Fish Migrations: Navigating the Oceans for Survival

The concept of migration is a fascinating subject. Animals leave their home in droves to go out and breed or settle in a different place, but only for a time. And if bird migrations seem at all captivating, fish migrations will seem even more intriguing. The complex phenomena that is fish migration is one of the most breathtaking visions ever witnessed in the animal kingdom, but it’s also a necessity for many species of fish.

As they swim, migrating fish cover vast distances, sometimes crossing entire oceans, in order to breed, find food, or escape seasonal predators. These journeys are not just essential to the shoals of fish that make the trip, they are a vital piece of maintaining the longterm health of marine ecosystems. Understanding why and how fish migrate offers scientists valuable insight into the delicate balance of oceanic ecosystems.

In this article, we will take a deep dive into the many reasons behind fish migrations. We will explore the challenges they face and examine the species known for remarkable migrations.

Why Do Fish Migrate?

Fish generally migrate for three main reasons: reproduction, feeding, and survival. One shoal might migrate in order to find suitable breeding grounds, while others do so because they are following seasonal food supplies. The last group may be driven to migrate because of environmental pressure within their current habitat. Such pressure could be something as simple as a temperature changes, an alteration of salinity, or threats from predatory animals in the area.

Reproduction and Spawning

Most fish choose to migrate because they feel the need to breed. The biological imperative to propagate the species is a powerful one and many species of fish can only copulate in specific spawning grounds. These grounds are idea for several reasons, but mostly because the conditions there are perfect for ensuring the survival of their offspring. Many breeding migrations are triggered by seasonal alterations, such as temperature changes, moon phases, or shifts in water currents.

The uphill journey of the salmon is perhaps the most well-known story of fish migration; and one of the most remarkable. Born in freshwater rivers, salmon migrate to the open ocean as they grow, ultimately spending majority of their lives there. Once it’s time to spawn, however, they make the long journey across thousands of miles back up into the rivers from whence they came. Once they have reached their birthplace—though many do not, incidentally—the salmon undergo physiological changes to prepare for spawning. Either way, most species of salmon die after laying and fertilizing eggs, having fatally exhausted themselves as they made the journey.

Feeding Migrations

Food availability is another unavoidable need that affects certain species of fish. The annual shift of ocean currents and temperatures can have a profound effect on the behavior of the creatures that fish feed upon, some of them even other fish themselves. When these shifts occur, fish must follow their prey to ensure their species’ survival.

Some of the most recognizable of these are large pelagic species like tuna and swordfish. These massive swimmers are known to undertake long migrations in search of fertile feeding grounds. Fish that live in regions with seasonal plankton blooms—such as the Arctic or Antarctic regions—must time their migrations perfectly. In these places, migrations coincide with the blooms themselves, offering fish more opportunities to feed.

Herring are one example of this type of migration. These salty little buggers travel vast distances to feed on plankton, which is more abundant in certain areas depending on the time of year. In all these cases, it isn’t just the migrating fish who take advantage of the journey. Migrations are also crucial for the predators that depend on the fish for meals, such as seabirds, marine mammals, and larger fish like sharks.

Environmental and Survival Pressures

Finally, fish migrate in response to environmental changes such as water temperature, salinity, oxygen levels, and predation from other animals. Each one of these pressures can influence where fish can survive and some of these conditions fluctuate quite often, sometimes even seasonally.

Until recently, people weren’t exactly aware how eels matured. Turns out it had a lot to do with migration. The European eel and the American eel undergo a life cycle migration that spans thousands of miles, from the Sargasso Sea in the Atlantic where the eels are born, to the freshwater rivers in Europe and North America, where they mature. Of course, the life cycle and the migration aren’t over yet!

After maturing in their new freshwater environments, the eels then make the long journey back to the Sargasso Sea to spawn and complete the cycle. This highly specialized lifecycle is a wonder to comprehend, but it’s also easily disrupted by environmental changes, such as shifts in ocean currents or temperature.

The Science Behind Fish Migrations

Though the reasoning behind these fish migrations were largely unknown for the bulk of human history, modern biology has taught us much about the complex mechanisms that guide these shoals to distant seas. The reasons behind these migrations include a combination of environmental cues, sensory abilities, and instinctive behaviors. Fish might seem like simpler creatures evolutionarily, but they use a variety of tools to navigate the oceans and rivers during migration.

Navigation Tools

Magnetic Fields: It is believe that many migratory fish species, including salmon and eels, use the Earth’s magnetic field as a means of guiding them on their journey. They do this in a very interesting way. The tiny particles of magnetite in the fishes’ bodies act as a sort of biological compass, helping them navigate long distances across the ocean.

Sun and Stars: There is another theory that some species of fish utilize the position of the sun and stars to orient themselves during migration, in the same way that birds and other migratory animals do.

Olfactory Cues: Despite living their lives underwater, fish can smell just like any other animal, in some cases better than other animals. Salmon, for example, use their sense of smell to find their way back to their birthplace using olfactory cues. They do this by memorizing the unique chemical signature of the river where they were born. When they make their way back many years later, they still remember that particular scent.

