What creatures live in Earth's oceans? Which are likely to survive into the future? What sorts of sea animals have inhabited the oceans in the past? Finding answers to these questions is the goal of the Census of Marine Life (CoML), a 10-year, $1-billion research project that reached its halfway point in 2005. This quest by at least 1,000 scientists from 70 countries may be the first great voyage of discovery of the 2000's.

Oceans cover nearly 71 percent of Earth's surface, and yet oceanographers have explored less than 5 percent of this volume. We don't know how numerous marine species are—estimates vary wildly from 1 million to 10 million—or exactly where most species live.

A census of marine life is timely, even urgent. Few countries have a reliable base of information about their marine resources, including their fisheries (areas where fish are caught commercially or recreationally). Such information is needed to help governments develop successful strategies for protecting and managing these resources. Accurate information about the distribution, abundance, and variety of marine species can also help scientists determine how marine ecosystems have changed over time and help predict future changes. (An ecosystem consists of an area's biological and physical environments.) Finally, basic information about the condition of the oceans is essential for tackling such pressing global issues as the loss of habitat, pollution control, and climate change.

The CoML has four main components, the largest of which consists of 14 field projects exploring ocean life from both a biological and geographic perspective. That is, scientists working on these projects are cataloging marine species ranging from the smallest microbes to the largest predators in ecosystems ranging from shallow coastal waters to the deep sea. By 2005, census scientists had recorded 15,300 new species of fish and thousands more new plants and other marine animals. By the time researchers conclude in 2010, they expect the number of new fish species to be closer to 20,000. They also expect to have identified hundreds of thousands more marine life forms.

The CoML would not be possible without exciting technological advances in marine research. These include robotic vehicles that can penetrate previously inaccessible depths and hydroacoustic (underwater sound) devices that can “hear” small fish 100 kilometers (60 miles) away. Census scientists are using new optical sensors and cameras to record animals' appearance and behavior with greater clarity. Smaller, lighter tracking tags that don't interfere with animals' normal behavior are providing more accurate information about habitats and migration routes.

This Science Year Special Report focuses on four CoML field projects that were well underway by 2005. They are the Mid-Atlantic Ridge Ecosystems (MAR-ECO) Project; the Tagging of Pacific Pelagics (TOPP) Project; the Pacific Ocean Shelf Tracking (POST) Project; and the Gulf of Maine Area (GoMA) Program.

Census researchers working on the History of Marine Animal Populations (HMAP) Project, a second CoML component, are compiling information on the numbers and diversity (variety of different marine species) of sea life since 1500. Researchers with the Future of Marine Animal Populations (FMAP) project, the third CoML component, are using the historical and field data to develop mathematical models to predict future changes in marine ecosystems. Finally, much of the information from all these projects is being collected into a fourth CoML component, a publicly accessible, interactive online database called the Ocean Biogeographic Information System (OBIS). By mid-2005, OBIS held more than 5 million records, including data on about 40,000 marine species. Both OBIS and HMAP are discussed in depth at the end of this article.

Coordinating all the work of the census is a secretariat based at the Consortium of Oceanographic Research and Education in Washington, D.C., an association of U.S. oceanographic research organizations. Financial support is coming from an international group of government agencies as well as from such private sources as the Alfred P. Sloan Foundation, a nonprofit philanthropic organization in New York City.

In June 2004, an international team of scientists set sail from Norway aboard the G. O. Sars, a Norwegian research vessel, to explore one of the most remote regions on Earth. Their destination was the Mid-Atlantic Ridge, a vast underwater mountain range that stretches along the floor of the Atlantic Ocean for about 10,000 kilometers (6,200 miles) from a point north of Iceland to the Azores. Their quest was to observe and sample life in a rugged landscape that exists in near-total darkness thousands of meters below the surface. When the research ship returned to port two months later, its scientists brought back an abundance of data and an astonishing specimen collection. Like the great voyages of the 1800's and 1900's that pioneered ocean research, the expedition of the G. O Sars returned with enough material to keep marine scientists busy for years.

The G. O. Sars is one of several major research vessels working with the MAR-ECO Project, each committed by a different country, including Iceland, Germany, Portugal, the United Kingdom, and the United States. These ships are state-of-the-art, ocean-going laboratories that provide launching platforms for deep-sea exploration instruments and vehicles as well as facilities where scientists can study the samples and data they collect. In 2004, MAR-ECO scientists also chartered a Norwegian fishing vessel to sample bottom-living fishes.

