The ocean provides most of the air we breathe.
The ocean makes Earth habitable for all species.
The ocean is a major influence on weather and climate.

The ocean supports a great diversity of life and ecosystems.
The ocean and humans are inextricably linked.
The ocean is largely unexplored.

While man has walked on the moon, he has yet to fully explore this planet of water right here on earth.

Those that are in a quest to do this are called ‘Oceanographers’… people that study ‘Oceanography’. The Oceans of our earth are home to an enormous amount of secrets yet untold. Plant life and animals, fish, and much, much more have yet to be discovered.

OCEAN FACTS (for facts around New Zealand click here)

Our Ocean is the source of life on planet earth, providing most of the air we breathe, a considerable amount of the food we eat and unbelievable reserves of energy needed to support life as we know it today. We, as humankind, need to better understand our relationship with Our Ocean and how we can live sustainably and in harmony with all of the lifeforms within. Learn about about the most important place on the planet below.

• Today 71% of the Earth is covered with water, 29% by the 7 continents.

• More than half of this area of salt water is more than 9000 feet deep!

• Our Ocean has 1,100 times the heat capacity of the atmosphere.

• It also contains 97% of the free water on the planet.

• More than one-half of the world's population lives within 100 km of the ocean.

One Planet, One Ocean. All the oceans and seas are actually one body of water, interconnected and reliant on each other. This great body of water embracing the continents of the Earth is also known as the world ocean. Its major subdivisions are the Pacific, the Atlantic, the Arctic, the Indian, and the Southern oceans.

Ocean Surface Area
Pacific - 60,060,000sqm or 155,557,000sqkm (46.3%)
Atlantic - 29,637,000sqm or 76,762,000sqkm (22.8%)
Indian - 26,469,000sqm or 68,556,000sqkm (20.4%)
Southern - 7,848,000sqm or 20,327,000sqkm (6.1%)
Arctic - 5,427,000sqm or 14,056,000sqkm (4.2%)

The average depth of planet Earth's oceans is 3,795 m. The average height of the land is 840 m.

Deepest Oceans and Seas
Pacific Ocean (35,837 ft) (10,924 meters)
Atlantic Ocean (30,246 ft) (9,219 meters)
Indian Ocean (24,460 ft) (7,455 meters)
Caribbean Sea (22,788 ft) (6,946 meters)
Arctic Ocean (18,456 ft) (5,625 meters)
South China Sea (16,456 ft) (5,016 meters)
Bering Sea (15,659 ft) (4,773 meters)
Mediterranean Sea (15,197 ft) (4,632 meters)
Gulf of Mexico (12,425 ft) (3,787 meters)
Japan Sea (12,276 ft) (3,742 meters)

Ocean Zones

Scientists have broken Our Ocean down into seven separate and distinct levels or zones, starting with the surface and going down to the very bottom in the order that follows...

Epipelagic - The top ocean layer, at or near the surface in open ocean where the water is warmer and sunlight and photosynthesis is most effective.

Mesopelagic - Mid-ocean, the layer of ocean water between 200m and 800m down, where sunlight pentrates the water enough to be beneficial to the species that live there.

Bathypelagic - Deep water, where penetration of sunlight is extremely low; this layer ranges from about 800m to 4,000 deep.

Benthopelagic - The layer of ocean that is just above the bottom; the sea floor, where the term benthic refrers to the sea floor itself.

Bathyal Zones - The benthopelagic areas of the ocean that are in the continental slopes. 

Abyssal Zones - The benthopelagic areas of the ocean that are on the sea floor plain. 

Hadopelagic - The places that are the deepest in the ocean. These are known as the trenches, such as the Challenger Deep, with a depth of 35,838 ft/10,923m.

The deepest trench is the Mariana Trench in the Pacific Ocean, at 11,033 metres deep, which is six times deeper than the Grand Canyon!!

