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In the marine ecosystem, migrations are a key to survival. By travelling to different
locations, animals stand better chances of finding food or a suitable place to breed and raise
their young. In the previous video, we explored the ecologically important 24 hour cycle of
predators and prey migrating between the surface and depths. But vertical migrations are not
the only journeys creatures of the deep undertake. In this video, we’ll take a look at the
migrations of nektonic organisms - animals that are able to actively swim and can undertake
large-scale journeys around the world, covering
larger distances than plankton and their predators.
With the ability to swim actively from one point to another, nektonic animals possess
the ideal adaptations needed to undertake extensive migrations. Recent technological
developments in wildlife tracking have given us an idea of when and why these migrations
occur. Notably, they appear to be seasonal, often taking place to and from annual feeding
or breeding grounds. But travelling such long distances comes at a cost. It requires a lot
of energy, and causes exposure to a number of risks. But such journeys allow animals
to make use of different habitats for different purposes. Often, the driving force behind
migrations is finding a breeding area safe from predators.
Whales infamously set out on these journeys. They are well-adapted to doing so, for growing
to such immense sizes enables them to lose less energy, and large fat reserves sustain
them for extended periods without needing to eat. A behavioural adaptation that whales
have developed is to travel in the direction of prevailing currents. A decision that greatly
minimises how much energy they use up. In a similar fashion, migratory fish like the
mako shark make use of ocean gyres. They adjust their depth to take advantage of counter-currents
that that run in different directions near the surface and at depth. This behaviour mirrors
aeroplanes saving time and fuel by flying with the jet stream.
Sometimes, marine animals might aim at specific targets such as oceanic islands. This is true
of sea turtles, which travel far and wide during their lives, but once mature they will
migrate thousands of miles to lay their eggs at the site where they themselves were hatched.
Studies into the long-distance movements of marine vertebrates have shown that oceanic
migrants likely rely on sophisticated means of navigation. Some, like the turtles, use
biological compasses to maintain direction in the open ocean. It has also been discovered
that sharks can detect and utilise the Earth’s magnetic field.
The largest vertebrates, the blue whale and the humpback whale, require enormous amounts
of food if they are to sustain themselves. This warrants a seasonal migration towards
polar regions. In particular, they travel to Antarctic waters to feed on the blooms
of krill that appear in the Spring and Summer.
They gorge themselves non-stop, putting on
vast amounts of fat as blubber. For the Winter, the whales then move to warmer but less food
abundant waters, where they survive on their reserves of body fat. It is during this period
of the year when calves are born, and the fat reserves are used to produce nutrient-rich
milk for the suckling juveniles. Without their seasonal migrations to the krill blooms, the
whales would find it difficult to build up these vital stores. Survival would be far
more difficult.
By suckling, the young whales are able to grow strong enough to make their own long
journey to the poles. By the time a calf is mature at 12 years old, it may have already
travelled further than 100,000 kilometres. A distance equivalent to travelling around
the world two and a half times.
But equally remarkable is the ability of the far smaller sardine to brave the open ocean
in search of spawning grounds. Between May and July each year, millions of these fish
travel north along the East coast of South Africa to release their eggs. This phenomenon
is the great Sardine Run, during which the migrating fish come under relentless attack
from sharks, dolphins, seabirds, and whales. Though it might seem like a doomed mission
for the sardines, they, like many migrating animals, are driven by a single objective.
Perpetuating the species. Their reproductive instinct takes over that for survival and
drives them to swim headlong into danger - guided only by the prospect of scattering their eggs
in the spawning grounds.
It is the changes in temperatures, food availability
and the need to mate that drive such a large-scale
movement.
Although most nektonic animals are vertebrates, like whales and sharks, there are a few groups
of invertebrates that are good-enough swimmers to undergo migrations. Adaptations are fundamental
to whether an invertebrate is nektonic or planktonic. In the case of cephalopods like
squid, the ability to forcibly eject water from a siphon makes them excellent swimmers.
Overall, the seasonal migrations of pelagic creatures allow them to make use of the resources
and conditions of different marine habitats. Consequently, they increase their chances
of survival, making use of the adaptations that allow them to swim actively in order
to chase prey, follow weather systems, or give their young the best chances of survival.
Both large-scale seasonal migrations, and daily vertical journeys of plankton and their
predators represent the incredible ability of creatures to use environmental changes
to their advantage, moving to different areas where they can meet their needs.