Why do marine animals migrate: Everything you need to know  

Migration across the Ocean is such an extraordinary phenomenon that scientists today are still trying to discover how and why it’s done. 

• How does a turtle find the same exact beach where it hatched after an epic journey across the Ocean? 
• How do gray and humpback whales navigate record-breaking migrations: 14,000 miles of deep blue sea over 172 days? 
•  Why do sardines, whales, turtles, hammerheads, great whites, manta rays and all manner of smaller creatures undertake these incredible journeys across our seas? 

Why do marine animals migrate across the open Ocean? 

Crossing an entire Ocean is extremely tiring. You could get lost or caught in a storm and you’re exposed to various risks along the way, so why do it? 

Migration comes down to a need for a resource that an animal doesn’t have in its current environment. They’re often seasonal, long-distance movements in search of food, mates, habitat or to escape predation.

Food: One of the biggest reasons for migration. 

Baleen whales, like humpbacks and gray whales, will migrate to northern latitudes during the spring and summer to feed in cold arctic waters, rich in krill and shrimp like crustacea. The long, tiresome journey from the south is made worthwhile for the feast of food that awaits them there.  

Turtles also make their way north, with species like leatherbacks spotted in the waters off Canada, Alaska or Nova Scotia. Leatherbacks are some of the most highly migratory animals on Earth, the longest recorded journey being 12,744 miles from Indonesia to Oregon, USA.

Here during the summer months, there is an increasing abundance of a turtle’s favourite food: jellyfish.

But of course, the food can move too.

Fish are one of the most important sources of food on Earth, preyed upon by numerous different animals, including humans. The KwaZulu-Natal sardine run, also known as the “greatest shoal on Earth,” is a mass migration of South African sardines to the sub-tropical waters of the Indian Ocean.  

Estimated to rival Africa’s wildebeest migration as being the largest biomass migration on Earth, this shoal becomes a ‘moveable feast’ for opportunistic predators like sharks, dolphins, gannets, seals and whales.  

Whales also migrate to find a mate.

Whales, like humpback and gray whales, feed in cold arctic and sub-arctic waters but that’s not a suitable place to find a mate and give birth to their offspring. They could breed here but there are serious risks to the mothers and their calves with the cold water and predation by animals like orcas. 

Instead they move from north to south during the winter months, giving birth to their young in shallow, warm waters such as lagoons. Popular destinations include Baja California, Mexico, Hawaii and Japan.  

Frodo the humpback whale, named after the Lord of the Rings character, underwent his record-breaking adventure to find a mate from the Mariana islands to Mexico covering around 7,000 miles. Check out his journey on Happywhale. 

Humpbacks will often migrate the same routes they were guided on by their mothers. Frodo’s unusually long journey may be relic behaviour of the whaling industry, where depleted numbers require males to travel further in search of a mate.  

Turtles will return to the exact same beach where they hatched to lay their eggs, known as natal homing. Most turtle species spend most of their time in the open Ocean, widely dispersed across the globe.  

But how do they know where they are and where they’re going? 

Turtles show remarkable navigation skills with pinpoint accuracy using a combination of external cues to calculate their position and route. When they are near the site of their hatching, turtles may use visual cues such as the incline of the beach or the smell of the water or air.  

However, in deeper water turtles must resort to other methods to find their way home.  Loggerhead, green and leatherback turtles have all demonstrated the use of a ‘magnetic map sense’ like other long-distance migrants such as bird and butterflies.  

Along a coastline, the inclination and intensity of the magnetic field will vary, giving rise to a unique magnetic signature at a precise location. Scientists suggest that hatchlings imprint on this unique magnetic signature and use it to navigate back across the entire Ocean years later.  

Long journeys come with obstacles that Ocean migrants must face.  

Our Ocean is becoming an increasingly treacherous place for its inhabitants, with threats from entanglement, ship strike, lack of jurisdictional protection and climate change. 

As these migrants make their way along vast journeys, they tend to cross paths with one of the most dominant and widely distributed animals on Earth: people.  

Many important migratory routes for whales and other surface-dwelling animals like turtles and sharks, converge with areas of heavy maritime traffic. This cross over can lead to ship strike, which is harmful if not fatal to an animal.  

Species like the endangered North Atlantic Wright whale are particularly vulnerable as their habitat and migration routes are close to major ports and shipping lanes. There were 37 whales were reported injured in this region between 2010 and 2014 and that is likely to be an underestimate. 

Furthermore, about 640,000 tonnes of discarded fishing gear, known as ‘ghost gear’, enters our Oceans every year, posing the major threat of entanglement.  

The animals who travel the most are at higher risk of such encounters. For instance, an estimated 30,000 whales and dolphins die from entanglement each year.

Rising sea surface temperatures due to climate change may also alter where migratory species find food and push them past their heat tolerance. This could disrupt the longstanding migration patterns between feeding and breeding grounds.

Nevertheless, there’s a push for the conservation of these migratory species and a desire to make the Ocean a safer place.

We’re constantly developing new technologies to help prevent animals from becoming entrapped in fishing gear. For example, Galvanic Timed Releases (GTRs) involve materials that disintegrate over time, opening doors or panels on the gear or allowing lines to break away. 

Restrictions such as vessel speed limits and altered ship routes help avoid collisions with endangered species such as North Atlantic wright whales, as well as establishing temporary precautionary zones around recently sighted whale groups.  

The migration of these marine travellers across the Ocean highway are some of the most extraordinary and treacherous journeys in the world.  

Continuing to learn and understand these journeys is essential for protecting Ocean life and reducing the threat that is posed by humans. 

Post by

Olivia Jones