The world’s longest mountain chain is more than 50,000km long and is also found underwater. This is the mid-ocean ridge, and extends from the Arctic Ocean, down the centre of the Atlantic, into the Pacific Ocean. It is four times longer than the Andes, the Rockies and the Himalayas put together.

The ocean is deeper than Mount Everest, at 8,850 meters (29,000 feet), is tall. In 1960 the U.S. Navy, using Auguste Piccard's bathyscaph "Trieste," reached the bottom of the Marianas Trench, 10,900 meters (35,800 feet) down.

At that depth, the temperature is always just above freezing, the pressure is more than 1000 times what it is on the surface, and many bottom-dwelling fish (example below) and invertebrates call it home!

The tallest mountain on Earth stands 33,465 feet tall and extends from the ocean floor to above water. This is Mauna Kea, an inactive volcanic island in Hawaii.

Over deep ocean canyons the sea surface dips because of the local gravity, leaving depressions as deep as 20 meters (65 feet) and as wide as 160 kilometers (100 miles).

An estimated 80% of all life on earth is found under the ocean surface and the oceans contain 99% of the living space on the planet. Less than 10% of that space has been explored by humans. 85% of the area and 90% of the volume constitute the dark, cold environment we call the deep sea.

Why is "Our Ocean" salty?

Our Ocean contains lots of different salts :  sodium, chloride, sulfate, magnesium, calcium, potassium, bicarbonate, and bromide. These salts enter the ocean through rivers, which, before entering pass over rocks and soil, and pick up salt along the way.

This salt builds up in the ocean because the only way water can leave the ocean is through evaporation. And when the water evaporates it doesn't take the salt with it. So you end up with less water, and the same amount of salt, resulting in a pretty salty sea.

The same thing can happen to a smaller, landlocked body of water, such as the Great Salt Lake in Utah, where water evaporates quickly in the desert climate--the lake has dropped 20 feet since 1849. A favorite pastime for visitors is to float on the lake like a cork, because the high salt content makes people more buoyant. You also float better in the Mediterranean Sea than in the waters off California because the Mediterranean is saltier and denser. Our Oceans, are on average, 3.5 percent salt.

Frozen Water

Although tap water freezes at 0 degrees Celsius (32 degrees Fahrenheit), seawater does not freeze until about minus -1.9 °C because 3% of it is salt. This is why people "salt the roads" after heavy snows or frosts. The density of sea water becomes more dense as it becomes colder, right down to its freezing point of -1.9 °C unlike fresh water which is most dense at 4 °C, well above its freezing point of 0 °C.

Antarctica has as much ice as the Atlantic Ocean has water.

The Arctic produces 10,000-50,000 icebergs annually. The amount produced in the Antarctic regions is inestimable. Icebergs normally have a four-year life-span; they begin entering shipping lanes after about three years.

90% of an iceberg is under water--boaties beware!

If all the ice in glaciers and ice sheets melted, the sea level would rise by about 80 meters, about the height of a 26-story building.

If sea level should rise by 3 meters (10 feet), many of the World's coastal cities, like Venice, London, New Orleans, and New York, would be under water.

Ocean Species

The variety of layers within the oceans of the world are home to the vast majority of the plant and animal species on earth, many of which have yet to be discovered by man. The plants and animals that live within the ocean often exist at a particular level or layer that can be divided into 3 categories. The Benthos is plants including kelp and animals including starfish, which depend on or live on the bottom. The Nekton is swimming animals including fish and whales, which move independently of the current. The current carries the Plankton, which consists of various microscopic creatures,.

Benthic plants and animals live in particular regions within the ocean. The cold region between the shoreline and continental shelf is home to sponges, crustaceans, attached algae and polyaete worms. The deepest parts of the ocean beginning at the continental slope and extending to the ocean floor make up the benthic zone. This region is sparsely populated with filter feeders and deposit feeders such as sea spiders and sea lilies.

