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Unlock This CourseMarine migration is one of nature's most incredible phenomena. Every year, billions of marine animals travel thousands of kilometres across our oceans, following ancient routes that have been used for millions of years. These journeys are essential for feeding, breeding and survival, but they're increasingly threatened by human activities.
Key Definitions:
Seasonal Migration: Regular yearly movements between feeding and breeding areas. Spawning Migration: Movements specifically for reproduction. Ontogenetic Migration: Different life stages living in different habitats. Feeding Migration: Following food sources as they become available.
Humpback whales undertake one of the longest migrations of any mammal, travelling up to 25,000 kilometres annually. Their journey demonstrates the incredible navigation abilities of marine animals and the importance of protecting migration corridors.
Humpback whales spend summer months in polar waters like Alaska and Antarctica, where they feed on krill and small fish. As winter approaches, they travel to warmer tropical waters near the equator to breed and give birth. This journey can take 2-3 months each way.
In polar waters, humpbacks consume up to 1.5 tonnes of food daily, building fat reserves for their long journey and fasting period during breeding.
Whales travel along coastlines, using magnetic fields, water temperature and underwater topography to navigate their 12,000km route.
In warm tropical waters, females give birth and mate. Calves are born with little blubber, so warm water is essential for survival.
North Pacific humpback whales travel from Alaska to Hawaii, Mexico and Japan. Scientists track them using satellite tags, discovering that some individuals show incredible site fidelity, returning to the exact same breeding lagoons year after year. This population has recovered from near extinction (1,400 individuals in 1966) to over 25,000 today, showing how protection can help migrating species.
Sea turtles are master navigators, using Earth's magnetic field as a GPS system. Their migrations span entire ocean basins and can last decades, making them one of the most remarkable examples of marine navigation.
Young loggerhead turtles from beaches in Japan and Florida embark on transpacific journeys that can last 20 years. They follow ocean currents in a giant loop, growing and feeding before returning to their birth beaches to nest.
Sea turtles can detect Earth's magnetic field and use it like a map. They learn the magnetic signature of their birth beach as hatchlings and can return to it decades later. Different magnetic signatures tell them their location and direction to travel.
Leatherback turtles make the longest migrations of any reptile, travelling up to 20,000km annually. They follow jellyfish blooms across entire ocean basins. Pacific leatherbacks nest in Indonesia and Malaysia but feed off the coasts of California and Oregon. Sadly, this population has declined by 95% due to fishing nets, plastic pollution and coastal development destroying nesting beaches.
Salmon demonstrate one of nature's most dramatic life cycle migrations, spending years at sea before returning to their exact birth stream to spawn and die. This anadromous lifestyle connects freshwater and marine ecosystems.
Atlantic and Pacific salmon are born in freshwater streams, migrate to the ocean to grow and mature, then return to their birth stream to reproduce. This journey requires incredible physiological changes as they adapt from fresh to salt water and back again.
Salmon eggs are laid in gravel nests called redds. Young salmon (fry) develop in freshwater, feeding on insects and growing for 1-4 years depending on species.
Juvenile salmon (smolts) undergo physiological changes to survive in saltwater. They spend 1-7 years in the ocean, growing rapidly on a diet of fish and crustaceans.
Adult salmon stop feeding and use stored energy to swim upstream, sometimes hundreds of kilometres, to reach their birth stream. They spawn and die, providing nutrients to the ecosystem.
Fraser River sockeye salmon in British Columbia undertake a 1,500km journey from the Pacific Ocean up the Fraser River system. Every four years, massive runs of over 10 million fish return simultaneously. However, climate change is warming rivers beyond the salmon's tolerance and in 2019, thousands died from heat stress. Dam construction has also blocked traditional routes, forcing expensive fish ladders and hatchery programmes.
Marine migration is driven by several key environmental factors that animals have evolved to respond to over millions of years.
Seasonal temperature changes trigger migration timing. Warming waters signal breeding time for many species, while cooling waters indicate it's time to move to feeding grounds. Climate change is disrupting these temperature cues, causing mistimed migrations.
Marine animals follow food sources that change seasonally. Polar waters become highly productive in summer due to long daylight hours, attracting whales and seabirds. Tropical waters provide consistent temperatures for breeding but less food, explaining why many species migrate between these regions.
Currents transport nutrients and provide 'highways' for migration. The Gulf Stream helps sea turtles travel across the Atlantic, while the California Current brings nutrients that support whale feeding.
Changing daylight hours trigger hormonal changes that initiate migration. This photoperiod response ensures animals migrate at the optimal time for survival and reproduction.
Internal biological clocks coordinate with environmental cues to time reproduction. Many marine animals breed annually or every few years, requiring precise timing to reach breeding grounds.
Human activities increasingly threaten marine migration routes through pollution, climate change, overfishing and habitat destruction. Understanding these impacts is crucial for conservation efforts.
Ship strikes kill thousands of whales annually, especially along busy shipping lanes that overlap migration routes. Plastic pollution causes sea turtles to mistake bags for jellyfish, leading to intestinal blockages. Fishing nets entangle marine mammals and sea turtles, while overfishing removes prey species that migrating animals depend on.
Eastern Pacific gray whales migrate 20,000km annually between Alaska and Mexico. Hunting reduced their population to just 2,000 individuals by 1946. International protection allowed recovery to over 26,000 whales by 2016, though recent unusual mortality events linked to climate change and reduced Arctic sea ice are causing new concerns. This shows both the potential for recovery and ongoing challenges facing migrating species.
Protecting marine migration requires international cooperation since these animals cross multiple national boundaries. Several conservation strategies are proving effective.
Creating protected areas along migration routes provides safe havens for feeding and resting. The Papahānaumokuākea Marine National Monument protects Hawaiian monk seals and green sea turtles, while seasonal fishing closures protect whale feeding areas.
Satellite tags and acoustic monitoring help scientists understand migration patterns and identify critical habitats needing protection. This technology also enables real-time alerts to ships when whales are present, reducing ship strikes.
Climate change is shifting ocean temperatures and currents, potentially disrupting migration patterns that have existed for millions of years. Scientists are working to understand how species might adapt and identify climate refugia - areas that will remain suitable as conditions change. International agreements like the Convention on Migratory Species help coordinate protection efforts across borders, but more action is needed to address the root causes of climate change and pollution.