As the G. O. Sars sailed along the northern part of the Mid-Atlantic Ridge, it stopped at about 40 locations called stations. At each station, the scientists sampled the water using a device that consists of a circular frame capable of carrying up to 36 bottles. Scientists can program the instrument to open the bottles, which each hold from 1 to 30 liters (0.26 to 8 gallons), at specific depths. At the heart of the device, called a CTD instrument, is a sensor that measures the water's conductivity (ease with which it conducts electric current), temperature, and density. Conductivity measurements help scientists determine the water's salinity (saltiness). The greater the water's conductivity, the greater is its level of salinity. The data are sent continuously to a computer aboard the ship. Information from different areas help oceanographers plot the location and movement of ocean currents and other water masses. The data also help scientists chart the distribution of species.

The movement of ocean currents may explain one of the most interesting discoveries made by scientists aboard the G. O. Sars—surprisingly large rings of plankton. (Plankton is the mass of tiny organisms that drift in the ocean.) To “count” plankton or other small fish that live at middle depths and to track their movement, scientists use sonar to record the sounds reflected by the animals as they move through the water. (A sonar is a device that uses sound energy to locate objects; measure their distance, direction, and speed; and even produce pictures of the objects.) In one area east of the Mid-Atlantic Ridge, MAR-ECO's echosounders discovered four enormous, donutlike rings of ocean plankton hundreds of meters in diameter. Although scientists had observed plankton rings before, they had never seen any so large. Scientists suggested that the rings may have formed from plankton communities caught up in eddies that were created as ocean currents collided. The rings may also have been created in eddies formed as currents were swept off course by their passage over a seamount (underwater mountain) or other undersea feature.

Among the many specimens brought back by scientists aboard the G. O. Sars was a major collection of zooplankton, small free-floating organisms that include such tiny crustaceans as water fleas and shrimps and many gelatinous (jellylike) forms. Interestingly, some of the gelatinous species had never been documented before. Scientists expect the collection to provide new insights into the zooplankton's range and the structure of their communities.

To collect the zooplankton and other ridge inhabitants, MAR-ECO scientists are employing a variety of instruments, including trawls and other traditional fishing gear adapted for scientific use. (Trawls are funnel-shaped nets that are open at the mouth and closed off at the tail end, where the fish or other marine life collects.) Some of the G. O. Sars's trawls could reach a depth of at least 3,000 meters (10,000 feet). Some trawls were also equipped with video and hydroacoustic equipment to help scientists control nets operating behind and below the ship.

Trawls and nets provide a “snapshot” of the creatures living in a particular water column, a top-to-bottom cross section of a body of water. Net samplers operated vertically provide another means of studying life in a water column. These devices contain a number of mesh nets, each designed to catch animals of a different size. After the scientists haul their catch aboard, they identify, count, measure, and weigh the specimens. Most specimens are frozen or preserved in formalin or another chemical solution for later study.

To photograph creatures living in a particular water column, MAR-ECO scientists rely on optical profilers. These instruments are essentially advanced video cameras. As the devices descend, they photograph the organisms that swim or float by at various levels or capture the flashes of bioluminescence the animals may produce. The MAR-ECO scientists also use free-fall photographic landers, which are metal frames containing cameras and other optical equipment that fall to the sea floor and photograph scavenging animals as they pass by.

To photograph and sample marine life at depths to 4,000 meters (13,000 feet), MAR-ECO scientists employ remotely operated vehicles (ROV's). These small, submarinelike “robots” are tethered by cable to a research ship. Although ROV's carry no crew, the video cameras and sensors they transport allow scientists to observe and record the appearance and behavior of sea life at lower depths. Mechanical arms attached to an ROV can be used to collect samples.

During their voyage, scientists aboard the G. O. Sars observed or collected at least 90 different fish species that live near the sea floor, another 200 species that live at a variety of other depths, and 50 species of squids or octopods. Scientists were surprised to find many of these species living in certain areas, and so the discoveries have increased our knowledge about the animals' habitats.

