A Look Inside Earth’s Marine Migrations

One of the most intriguing and magical aspects of our marine animals is their long, arduous journeys that circumnavigate the globe, taking them through coral reef jungles, barren stretches of open ocean and dramatic sea ice sculptures. These voyages allow them to take in the extremes of our ocean, for us a once in a lifetime experience but for most, this annual trek is a matter of survival as they search for food, a mate or safe waters in which to raise young. No matter where they go, migrations require an incredible amount of energy and, in some cases, luck to escape the variety of threats which line the path; predators, busy shipping routes, fishing nets and a lack of food. Yet these migrations mean they can reap the benefits of each different habitat they visit, optimising their chance of survival. Let’s take a look at some of the most incredible marine migrations on the planet.

If you believe the most impressive migration to be the longest, then you need look no further than the small unassuming Arctic tern. This slight seabird with a black cap and red beak can be seen all over the world as it completes its annual journey from the Arctic to the Antarctic. This is roughly a whopping 35,000 kilometres, making it the longest migration on Earth. The reason for this incredible way of life is the same reason many of us travel; sun seeking. In the Arctic summer, there are almost 24 hours of daylight, allowing these birds to feed for long hours as the sun illuminates the land and the sea making it easier to spot fish and insects. However, if they stayed in the Arctic all year, they would face several months of almost complete darkness and so they head to the Antarctic to make the most of the southern summer when the days turn shorter in the north. Terns tend to travel with their colony and just before they set off, the usually noisy nest falls silent in a phenomenon scientists have called ‘the dread’. Travelling south has been documented to be a more slow-paced, wandering journey as the birds follow the coastlines of Europe and Africa, detouring to areas with better weather and more food. The journey will take around three months in total with the birds gliding on currents in the air, sleeping and eating as they glide. In fact, if they didn’t need to refuel, these birds could cover an astonishing 1,000 miles each day! The journey back from the Antarctic is more rushed however, with the terns covering the colossal distance in around 40 days as they prepare to mate and nest.

Many tens of seabirds take part in migrations, most moving between areas filled with food to locations which provide sheltered safe nests for their chicks to grow. Recently, a seemingly random patch of open ocean was found to support around five million seabirds, all on a part of their migration. This area of the ocean lies halfway between Scotland and Bermuda and is where several ocean currents meet, allowing a swirling mass of food and nutrients to be gathered together. This attracts millions of birds from around 20 species, including Atlantic puffins and Bermuda petrels, some of which travel an astonishing 13,000 kilometres to reach the feeding ground. Many of these birds are known to have declining populations and whilst they may have their foraging grounds protected on land, out here past countries’ jurisdictions, they are not protected by law at all. This recent data is therefore incredibly important in helping scientists prove the need for marine protected areas in wide open oceans, something which will require international agreement.

Whales are probably one of the marine creatures most famous for their migrations as people hope to catch a glimpse of them on their journey from coastlines around the world; from Canada to Sri Lanka, Australia to Iceland. These giants are adapted for long journeys as their size allows them to lose less energy along the way whilst they are also able to build up enormous stores of fat to sustain them on their journey. For example, one population of humpback whales migrates to and from Australia and Antarctica. They move south to cold waters to feed on the large volume of krill which accumulate around the sea ice each summer, building up essential fat reserves which will carry them up to warmer waters. Once back in Australia, they will give birth to calves which will grow in tropical waters fed by the highly nutritious rich milk which has come from the Antarctic feeding. The calf will then be strong enough to make the return journey themselves. By the time they are considered mature at 12 years old, a whale will have travelled 100,000 kilometres, the equivalent of circumnavigating the world 2.5 times! Studies have been following this incredible migration for many years and have discovered that the ratio of males to females migrating each year fluctuates in line with polar fish populations. In years when the availability of fish in Antarctica is low, females who are not pregnant may decide to forgo the long and arduous journey as they simply don’t have enough energy to make the trip. Scientists are now monitoring the number of female humpbacks spotted in Australian waters as a way of gauging the health of the Antarctic ecosystem and the krill which live within it.

Turtles are another of our iconic species which make trans-world voyages although many of these take a less traditional east-west route rather than north-south. Mature turtles are known to travel thousands of kilometres a year between foraging grounds searching for tasty jellyfish before swimming back to the beach where they were born in order to mate and lay their eggs. Scientists are still not entirely sure how they find their way back but believe it is a combination of registering changes in temperature and daylight hours, which tell turtles when it is time to leave, as well as use of Earth’s magnetic field to navigate. Once a female reaches her destination, she will come ashore on a dark moonless night and dig a hole in the sand in which to lay her eggs. She will then cover these back over so they are protected from predators and the waves. Most incredibly however, is how the new-born two inch long hatchlings dig their way out of the sand and rush towards the sea. Once in the sea, they immediately head to deeper waters where there will be fewer predators, giving them chance to feed and grow more safely. How do they know where they are going though? A recent study shows that turtles may be born with an inherited magnetic map, meaning they are hardwired to navigate along their ancestral migratory route from birth; an incredible evolutionary trait that seems hard to imagine. Following the lines of the magnetic field, they head into deeper water by drifting in ocean currents, however if they start to be pulled in the wrong direction, they have been seen to swim actively against the current to get back on track. Scientists believe this could cause conservation problems down the line however as if, for example, one turtle population were to become extinct, individuals from another population could not be relocated easily as they would not have the inherited magnetic map for that region telling them instinctively where to travel to find safe, warm, nutritious waters.

Similarly, sharks are also believed to use Earth’s magnetic field to help them navigate across large patches of the ocean, whether that is a great white shark crossing the Indian Ocean from South Africa to Australia or a lemon shark heading back to the tiny island on which it was born. Each location on Earth has a different magnetic signature, allowing sharks to know whether they should be heading north or south using a system much more complex than a human compass. To make these journeys easier, they use circular swirling gyres to travel around ocean basins, sinking or rising in the water to take advantage of counter currents which run in the opposite direction.

Whilst many take part in annual migrations, there are also daily migrations that take place in every ocean around the world involving some of the oceans’ smallest creatures as well as some of the larger ones too. When night falls, thousands of creatures, from zooplankton to sharks, ascend from the dark depths to the surface where they feed on more abundant food sources under the cloak of darkness to protect themselves from predators. They also use this opportunity to mate and this daily routine is thought to be the largest synchronous migration on Earth; it even takes place in the Arctic winter when the day is pitch black. This event was actually researched and better understood due to the technology used in World War II. It was found that sonar pings were unexpectedly being deflected at a certain level of the ocean but each night this phenomenon disappeared as if the seabed had vanished. It turned out that during the day, the sonar pings were deflecting off all the creatures lurking in the deep waiting to rise to the surface.

Most of the current migrations embarked upon are ancestral routes that have been followed for centuries, however with the effects of climate change taking place, there are some new migrations forming; journeys where the creatures aren’t making the return trip. With waters warming, many are moving towards the poles to seek colder waters which are more suitable for them to live in and contain more nutrients than warm waters. Predators are following in their footsteps with some species already having migrated 600 miles from their territory over the last few decades. Scarily, ocean species are migrating in response to climate change 10 times faster than terrestrial animals. With Arctic sea ice melting, this is also breaking down the barrier between the Atlantic and Pacific Ocean, meaning some famous migration routes could soon be shifting. Aside from being fascinating, it is actually incredibly important for scientists to track and detect popular migration corridors so they can ensure these areas are protected from threats such as busy shipping lanes and fishing hotspots which threaten the lives of many of these incredibly hardy and inspiring creatures.

By Neve McCracken-Heywood