Marine Science (0697) | Migration

🌊 Marine Organisms: The Great Journeys - Migration (3.7)

Hello Marine Scientists! This chapter explores one of the most incredible phenomena in the ocean: migration. Migration is essential for survival—it helps marine organisms find food, reproduce, and stay safe. Don't worry if this seems like a huge topic; we will break down these amazing journeys into simple, clear steps!

1. Defining Migration and the Reasons Behind the Journey (3.7.1)

Migration is the regular, often seasonal, movement of an animal from one region or habitat to another. Unlike just swimming around, migration follows a predictable route and serves a vital purpose for survival.

Why do marine organisms migrate?

Marine life doesn't travel just for fun! They undertake these sometimes exhausting journeys for four main reasons (Think of the mnemonic F-M-R-P: Food, Mates, Reproduction, Predators):

• Finding Food (Feeding Grounds): Organisms often move to areas where food is abundant, especially seasonally. For example, many whales migrate to nutrient-rich polar waters during the summer to feed.
• Finding Mates and Reproducing: Many species travel to specific locations (often warm, safe, shallow waters) to breed and lay eggs or give birth. These locations, known as spawning grounds, offer ideal conditions for the survival of their offspring.
• Moving to a Different Habitat to Reproduce: This overlaps with finding mates, but focuses on the need for a specific, safe nursery environment. Sea turtles return to the exact beach where they hatched to lay their own eggs.
• Avoiding Predators: Moving to areas where predators are less common, or using the cover of depth or darkness to avoid being eaten.

2. The Three-Dimensional Nature of Migration (3.7.2)

Unlike land animals, marine organisms live in a three-dimensional environment (length, width, and depth). This means migration can happen in two major ways:

(a) Horizontal Migration

This is movement across the surface of the ocean or along the coastline, covering large distances (latitude and longitude).

• Distances: Can be over very long distances, sometimes spanning entire ocean basins (e.g., from the Arctic to the tropics).
• Examples: *Tuna*, *whales*, and *sea turtles*.

(b) Vertical Migration

This is movement up and down the water column (changing depth).

• Distances: Often over short distances, sometimes happening daily.
• Examples: *Plankton*, *squid*, and *shrimp*.

3. Daily Vertical Migration (DVM) (3.7.3)

Many smaller marine organisms perform a massive, coordinated daily journey called Daily Vertical Migration (DVM). This is the largest biomass movement on Earth!

Which organisms perform DVM?

*Plankton* (both phyto- and zoo-), *shrimp*, *squid*, and small *mesopelagic fish* undertake DVM.

The Process: Twilight Zone vs. Sunlight Zone

These organisms move between the upper layer of the ocean (the Sunlight Zone) and the deeper, darker layer (the Twilight Zone).

Step 1: The Ascent (Night-time)
When the sun sets, organisms move up from the deep Twilight Zone (200m–1000m) into the shallow Sunlight Zone (0m–200m).

• Why? The Sunlight Zone is where the primary food source (*phytoplankton*) lives and photosynthesises. They feed in the rich surface waters.

Step 2: The Descent (Day-time)
When the sun rises, the organisms move down from the surface waters back into the deep, dark Twilight Zone.

• Why? The most crucial reason for DVM is avoiding predators. Larger predators (*like tuna or marine mammals*) hunt by sight. By staying in the dark depths during the day, the small organisms reduce their risk of being eaten.

4. Very Long Horizontal Migration (3.7.4)

Large, powerful swimmers undertake extremely long horizontal migrations, sometimes traveling thousands of kilometres.

Examples and Reasons for Long-Distance Travel:

• Tuna (e.g., Bluefin Tuna): Known for crossing entire oceans (Atlantic or Pacific) to find optimal feeding grounds or warm spawning areas. They need to move vast distances to sustain their high metabolic rate by following food sources.
• Turtles (e.g., Leatherback Turtles): Migrate huge distances between cold, productive feeding grounds (like Canadian coasts) and warm, tropical nesting beaches. They must return to specific natal beaches to reproduce.
• Whales (e.g., Humpback Whales): Undertake annual migration from polar regions (where they feed on abundant krill during summer) to tropical breeding grounds (where their calves are born in warm, protected waters during winter).

5. Navigating the Open Ocean: How Organisms Find Their Way (3.7.5)

Imagine trying to cross an entire ocean without a map or GPS! Marine organisms use sophisticated, natural methods to navigate their long and complex routes.

Methods Used for Navigation:

1. Magnetic Field of the Earth

• The Earth generates a weak magnetic field. Animals like sea turtles, whales, and some fish can sense this field.
• It acts like an enormous, internal compass, allowing them to maintain a consistent bearing over long distances, even in featureless open water.

2. Location of the Sun, Moon, and Stars (Celestial Cues)

• Organisms that travel near the surface, or at night, can use the position of celestial bodies (Sun, Moon, and constellations) to maintain their direction, similar to ancient human sailors.

3. Olfaction (Sense of Smell)

• Olfaction is vital for species that return to a highly specific location, such as a river mouth or a nesting beach.
• Salmon use their powerful sense of smell to detect the unique chemical signature of the freshwater stream where they were born, guiding them back years later to spawn.
• Turtles may use olfactory cues from the beach sand or near-shore waters.

4. Mental Maps and Landmarks

• For shorter, coastal, or repeated migrations, organisms build a "mental map" of their surroundings.
• They use recognizable landmarks, such as specific seabed topography (*seamounts, canyons*), coastal outlines, or islands, to follow a known route.