Nekton fish are generally found swimming freely in the warmer well-lit waters above the continental shelf. In the darker regions beginning at the continental shelf giant squid can be found.

Plankton is the most prevalent life and food source in the ocean. Phytoplankton, which carries on photosynthesis near the ocean surface, serves as food for the zooplankton and fish.

Plankton (pictured above) are microscopic animals (zooplankton) and algae (phytoplankton) that live in the ocean. They drift on currents and provide food for many ocean residents.

A mouthful of seawater may contain millions of bacterial cells, hundreds of thousands of phytoplankton and tens of thousands of zooplankton.

Fish have been around for 500 million years. All fish live in water and breathe with gills. Fish are cold-blooded, which means their internal body temperature changes as the surrounding temperature changes. All fish have a backbone.

Only one in every million fish eggs survive to adulthood.

Many fish can change sex during the course of their lives. Others, especially rare deep-sea fish, have both male and female sex organs

There are about 25,000 different species of fish alive today with another estimated 15,000 waiting to be discovered.

The 25,000 known species of fish are divided into three main groups. They are the jawless, the cartilaginous and the bony fish. Jawless fish are the last survivors of the world's first vertebrate animals, which means "back-boned". They lack both scales and jaws. Dating from over 5000 million years ago, only the hagfish and lampreys remain.

Cartilaginous fish developed about 100 million years later, ancestors of today's sharks. The skeleton of these fish is made of cartilage (KART-laj), which is not as hard as bone. These fish have jaws, as well as teeth which are usually hard and sharp. Their bodies are covered with hard scales.

Bony fish—fish with bony skeletons—appeared at the same time as cartilaginous fish. They are the largest group, with about 20,000 species. These fish have an organ called a swim bladder which gives the animal buoyancy, the ability to float.

The blue whale, Balaenoptera musculus, is the largest known animal ever to have lived on sea or land. Individuals can reach more than 110 feet and weigh nearly 200 tons more than the weight of 50 adult elephants. The blue whale's blood vessels are so broad that a full-grown trout could swim through them, and the vessels serve a heart the size of a small car. Its food consists of tiny shrimps called krill. Owing to its sheer size, it is no surprise that it can eat 400,000 krills a day. It is a mammal which means it gives birth to its young ones.

Penguins "fly" underwater at up to 25 miles per hour.

The swordfish and marlin are the fastest fish in the ocean reaching speeds up to 121 kph in quick bursts.

Bluefin tuna, Thunnus thynnus, are also amongst the largest and fastest marine fish. An adult may weigh 1,500 pounds and swim up to 55 miles per hour. Prized as sushi in Japan, bluefins are also among the most valuable fish: individual bluefins can bring as much as $20,000 at U.S. docks.

The largest invertebrate in the world is the giant squid. An invertebrate is an animal without a backbone. Giant squids grow up to 20 metres long and can weigh over two tonnes. They can be found in the Atlantic as well as Pacific oceans.

The oarfish, Regalecus glesne, is the longest bony fish in the world. With its snakelike body sporting a magnificent red fin along its 50-foot length_horselike face and blue gills, it accounts for many sea-serpent sightings.

Commerson's dolphin (above) is the smallest mammal at the sea. The adult weighs 25-35 kilograms. It is almost 3500 times smaller than the blue whale, which is the largest mammal

Green turtles can migrate more than 1,400 miles to lay their eggs.

In the turtle family, the leatherback turtle is the largest. It can be up to 1.8 metres long and can weigh over 650 kilograms. It has a grooved leathery shell. These grooves aid the turtle in swimming fast. Interestingly, sea turtles go to their favourite beaches to breed. They mate off-shore and then the female digs a pit on the beach to lay the eggs. Once the eggs hatch the young turtles climb to the surface and crawl down the beach to the sea.