In addition to providing scientists with a treasure trove of data, the ROV dives left G. O. Sars scientists with at least one mystery. North of the Azores, an ROV's camera revealed a strange line of burrows on a seamount about 2,000 meters (6,500 feet) down. The burrows, which extended for several meters in different directions, consisted of an odd grouping of circular and irregularly shaped holes about 5 centimeters (2 inches) wide. The holes were so evenly spaced that one scientist compared them to lines of stitching made by a sewing machine. The intrigued scientists watched the burrows for some time, hoping to catch sight of the digger and trying to figure out if the holes represented many burrows or only one burrow with many entrances. But the animal and the nature of the burrows remained a secret.

Mapping the “highways” in the North Pacific Ocean that pelagic animals travel to feed, breed, and migrate is a focus of two census projects—the Tagging of Pacific Pelagics (TOPP) project and the Pacific Ocean Shelf Tracking Project (POST). (Pelagic means of the open ocean). Thanks to sophisticated electronic tags—which are implanted or attached to the animals without causing harm—the animals themselves are helping to provide the most detailed information ever collected about their movement and behavior in the open ocean. By 2005, TOPP scientists had tagged about 1,250 animals representing 20 species. POST had tagged or detected 3,000 members of the 7 species of salmon and sturgeon.

One type of tag, called an archival tag, can collect and store a variety of data over periods lasting two years or more. Implanted archival tags, for example, can record and store information on the internal temperature of tuna and swordfish as well as the temperature of the surrounding water. Such tags can help scientists track the animals' feeding behavior. Because a tuna is warmer than the fish it preys on, its internal temperature drops when it eats. Both implanted and attached archival tags also track an animal's depth—to learn about its diving behavior—and light levels in the surrounding water, which can be used to estimate location.

Archival tags, however, can reveal their data only if the tagged animal is caught. As a result, TOPP scientists are implanting these tags only in tuna and other commercial fish that are caught in large numbers or attaching the tags to sea turtles and other animals that regularly return to certain locations.

A more sophisticated form of archival tag contains a satellite transmitter and a “pop-up” device. TOPP scientists are using these pop-up tags with sharks, swordfish, ocean sunfish, and other animals that live below the surface, where the antenna would not be able to transmit properly. After collecting data for a predetermined period, these tags separate from the animal and pop to the surface. The tags then transmit their data to a system of remote-sensing satellites, which, in turn, send the information to TOPP scientists.

Some TOPP scientists are tracking several species of air-breathing sea animals daily with tags that communicate with satellites while still attached to the animals. The data transmitted by the tags, relayed to scientists by the satellites every 24 hours, are helping the scientists create maps showing exactly where the animals have traveled since being tagged.

By 2005, TOPP researchers had made fascinating discoveries about some of these movements. One of the most interesting finds involved a surprisingly well-traveled bluefin tuna. Scientists had known that bluefin tuna, the most commercially valuable fish in the ocean, spawn (reproduce) only in Asian waters. They had thought that a year or two after hatching, the young tuna traveled eastward across the Pacific to North America, returning westward some years later to spawn and die. To their surprise, however, TOPP scientists tracked one young bluefin tuna as it crossed the Pacific Ocean three times and traveled up and down the West Coast several times—all within about 600 days. Scientists are unsure whether a search for food motivated the tuna's journey. But the findings have provided new data on tuna habitats and migratory highways that may one day be used to help shape the management of tuna fisheries in the Pacific.

Learning more about the behavior of salmon, another commercially valuable fish, is the focus of the POST project. POST scientists use implanted acoustic tags that transmit coded signals unique to each fish. To receive the signals, the scientists have established a network of underwater listening stations. When a tagged fish swims by, tracking sensors in the stations pick up its signal, storing it along with the date and time in its memory.

In 2004, POST scientists made a surprising discovery about the dangers salmon face as they try to leave their hatching grounds in rivers for the open ocean. Before the study, scientists believed young salmon were most vulnerable while swimming through rivers and estuaries (coastal river valleys flooded by an ocean). Scientists thought that the dangers in these areas—human-related pollution, predators, and the stress of moving from a freshwater to saltwater environment—outweighed the perils awaiting them in the open ocean. In fact, POST scientists found that the salmon they tracked died at about the same rate along their entire migration path. This finding suggests that salmon conservation measures, which currently focus on rivers and estuaries, should include ocean habitats as well.