Currents and Temperature: Many fish are sensitive to the change in flow and temperature of ocean currents to make their journey. Herring seek out and follow temperature gradients along their migration path to guide their migrations and find plankton, which thrive in warmer waters.

Species Known for Remarkable Migrations

Salmon

As we discussed earlier, salmon migrations are among the most iconic and recognizable in the animal kingdom. In the course of their migration, these tenacious fish travel as babies from their freshwater birthplaces to the open ocean, where they grow and mature. When they have reached spawning age, they travel all the way back upstream to spawn, using their amazing sense of smell to find their familiar way to that same green river of their birth. Salmon demonstrate incredible endurance as they travel upstream, swimming against strong currents, leaping over waterfalls, and navigating obstacles to reach their destination.

They also play a vital role within the marine ecosystem, as they provide nutrients to both marine and terrestrial animals. Bears and other animals eat some of them as they travel, as do fish and other predators. Also, when everything is said and done, their bodies decompose and release nutrients that enrich the soil and water, benefiting plants and animals in the surrounding area.

Atlantic Bluefin Tuna

The Atlantic bluefin tuna is one of the most powerful migratory fish in the world, as well as one of the fastest and the most agile. These mighty fish are so large and so strong that they are capable of crossing entire oceans in their epic search for food and ideal breeding grounds. These fish are also highly prized by commercial and recreational fishers, and are a real culinary delicacy the world over.

Bluefin tuna migrations are tied to temperature and current, they follow schools of smaller fish, such as herring and mackerel, and migrate to warmer waters when they want to spawn. Unfortunately, overfishing has severely impacted the bluefin tuna populations, as has climate change, both of which have impacted the tuna themselves and the creatures that they encounter along their ocean voyages.

Challenges Faced by Migrating Fish

Fish migrations are fraught with natural challenges, but there are many man-made ones as well. From climate change to overfishing, these human challenges have threatened the longterm health and survival of migratory fish and the ecosystems in which they live.

Climate Change

Rising ocean temperatures and changing currents are a big problem these days, and they are exacerbated every year by the increased rate of climate change. All of these factors are causing huge disruptions to the timing and routes of fish migrations. Some species are being forced to migrate earlier than usual, while some are pushed back. In either case, these setbacks can result in missed opportunities for feeding or breeding. There is also the added problem of fish migrating to new areas to find cooler waters, which can lead to competition with other native species.

Overfishing

We eat a lot of fish on this planet and the result is rampant and unsustainable overfishing. Overfishing is one of the biggest threats to migratory fish today. Eels, tuna, salmon, and many other commercially valuable species are being harvested at alarming rates, which leads to full population declines in some ocean habitats. What many people don’t realize is that overfishing not only threatens the fish themselves, but the surrounding ecosystems and coastal communities that depend on them for survival.

Habitat Destruction

The destruction of critical habitats, such as coral reefs, mangroves, and estuaries, have been going on for years, and they are only getting worse. These habitats are an essential part of marine life, providing shelter, breeding grounds, and feeding areas for many species. Dams and other manmade barriers are equally culpable though, as they can prevent fish like salmon and eels from reaching their spawning grounds.

Pollution

Pollution is always a problem and the increase in plastic waste, chemicals, and oil spills, have already done incalculable damage to marine ecosystems. Microplastics are ingested by fish and smaller organisms, poisoning them and disrupting metabolic processes. The same is true of chemicals contaminants in the water, which affect fish health, reproduction, and navigation, making it harder for them to complete their migrations successfully.

Protecting Fish Migrations for Future Generations

The survival of migratory fish species depends on the protection of their habitats but it also means that humans need to be more mindful of the way we interact with the natural world. We need to implement sustainable fishing practices, and make efforts to mitigate the impacts of climate change more effective. Conservation organizations, governments, and local communities have already begun this process and have thus far managed to are create marine protected areas, restore damaged habitats, and reduce pollution. Unfortunately, more needs to be done.

Marine Protected Areas

Marine protected areas (MPAs) are designated regions of the ocean where human activities, such as fishing and development, are restricted or regulated by local governments. Such places provide safe havens for migratory fish, allowing them to feed, breed, and grow without the threat of overfishing or habitat destruction.

Sustainable Fishing Practices

Sustainable fishing practices must be adopted in order for animals to continue to thrive and be available for further generations to come. This means ensuring the long-term survival of migratory fish species by setting catch limits, reducing bycatch, and protecting spawning grounds.

Addressing Climate Change

Mitigating the impacts of climate change is essential not only for protecting migratory fish, but the whole world. Though the results vary around there world, there are already efforts and initiatives in place that seek reduce carbon emissions, protect marine habitats, and restore damaged ecosystems in a meaningful way. The real work has yet to be done, however, as many of climate change’s greatest culprits, namely large corporations, are still not being held responsible or helping to fix the problem.

True Investigator Says…

Fish migrations are a wondrous thing to behold, but they are in grave danger. Even if the fish species that do migrate do show incredible resilience and adaptability, they cannot fight off the life-changing effects of pollution, climate change, overfishing, and habitat alteration all on their own. Therefore, we must endeavor to do all we can to help them, so that these fantastic fish can continue to make their amazing journeys for centuries to come.