed on fields should not have been enough to harm birds. But a falcon eats many small birds, and each small bird has eaten many insects. At the top of the food chain, the chemical had become concentrated enough to upset the way shells are made.
In Japan around the same time, in Minamata Bay in Kumamoto Prefecture, methylmercury in factory wastewater built up in fish. People who ate those fish almost every day suffered serious health damage. This was Minamata disease, officially recognised in 1956. Records say that cats and seabirds showed signs before people fell ill. This too was because animals that eat a lot of fish are the first to get high concentrations.
Fish are rich in good-quality protein and fats thought to be good for the body (such as DHA). The Ministry of Health, Labour and Welfare's advice does not ask people to stop eating fish. Because unborn babies are sensitive to mercury, it asks pregnant women to choose the kinds and amounts. Eating a variety of fish, mainly small ones lower in the food chain, is considered best. Methylmercury that gets into the body is said to fall by half in about two months.
Summary
Most of what an animal eats is burned off as energy, but substances that are hard to get out of the body simply stay. So with each step up the food chain, the same substance becomes several times more concentrated. Big fish have more mercury not because they are bad, but because in the sea they are on the "eating a lot" side. This mechanism was uncovered through broken falcon eggs and old shells sleeping in a museum.
What you eat burns away, but what can't burn away stays.
The top of the food chain is where the "leftovers" of the whole sea collect.
An example where the ocean food chain shows up in plain view is in "Why can fishermen tell where the fish are just by looking at a flock of seabirds?", and the pyramid of living things is covered in "If you put every living thing on Earth on a scale, what would be the heaviest?".
- At the fish counter of a supermarket, pick three or four kinds of fish and look up what each one eats (sardines eat plankton, bonito eat small fish, and so on).
- Counting from plant plankton, write down on paper which step each fish is on.
- Compare with the Ministry of Health, Labour and Welfare's list "About mercury in fish and shellfish" and check whether fish on higher steps have stricter guidelines.
Even within one species, larger fish have been feeding for longer, so they tend to carry more mercury. Paying attention to size can be interesting too.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Basic Biology" and "Biology"
- High school+Advanced high school material, or textbook sidebars
- UniversitySpecialist university courses not taught in high school (ecology, environmental chemistry, toxicology)
- ResearchNot yet settled even at university; researchers are still working on it
Junior highTerms: this phenomenon has a name
- Biomagnification: a phenomenon in which a substance that is hard to get out of the body builds up through the food chain, reaching higher concentrations in animals higher up.
- Food chain: the linked relationships of who eats whom. In the sea, phytoplankton is the starting point.
- Methylmercury: a form of mercury with a small cluster of carbon and hydrogen attached. It is easily absorbed by living things and hard to get rid of.
Junior highHigh schoolChecking with a formula: how many times per step, and over four steps
We estimate the multiplier per step from the balance between the mercury coming in with food and the weight the body gains. The numbers are assumptions to show how the mechanism works.
| In symbols | C_up = C_down × ( α ÷ ε ) |
| In words | Concentration in the eater = concentration in the food × (share of mercury kept in the body ÷ share of the food eaten that becomes body) |
| Where it comes from | A balance sheet of mercury in and out (conservation of matter). Of the mercury in 1 kg of food, a share α stays in the body, and 1 kg of food adds only ε kg of body, so mercury per kilogram of body becomes α ÷ ε times higher |
| Symbols C_down, C_up | Mercury concentration in the food and in the animal that eats it (in units such as milligrams per kilogram of body) |
| Symbol α (share of mercury kept in the body) | Assumed to be 0.8 (most methylmercury eaten is said to be absorbed) |
| Symbol ε (share of food eaten that becomes body) | 0.1 (the rough "10% rule" of ecological pyramids) |
| Steps from phytoplankton to tuna | 4 steps |
| Multiplier per step | 0.8 ÷ 0.1 ＝ 8 |
| Step 2 (sardines etc.) | 8 × 8 ＝ 64 |
| Step 3 (mackerel etc.) | 64 × 8 ＝ 512 |
| Step 4 (tuna) | 512 × 8 ＝ 4096 |
| Ratio of tuna to sardines | 4096 ÷ 64 ＝ 64 |
In this simple estimate, tuna carry 64 times the mercury of sardines and about 4,000 times that of the plankton at the start. In reality mercury leaves the body little by little, so the multipliers are smaller, and the gap between tuna and sardines is said to be dozens of times. Even so, the fact that each added step acts by multiplication is what lies behind this gap.