The blobfish, aka Psychrolutes marcidus (above), is a deep water fish that lives off the coasts of Australia and Tasmania. Due to the extreme deeps that Blobfish are found at, the pressure is several dozens of times higher than at sea level, enough to crush a man under his own body weight. As such, the flesh of the blobfish is primarily a gelatinous mass with a density slightly less than water. This unique feature allows the fish to float above the sea floor without expending energy on swimming. The relative lack of muscle is not a disadvantage as the blobfish primarily swallows edible matter that floats by in front it.

The sponge is known as the vacuum cleaner of the sea. In fact, a sea sponge the size of your fist can filter as much as 1,100 gallons of water every day.

A group of herring is called a seige.

A group of jelly fish is called a smack.

Horseshoe crabs have existed in essentially the same form for the past 135 million years. Their blood provides a valuable test for the toxins that cause septic shock, which previously led to half of all hospital-acquired infections and one-fifth of all hospital deaths.

One study of a deep-sea community revealed 898 species from more than 100 families and a dozen phyla in an area about half the size of a tennis court. More than half of these were new to science.

Sharks attack some 50-75 people each year worldwide, with perhaps 8-12 fatalities, according to data compiled in the International Shark Attack File (ISAF). Although shark attacks get a lot of attention, this is far less than the number of people killed each year by elephants, bees, crocodiles, lightning or many other natural dangers. On the other side of the ledger, we kill somewhere between 20-100 million sharks every year through fishing activities.

Of the 350 or so shark species, about 80% grow to less than 1.6 m and are unable to hurt people or rarely encounter people. Only 32 species have been documented in attacks on humans, and an additional 36 species are considered potentially dangerous.

Almost any shark 1.8 m or longer is a potential danger, but three species have been identified repeatedly in attacks: the Great white, Tiger, and Bull sharks. All three are found worldwide, reach large sizes and eat large prey such as marine mammals or sea turtles. More attacks on swimmers, free divers, scuba divers, surfers and boats have been reported for the great white shark than for any other species. However, some 80% of all shark attacks probably occur in the tropics and subtropics, where other shark species dominate and Great white sharks are relatively rare.

Lantern fishes use bioluminescence to protect themselves from predators. By lighting their bodies, their silhouette is less visible from below. Bioluminescence is a natural glow on fish in the deep ocean depths. Chemicals mix together to produce "cold light," which doesn't give off any heat. Some bioluminescent animals get their glow from tiny microbes that live inside special light organs in their bodies.

Flashlight fish (above) have these special pouches of glowing bacteria under their eyes. When they want to turn on their lights, they lower folds of skin below each eye. By using a technique known as "blink and run," they can swim in one direction with their lights on, then quickly turn them off and swim in another direction. Eventually their predator gets disoriented and the flashlight fish escapes.

The pistol shrimp (above) is so called because it snaps a specialized claw shut to create a cavitation wave that generates acoustic pressures of up to 80 kPa at a distance of 4 cm from the claw. The pressure wave is strong enough to kill small fish! The duration of the click is less than 1 millisecond. The snap can also produce a collapsing bubble. As it collapses, the cavitation bubble reaches the surface temperature of the Sun, though the light is of lower intensity than the light produced by typical luminescence and is not visible to the naked eye.

Some species of cone snails (above) can grow up to 23 cm in length and are found in tropical or subtropical waters. There are about 500 different species. They are carnivorous, generally eating marine worms, small fish, molluscs, and even other cone snails. Because cone snails are slow-moving, they use a venomous harpoon (called a toxoglossan radula) to capture faster-moving prey such as fish. The venom of a few larger species is powerful enough to kill a human being.

The harpoon is loaded with venom and, still attached to the radula, is fired from the proboscis into the prey by a powerful muscular contraction. The venom paralyzes small fish almost instantly. The snail then retracts the radula, drawing the subdued prey into the mouth. After the prey is digested, the cone snail will regurgitate any indigestible material such as spines and scales, along with the disposable harpoon.

Did you know?