The Gulf of Maine Area Program (GoMA) taking place off the coast of New England is the census demonstration project focusing entirely on a single ecosystem. Long famous as one of the world's richest fishing grounds, the Gulf of Maine has suffered since the early 1990's from the collapse of many of its fisheries, particularly the cod fishery, because of overfishing and improper management of fish stocks.

GoMA's first goal is to gain enough knowledge to help the governments of the United States and Canada establish ecosystem-based management within the region. Ecosystem-based management means regulating the activities of human beings so that we do not disrupt the web of relationships that exist among the organisms of the ecosystem and between the organisms and their environment. To implement such a system, however, scientists need an adequate knowledge of the diversity and distribution of life in the ecosystem and of the ecological processes that govern that life. GoMA's second goal is to make available, to areas with similar bodies of water around the world, information about how biodiversity can aid in ecosystem-based management.

The Gulf of Maine Area, which is bounded by land to the west and banks (underwater plateaus) to the east, is often called a “sea within a sea.” It extends from Cape Cod (a peninsula on the coast of Massachusetts) in the south to the Bay of Fundy (which divides the Canadian provinces of New Brunswick and Nova Scotia) in the north. It also extends seaward to the rich fishing regions of Georges Bank and Browns Bank, which divide the Gulf of Maine from the rest of the Atlantic Ocean. The area also includes a section of the continental slope, which begins at the outer edge of North America's continental shelf, the submerged land at the continent's edge.

An unusual combination of geographic factors accounts for this ecosystem's staggering diversity. For example, the gulf includes both subarctic and temperate climate zones. This makes it the southernmost range for some Atlantic sea life and the northernmost range for others. Such temperature differences make the Gulf of Maine an excellent laboratory for studying the effects of climate change on the ocean. The gulf also encompasses a wide variety of habitats, including salt marshes, mudflats, and rocky coastlines as well as banks, deep basins, and seamounts. In addition, the gulf has the highest tides in the world, which help churn up and distribute nutrients through the water.

Because of these features, the area supports rich communities of marine life. It offers a bounty of zooplankton, which draws many species of fish and other larger animals, including endangered right whales. Thanks to the abundance of sea life, the gulf is also a migratory corridor for both sea birds and songbirds. The area is, perhaps, most renowned for its formerly rich fisheries of cod, haddock, herring, Atlantic salmon, flounder, and hake as well as shrimp, lobster, and other shellfish.

Census scientists working in the Gulf of Maine have two main tasks. The first is to collect data on the physical and biological characteristics of the region. To accomplish this, scientists go into the field—and sometimes into the water—to collect samples and learn more about the interaction of the area's species. Sometimes, scientists take a basic approach to collecting their samples. Along the shoreline, for example, they may crawl along on their hands and knees in intertidal zones to identify and count the number of the types of animals that reside there. (The intertidal zone is that part of the shoreline between the high-water mark and the low-water mark.)

Observing marine life at sea, however, requires a more complex and sophisticated approach. For that, scientists use a variety of remote-sensing devices. These include the video plankton recorder, for example, an underwater video microscope that captures images of small organisms and other particles. Another device, the laser-optical plankton counter, detects, sizes, and counts individual particles based on light measurements, usually as it is towed behind a boat. In this device, plankton particles passing through a light beam block the light falling on an array of sensors, registering the outlines of the particles.

To locate schools of larger fish, GoMA scientists use a number of acoustic devices, including Doppler-based sonars. These devices send out sound waves, which travel through the water until they hit a fish or other object. The signals reflect off the object and then return to a receiver, which analyzes the signals. Scientists can then use the signals to identify the location of the fish and determine the direction in which they are traveling.

GoMA scientists are also using instruments commonly employed by other CoML scientists to capture specimens. These devices include trawls, dredges (nets dragged along the sea floor), and plankton nets. ROV's and computer-controlled autonomous underwater vehicles carry cameras and other video equipment to deep waters. Piloted submersibles carry scientists there.

By 2005, GoMA scientists had accomplished a great deal. While most of the research is still preliminary and will require additional analysis and confirmation, scientists have made some exciting discoveries. For example, by closely studying data from past trawl surveys, GoMA scientists have found that nearly half of every sample load collected in their nets includes species previously unknown on the continental slope.