High schoolHigh school+Why only substances that are "hard to get out" become concentrated
High schoolIn Basic Biology you learn that matter circulates within an ecosystem while energy flows away. Organic matter that is eaten leaves the body as carbon dioxide and water through respiration, but substances that are neither broken down nor easily excreted stay in the body. This is the core of biomagnification.
High school+Water-soluble substances leave the body in urine and so on, but fat-soluble substances (such as DDT) stay in fat, and methylmercury stays attached to the sulphur-containing parts of proteins. Also, bigger fish live longer and keep feeding, so even within one species, concentrations tend to rise with age.
UniversityBioconcentration factor and trophic level
In ecology and environmental chemistry, the degree to which a substance is taken up directly from water and concentrated is called the "bioconcentration factor", and the multiplier per step of the food chain is called the "biomagnification factor", and the two are kept apart. The α ÷ ε in the text is the simplest estimate of the latter. How many steps up an animal sits (its trophic level) is often estimated from the stable nitrogen isotope ratio in its body. Methylmercury itself is thought to be made when microbes attach a methyl group to inorganic mercury in seafloor mud and in mid-depth seawater.
📖 Derivations and further reading: Biomagnification (Wikipedia, in Japanese) ／ Minamata disease (Wikipedia, in Japanese)
ResearchWhat is still not clear
- Where in the sea is methylmercury made? It is thought to form not only in seafloor mud but also in low-oxygen mid-depth seawater, but the share for each place is still only an estimate.
- Will warming raise mercury in fish? When water warms, fish metabolism changes, and so does how they feed. Some studies suggest mercury in fish could rise as a result, but conclusions differ between sea areas.
- Do new chemicals become concentrated? Some fluorine-containing substances used in water repellents and the like are said to collect in blood proteins rather than fat, and the fact that older prediction formulas fit them poorly is a challenge.
So the contents of this article too are "an explanation of what is known now". Just as Ratcliffe weighed eggshells, steady records kept over a long time are thought to provide clues to the next discovery.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: "Balance in nature", "Food chains" | The steps of who eats whom, and the pyramid of quantity |
| High school | Basic Biology: "Ecosystems and the cycling of matter" | Matter stays, energy flows away |
| High school+ | Basic Biology sidebar: "Biomagnification" | Fat-soluble substances and substances that stick to proteins |
| University | Ecology, environmental chemistry, toxicology | Biomagnification factor, trophic levels and isotopes |
| Research | Ocean mercury cycle, new pollutants | Where methylmercury forms, effects of warming |
| ― | Links to daily life | Choosing fish in pregnancy, spreading out kinds with small fish as the mainstay |
- Ratcliffe, D. A. (1967) Decrease in Eggshell Weight in Certain Birds of Prey. Nature 215, 208–210
- Ministry of Health, Labour and Welfare (厚生労働省), "About mercury in fish and shellfish" (魚介類に含まれる水銀について)
- Rachel Carson, Silent Spring, Japanese translation by Aoki Ryoichi (『沈黙の春』青樹簗一 訳), Shinchosha Bunko (新潮文庫)
- Ministry of the Environment (環境省), "Lessons of Minamata disease and Japan's mercury measures" (『水俣病の教訓と日本の水銀対策』)
※This article is a general-audience science explainer. The numbers given are rough guides to help you understand the mechanism. For diet during pregnancy, please follow the latest advice from the Ministry of Health, Labour and Welfare and the guidance of your doctor or local health authority.