Life began in the seas 3.1 billion to 3.4 billion years ago. Land dwellers appeared 400 million years ago, relatively recently in geologic time

The greatest diversity of life on Earth is in the oceans. Scientists believe that life started in the oceans because the oldest fossils ever found seem to verify this.

80 percent of all ocean life is found on the continental shelves. This is because shallow water and the closeness to land provide food, light, and shelter-the conditions needed to support large quantities of life.

Home to an array of life like no other surface of the earth, within depths of unbelievable reaches, combined with frigid temperatures, and tremendous water pressure, life still exists. With sunlight having no chance of reaching even a depth of about 650 feet, it is easy to see that most of the world’s oceans exist in a land of darkness.

Below 2,000 feet, the ocean is completely dark. When you go below 2,000 feet underwater, sunlight cannot penetrate and you would find yourself in total darkness. Fewer animals live in this "twilight zone." Most of their food is provided by sea snow and zooplankton.

Less than 1% of the world's water is fresh water, and 2-3% of that is contained in glaciers and ice caps.

The average temperature of all ocean water is about 3.5 °C

Dissolved gold is found in the water of all oceans.

The most abundant major element dissolved in ocean water is chlorine.

Ocean tides are created by the pull of gravity from both the Sun and the Moon create ocean tides. The tides create the currents that many ocean animals use to travel.

The highest tides in the world are at the Bay of Fundy, which separates New Brunswick from Nova Scotia. At some times of the year the difference between high and low tide is 16.3 m, taller than a three-story building.

The Kuroshio Current, off the shores of Japan, is the largest current. It can travel between 40-121 km/day at 1.6-4.8 kph, and extends some 1,006 m deep. The Gulf Stream is close to this current's speed. The Gulf Stream is a well known current of warm water in the Atlantic Ocean. At a speed of 97 km/day, the Gulf Stream moves a 100 times as much water as all the rivers on earth and flows at a rate 300 times faster than the Amazon, which is the world's largest river

Ten meters (33 feet) of ocean depth has the same mass as the whole atmospher; 2.5 meters (8 feet) of ocean depth holds as much heat as the whole atmosphere; 2.5 centimeters (1 inch) of the ocean depth has as much water as the whole atmosphere.

The pressure at the deepest point in the ocean is more than 11,318 tons/sq m, or the equivalent of one person trying to support 50 jumbo jets.

The top ten feet of the ocean hold as much heat as the entire atmosphere.

The speed of sound in water is 1,435 m/sec - nearly five times faster than the speed of sound in air.

An ecosystem is made up of communities of organisms that depend on each other and their surroundings. Kelp forests, continental shelves, and mangrove forests are all examples of ocean ecosystems.

A small area of ocean that is partially surrounded by land is called a sea.

A large flat area on the ocean floor is called an abyssal plain.

A thermocline is an ocean zone that separates the warm surface water from the colder deep water.

The boundary between the continental crust and the oceanic crust occurs at the base of the continental slope.

A rift is a feature formed where oceanic plates are separating.

Hydrothermal vents, fractures in the sea floor that spew sulphur compounds, support the only complex ecosystem known to run on chemicals, rather than energy from the sun. Gigantic tubeworms and mussels thrive in densities of up to 65 pounds per square foot around vents.

The Dead Sea is the Earth's lowest land point with an elevation of 396 m below sea level.

Isolated mid-ocean volcanoes are known as seamounts.

Ninety per cent of all volcanic activities on Earth actually occur in the oceans.

Undersea earthquakes, volcanoes and landslides can cause tsunamis (Japanese word meaning "harbor wave"), or seismic sea waves. The largest recorded tsunami measured 60 m above sea level caused by an 8.9 magnitude earthquake in the gulf of Alaska in 1899 .

Tsunamis can travel at up to 800 kilometers (500 miles) per hour. At sea, they are hard to "see" because they're no more than 10 centimeters (4 inches) high! As they come toward the shore, tsunamis build up many tens of meters high, and can wash inland more than a kilometer.