Scientists have also confirmed that fish diversity is widespread across different regions of the gulf. For example, fish are most diverse in regions near the shore and around the edge of Georges Bank. In these areas, different types of habitat converge to create overlapping areas where many types of species congregate to feed and breed.

Along the northern edge of the New England Seamounts, researchers have identified 30 new species of coral. GoMA scientists using multibeam sonar have also mapped enough of the gulf's sea floor to develop a reasonably complete atlas of the northern part of the chain. (Multibeam sonar records the travel time of acoustic signals from a transmitting device to an object and back again.) One group of GoMA scientists has theorized that the seamounts serve as “stepping stones” for some organisms to spread their offspring across the Atlantic Ocean. That is, they have found that certain species known to inhabit the eastern side of the Atlantic have unexpectedly taken up residence on the western side.

At the other edge of the gulf, at the intertidal zone, preliminary data suggest that the number of species has dropped significantly. Scientists speculate that one cause may be the Asian shore crab, an omnivorous (plant- and animal-eating) crustacean first identified along the eastern coast of North America in 1988. The crab may be crowding other species out of its new territory.

These data and additional studies will enhance our understanding of how various parts of the Gulf of Maine fit together to form an ecosystem. By 2010, researchers hope to be using these data to develop more effective ways to protect the various parts of the gulf without harming the ecosystem as a whole. If they are successful, their plan will serve as a model for water bodies worldwide.

Some of the world's oldest documents record human efforts to harvest the bounty of the sea. Scientists working on the History of Marine Animal Populations (HMAP) project are gathering historical records about 12 ocean ecosystems in order to chart the changes in the abundance and diversity of marine life there for the past 500 years. In particular, HMAP aims to document the effects of human activities and climate change on ocean ecosystems.

The HMAP project employs what may, at first, seem to be an unusual combination of researchers that includes ecologists, biologists, oceanographers, historians, archaeologists, and mathematicians. The records being analyzed are equally varied. These include modern fishery statistics as well as ships' logs and records kept by monasteries. These religious communities of men, which often served as trading places, kept accurate records of fish brought in, sold, and taxed. HMAP scientists are also examining environmental records for clues about changes in marine ecosystems. Sediment cores (cylindrical samples of material from various layers of the ocean floor), for example, can reveal how marine life has changed in a given area over time.

In 2005, HMAP scientists were expressing surprise at the change in marine life in every region under study. For example, in March, HMAP scientists reported that the volume of North Atlantic cod living on the heavily fished Scotian Shelf, which surrounds the Canadian province of Nova Scotia, had declined by 96 percent since the 1850's. HMAP researchers found that the waters of the shelf contained 1.26 million metric tons (1.38 million tons) of cod in 1852. By 2000, the total had fallen to less than 50,000 metric tons (55,000 tons), chiefly because of overfishing.

Another HMAP study, released in April 2005, linked the decline of coral reefs in the Caribbean Sea, in part, to the overfishing of that region's sharks. Researchers found that fewer sharks led to a rise in the number of smaller carnivorous fish and a corresponding drop in the number of plant-eating fish that feed on the algae often found on coral reefs. The overabundant algae are, in turn, smothering the coral.

What is happening to the flood of information coming from CoML scientists? After all field projects conclude in 2010, the census will publish an in-depth report on its activities. It will examine how the diversity, distribution, and abundance of marine species in the oceans have changed since the 1500's and provide forecasts about the future of ocean life. It will also include descriptions of new species discovered by CoML scientists and discuss how these creatures fit into their ecosystems.

In the meantime, the data emerging from several census projects is being entered into the Ocean Biogeographic Information System (OBIS). Established by the CoML in 1999, OBIS is an Internet portal (gateway) to data on the distribution of marine species contained in many databases around the world. OBIS is freely available on the World Wide Web to researchers, students, or any other interested person. OBIS data from its many sources can be used interactively for such purposes as finding a species' scientific or common name, predicting a species' geographic range and distribution, and generating new hypotheses about marine ecosystems and then proving or disproving them. By mid-2005, OBIS users could access 4 million records from 45 databases on the distribution of 40,000 species, including at least 240,000 records dating from 1611 to 2000.

Another census legacy will be innovative technology for studying marine life. In addition, the census has created scientific connections that cross political boundaries. These links will make future studies of the oceans easier and more complete. Finally, this collaboration is strengthening our appreciation of Earth's oceans as a global resource.