As land-living animals, we are used to seeing habitats shaped and defined by fixed geographic features like hills, valleys, lakes and rivers. We also recognise changing seasons by looking at atmospheric weather and plants. By contrast, to our eyes, the open ocean can look uniform, constant and even boring. But animals living in the open ocean experience a changing and variable environment where physical conditions and habitats are shaped more by fluid dynamics than fixed geographic features. Because habitats and environments in the open ocean are not fixed in one place, the lives of many oceanic animals are characterised by movement. The longest migration on Earth is conducted by an oceanic seabird (the Arctic tern, migrating over 90,000km between Arctic and Antarctic waters), and the largest migration by volume of animals also occurs every day in oceanic waters.Terrestrial animals wanting to migrate may face barriers to movement such as mountain chains or seas, but in the open ocean movement and dispersal is generally easier, particularly when assisted by ocean currents. Ocean fronts, places where waters of different temperatures meet, or upwelling areas where cold, nutrient rich waters are brought to the surface create transient patches of productivity – oases in a wider desert. Mobile marine animals in the open ocean navigate between these regions of high productivity.
In common language we often think of migration as seasonal or large-scale movements of animals between habitats. A familiar example is the seasonal movements of songbirds between warmer regions in winter and cooler areas in summer. But ecologically speaking, migration is defined as:
Directed and sustained movement of an individual typically at the expense of behaviours such as foraging that would otherwise keep the animal in a local area.
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