The largest coral atoll complexes occur in the Maldive-Lakshadweep ecoregion of the central Indian Ocean and in Micronesia.

The Great Barrier Reef, measuring 2,300 km in length covering an area more extensive than Britain, is the largest living structure on Earth and can be seen from space. Its reefs are made up of 400 species of coral, supporting well over 2,000 different fish, 4,000 species of mollusc and countless other invertebrates. It should really be named 'Great Barrier of Reefs', as it is not one long solid structure but made up of nearly 3,000 individual reefs and 1,000 islands. Other huge barrier reefs include the barrier reefs of New Caledonia, the Mesoamerican (Belize) barrier reef, and the large barrier reefs of Fiji.

Because the architecture and chemistry of coral is so similar to human bone, coral has been used to replace bone grafts in helping human bone to heal quickly and cleanly.

Oils from the orange roughy, Hoplostethus atlanticus, a deep-sea fish from New Zealand, are used in making shampoo.

Alginates, derived from the cell walls of brown algae, are used in beer, frozen desserts, pickles, adhesives, boiler compounds, ceramics, explosives, paper and toys.

The remains of diatoms, algae with hard shells, are used in making pet litter, cosmetics, pool filters and tooth polish.

A given area in an ocean upwelling zone or deep estuary is as productive as the same area in rain forests, most crops and intensive agriculture. They all produce between 150-500 grams of Carbon per square meter per year.

The sea level has risen with an average of 10-25 cm over the past 100 years and scientists expect this rate to increase. Sea levels will continue rising even if the climate has stabilized, because the ocean reacts slowly to changes. 10,000 years ago the ocean level was about 110 m lower than it is now.

More than 90% of the trade between countries is carried by ships and about half the communications between nations use underwater cables.

A fathom is a unit of length in Standard English that is used to measure ocean depths. It is approximately 6 vertical feet. To get the total depth in feet from fathoms given, just multiply by 6. For example, 500 fathoms = 500 x 6ft. = 3,000 feet.

A league is also a unit of length (or distance) that is used to measure ocean depths. It is not used in science, but in literature. One league = 3 miles (or 4.8 km).

Ocean Threats

see our other page on Ocean Alerts here...

The biggest threat to Our Ocean is human beings. Examples follow;

• Fish supply the greatest percentage of the world's protein consumed by humans and most of the world's major fisheries are being fished at levels above their maximum sustainable yield; some regions are severely overfished. Expansion of commercial and sport fishing is depleting the ocean of its big fish, disrupting natural predator-prey relationships. Nets, lines and other devices injure and kill sea life.

• Loss of habitat, including estuaries and tidal marsh, removes spawning and feeding grounds for marine species that depend on subtle marine conditions to complete life cycles.

• Oil spills from drilling operations and vessels contaminate kelp beds and kill fragile life. Otters, common murres and other species have been among those hit hardest by crude oil.

• Rising ocean temperatures linked to global warming are reducing the ocean's productivity, including the vast stores of plankton and krill at the base of the food web. In other parts of the world, tropical corals die during warm events.

• Each year, three times as much rubbish is dumped into the world's oceans as the weight of fish caught. Trash, particularly nondegradable plastic, floats in the ocean where it chokes and kills fish and other wildlife. Plastic crumbles into tiny particles, which are consumed by animals and replace nutritional food. Discarded plastic, pouring into the ocean from cities and populated area's is turning into huge floating islands in the Pacific. Plastic is just one of the pollutants being disposed of in the ocean, with the world's un-restrained toxic waste putting the worlds life source, Our Ocean, at risk.

Watch a video documentary on the subject here.....

Sewage - treated and untreated - runs into the ocean from municipal and industrial plants, polluting water with bacterial waste, chemicals and metals. Storm water rushes off city streets and farmlands and into the ocean. That water carries a nasty collection of motor oil, cigarette butts, pesticides and animal waste. Old garbage dumps leak a stew of biological and chemical residue.

