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Unlock This CourseEvery single day, billions of marine animals perform the largest migration on Earth. But this isn't a journey across oceans - it's a journey up and down through the water column. Daily vertical migration (DVM) is one of the most fascinating and important behaviours in marine ecosystems, yet most people have never heard of it!
Imagine if every night, most of the animals in your neighbourhood moved to a completely different area, then returned home before sunrise. That's essentially what happens in our oceans every 24 hours, creating a massive daily shuffle of marine life.
Key Definitions:
Most vertical migrators follow a predictable schedule. During the day, they stay in deeper, darker waters (often 200-1000 metres deep). As the sun sets, they swim upwards to feed in the nutrient-rich surface waters. Before dawn, they return to the depths to hide from predators.
The main reason for this daily journey is a trade-off between finding food and avoiding being eaten. The surface waters are like a busy restaurant - lots of food, but also lots of predators. The deep waters are like a safe hiding place - fewer predators, but also less food.
Vertical migration is essentially a clever survival strategy. Animals get the best of both worlds: they feed in the productive surface waters when it's dark (and safer), then hide in the deep waters during daylight when visual predators are most active.
Surface waters contain abundant phytoplankton and small prey. Animals can feed efficiently in these nutrient-rich waters during the cover of darkness.
Many predators rely on vision to hunt. By staying deep during daylight hours, migrators avoid being seen and eaten by fish, seabirds and marine mammals.
Deeper waters are cooler, which helps some animals conserve energy. The metabolic cost of the journey is offset by energy savings and better feeding opportunities.
What tells these animals when to start their daily journey? The main trigger is light - specifically, the changing light levels at dawn and dusk. This is why scientists call it "following the isolume" - animals try to stay at depths where the light level remains constant.
Research shows that many migrators follow specific light intensities. As the sun rises and light penetrates deeper, animals swim down to maintain their preferred light level. As the sun sets and light fades, they follow the dimming light back towards the surface.
While light is the primary trigger, other factors also influence vertical migration patterns:
The diversity of animals involved in daily vertical migration is astounding. From tiny zooplankton to large fish and squid, this behaviour spans across many different groups of marine life.
Zooplankton: Copepods, krill and other small crustaceans form the backbone of vertical migration. These tiny animals can travel hundreds of metres twice daily.
Jellyfish: Many species of jellyfish and other gelatinous animals participate in vertical migration, following their zooplankton prey.
Vertical migration creates a complex community of interconnected species, each playing different roles in this daily movement.
Lanternfish, hatchetfish and other small mesopelagic fish are major participants. Some species can travel 400-800 metres twice daily.
Many squid species migrate vertically, following their prey. Some large squid can travel from depths of 1000 metres to near the surface.
Krill, copepods and mysid shrimp are key migrators. Antarctic krill can form massive swarms during their nightly migrations.
Antarctic krill (Euphausia superba) perform some of the most spectacular vertical migrations. These small shrimp-like creatures can travel from depths of 3000 metres to the surface - a journey equivalent to climbing Mount Snowdon twice every day! During summer, billions of krill migrate upward each night, creating dense swarms that can be detected by sonar. This migration is crucial for the Antarctic food web, supporting whales, seals, penguins and fish.
Daily vertical migration isn't just interesting behaviour - it's absolutely crucial for ocean ecosystems and even global climate patterns. This daily movement of billions of animals has far-reaching effects on marine food webs and ocean chemistry.
Vertical migration acts like a massive biological pump, moving nutrients and carbon between different ocean layers. Animals feed at the surface, then transport nutrients to deeper waters through their waste and when they die.
Scientists estimate that vertical migration transports 1-2 billion tonnes of carbon to deep waters annually. This is equivalent to the carbon stored in all terrestrial vegetation! This process helps regulate Earth's climate by removing carbon dioxide from the atmosphere.
How do scientists study something that happens in the dark depths of the ocean every single day? Modern technology has revolutionised our understanding of vertical migration patterns.
Scientists use various tools to study vertical migration:
The development of autonomous underwater vehicles (AUVs) has revolutionised vertical migration research. These robot submarines can follow migrating animals for days or weeks, collecting continuous data on their movements, behaviour and environment. Some AUVs can dive to 6000 metres and operate for months at a time.
Human activities are beginning to affect vertical migration patterns in concerning ways. Understanding these impacts is crucial for marine conservation efforts.
Several human activities can disrupt natural migration patterns:
Artificial lights from ships, oil platforms and coastal cities can confuse migrating animals, disrupting their natural light cues.
Warming oceans and changing currents can alter migration timing and routes, affecting food webs and ecosystem balance.
Shipping noise and sonar can interfere with the acoustic cues some animals use for navigation during migration.
Daily vertical migration represents one of nature's most remarkable phenomena - a daily dance of billions of marine creatures that connects ocean layers, drives global nutrient cycles and supports marine food webs. As we continue to explore and understand this behaviour, we gain deeper insights into the complex workings of our ocean ecosystems and the importance of protecting these vital migration patterns for future generations.