Sewer and stormwater outlets purge into Our Ocean millions of litres of polluted water every second around the world, increasing algal blooms and deadzones in the sea.

Refined oil is a key element of pollution in Our Ocean. More oil reaches the oceans each year as a result of leaking automobiles and other non-point sources than the oil spilled in Prince William Sound by the Exxon Valdez. Oil spills from drilling operations and vessel's contaminate kelp beds and kill fragile life. Otters, common murres and other species have been among those hit hardest by crude oil.

Air pollution is also responsible for 33% of the toxic contaminants that end up in oceans and coastal waters. About 44% of the toxic contaminants come from runoff via rivers and streams.

How much mercury is in the fish we eat?

Fish are one of the healthiest foods on Earth, but many are brimming with toxic mercury.

Global warming isn't fossil fuels' only dirty trick. While it is the most sweeping and civilization-threatening side effect of our carbon economy, there are also a variety of toxins lurking in every lump of coal and drop of oil. And one especially scary fossil-based toxin is also now also embedded in your mahi-mahi: mercury.
 
While carbon dioxide emissions feed a worldwide chain reaction that disrupts all kinds of natural processes, mercury is a more personal, hands-on pollutant. It gets sucked up into the food chain, accumulates as it moves up and then attacks our bodies directly when we eat contaminated animals, namely fish.

Mercury poisoning can cause severe brain, kidney and lung damage to children and adults, but it's even more dangerous to fetuses. It prevents nerve cells in the brain from forming correctly, which can damage attention span, fine motor function, language skills, visual-spatial abilities and verbal memory. That's why the United States FDA and the EPA suggested in a joint 2004 report that women who are pregnant or plan to become pregnant avoid fish with the highest levels of mercury.
 
The report offers the following three pointers for women and young children:
 
1) Don't eat these fish, which are notorious mercury smugglers: king mackerel (averages 0.730 parts per million), swordfish (0.976 ppm), shark (0.988 ppm) and tilefish. (1.45 ppm).
 
2) Eat up to 12 ounces (about two average meals) a week of various fish and shellfish that are low in mercury, such as shrimp, canned light tuna, salmon, pollock and catfish.
 
3) Check local advisories about the safety of fish caught in lakes, rivers and coastal areas. If no information is available, eat up to 6 ounces (about one meal) per week of locally caught fish, but don't eat any other fish that week.
 
The EPA and FDA are both careful not to ignore all the health benefits of fish — a low-fat protein source full of omega-3 fatty acids — and they stress the importance of eating limited amounts of certain species that are less contaminated.
 
But why do some fish have more mercury than others? They are what they eat.

Mercury is released into the air just like CO2 when fossil fuels are burned, and coal-fired power plants are the main source. Rather than drifting up into the atmosphere, though, the heavy metal accumulates in clouds and falls to the earth with rain. Most of it drains into streams, rivers, lakes and oceans, where microorganisms absorb it and convert it to an even more toxic form called methylmercury. Minnows and other small fish spend their lives eating these microbes, and the methlymercury builds up in their fatty tissues. Bigger fish eat the minnows, and each fish up the food chain accumulates more and more of it; that's why smaller fish like anchovies and tilapia have the lowest mercury levels and predator fish like shark and swordfish have the highest. That's also why women who aren't yet pregnant but plan to eventually have kids should also avoid predator fish, because mercury builds up in humans, too.

 
 
Any amount of the potent neurotoxin is cause for concern, since it adds up over time while passing from plankton to people. What began as tiny concentrations can be magnified to dangerous levels by the time it reaches us and other fish-eating predators like loons, herons and eagles. A 2008 study found that bald eagles in New York's Catskill Mountains were showing increasing levels of mercury — not enough yet to further threaten that species' survival, but a sign nonetheless that the toxin is still contaminating wild fish and moving up the food chain. (Source)

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