- April 21 marks World Fish Migration Day, a biennial event that strives to foster appreciation for the importance of migratory fish and their aquatic swimways.
- Healthy fish stocks with unimpeded migrations are essential to feeding humankind and maintaining the ecological equilibrium of the world’s waters.
- But fish migrations are being increasingly stressed by a worldwide boom in the building of dams that block their essential riverine passage, pollution, overfishing, lowering of water levels for agriculture and drinking water, and climate change.
- Here are six notable fish migrations to consider on this day.
Tomorrow, April 21, marks World Fish Migration Day. Many animal species make lengthy migrations as one-time or annual portions of their life cycle. Whereas journeys by birds between North and South America and large mammals across Africa’s Serengeti are highly visible, the migrations of countless fish, often over great distances and across disparate habitats, go on largely hidden from view.
The impetuses for fish to swim long distances are manifold, among them, differences in food availability, water levels, temperatures, spawning habitat, and vulnerability of their young to predation. A noteworthy category of fish migration is diadromy, the movement between fresh and salt waters. Anadromous fish spawn in freshwaters and go to sea, whereas catadromous species spawn in the sea but live most of their lives in fresh waters. Most anadromous species “home” to the rivers they were born in to mate; catadromous species reproduce in the same general marine waters where they originated. But many other fish make long excursions within either fresh or salt waters: along vast reaches of the Amazon and other large rivers, or across immense swaths of ocean.
Healthy fish stocks with unimpeded migrations are essential to feeding humankind and maintaining the ecological equilibrium of the world’s waters. But fish migrations are being increasingly stressed by a worldwide boom in the building of dams that block their essential riverine passage, pollution, overfishing, lowering of water levels for agriculture and drinking water, and climate change.
This year marks the third World Fish Migration Day, a biennial happening coordinated by the Netherlands-based World Fish Migration Foundation that strives to foster appreciation for the importance of migratory fish and their aquatic swimways. Organizations across six continents and 62 countries have planned well over 500 events around the theme of “connecting fish, rivers and people,” according to the World Fish Migration Day website.
Here are six notable fish migrations to consider on this day.
1) European eel (Anguilla anguilla)
The Atlantic, Pacific and Indian oceans all have eels that make prodigious catadromous migrations from salt water to fresh water, and then back again. European eels make especially epic journeys as long as 16,000 kilometers (10,000 miles). They are spawned in the Sargasso Sea, a huge area of deep ocean east of the Bahamas, and spend about two years riding the Gulf Stream clockwise around the North Atlantic as larvae and then as transparent “glass eels.” Then they detrain, transforming to their dark adult form to swim their way up rivers from northern Russia to northern Africa. After 10 to 20 years, European eels turn silver and their eyes enlarge to prep for an extended downstream migration into deep water and back to the Sargasso, where they mass with untold numbers of their species to spawn and then die.
The spawning grounds of the European eel are near those of the American eel (Anguilla rostrata), which has a similar life history. Some European eels overlap in their location and timing with American eels, producing hybrids that penetrate rivers in Iceland, halfway between Europe and North America.
When I visited Iran I was surprised to learn that European eels even reach the Caspian Sea, a water body without a natural outlet that exists well below sea level. This is an extraordinary one-way trip of some 12,000 kilometers (7,500 miles): around the North Atlantic on the Gulf Stream, through the Strait of Gibraltar and down the Mediterranean, through the Bosporus Strait, across the Black Sea and the Sea of Azov, into a manmade canal and then into the Volga River, and, finally, into the Caspian.
European eels once occurred in vast numbers as an esteemed, oily fish that helped feed the continent and graced the holiday menus of many cultures. But pervasive damming of Europe’s rivers greatly restricted their freshwater habitat. More recently, glass-eel fisheries for Asian markets have contributed to declines that may have reached 95 percent across Europe. As is often true for fisheries, it took a crisis to spur effective conservation actions, such as restrictions on fishing. Today the abundance of European eels, still extremely low, may be increasing.
2) Sea lamprey (Petromyzon marinus)
Atlantic sea lamprey have almost the reverse migratory habits of the European and American eels. They also occur naturally in rivers of North America and Europe. But sea lamprey are anadromous, and unusually so in that they don’t home to their natal rivers for spawning; instead they seek the nearest suitable river.
The sea lamprey strikes many as gruesome, with an eel-like body and a rasping disc for a mouth that enables it to suck blood and tissue from living fish. Once attached to a host, the parasitic lamprey draws sustenance for a week or two before detaching and finding a new victim. Not all hosts die; it is not unusual to see living fish with lamprey scars on their flanks. Sea lamprey sometimes latch on to cetaceans and even human swimmers.
If they don’t home to their birth rivers, how do sea lamprey know a river is apt for reproduction? Before traveling downriver to the sea, lamprey larvae spend as long as seven years living like clams in river sediments. While in their burrows they secrete bile chemicals that signal a decent spawning river to adult lamprey along the coast.
Sea lamprey have starkly divergent conservation statuses across their range. In Europe, they are a delicacy and have become scarce. Along the east coast of North America they are largely ignored. And in four of the U.S. Great Lakes, where they are non-native and invasive and have substantially harmed trout and salmon fisheries, there are expensive ongoing efforts to control them.
3) Pacific salmon (genus Oncorhynchus)
The Pacific salmon are among the most iconic of migratory fishes. The seven species that spawn in rivers from eastern Asia to Alaska and as far south as California are important sport and commercial fish that make lengthy oceanic circuits before returning to their home rivers to reproduce. Their upriver runs are often dramatic, involving soaring leaps up steep rapids and waterfalls.
One characteristic differentiating these salmon from the Atlantic salmon (Salmo salar) is that they are programmed to die after spawning. In fact, their bodies begin to break down during their spawning runs. I’ve seen many still-actively spawning salmon with fungal infections and pieces of flesh hanging off. This mass mortality is the key to their sustaining spectacularly large (if left undisturbed) populations in often modest-sized rivers.
Last September I fished the Nome River in Alaska. The pink salmon (Oncorhynchus gorbuscha) run had just ended but their dead bodies littered the river bottom. The Nome River is middling in size, ultra-clear, icy cold as it drains the tundra, and has a short growing season — all factors of low productivity. And yet it supports a run of 700,000 pink salmon in an off year and more than a million in peak years. Nutrients contained in the fish, squid and zooplankton that the adult salmon consumed in rich marine waters hundreds of kilometers away slowly leach out of all the decaying cadavers, subsidizing the otherwise weak food chain and nourishing their young — in a sense, paying it forward for the next generation.
Although salmon stocks in the largely pristine rivers of Alaska and western Canada remain robust, damming and water withdrawals in rivers from Washington to California have severely reduced many populations.
4) ‘O’opu ‘alamo’o (Lentipes concolor)
There are few freshwater fish species on most oceanic islands, given the impossibility of their swimming across long stretches of salty water. Those that do live on distant offshore islands evolved from marine ancestors, and many still spend part of their life cycle in saltwater. None so dramatically, however, as the ‘o’opu ‘alamo’o of Hawaii, and four other species of Hawaiian stream gobies.
These gobies, reaching only a few inches in length, are spawned high in mountain streams, but their fertilized eggs wash down-current to where the river meets the sea, where they develop and live as plankton eaters. But then they work their way back upstream, physically transforming as they go. Their mouth becomes sucker-like and their pelvic fins partially fuse and become capable of suction. With these two adhesive discs the fish can slowly inch their way up cascading waterfalls to reach the headwaters, streams where their new form is well adapted to feed on algae. How high can the ‘o’opu ‘alamo’o go? At Akaka Falls on Hawaiʻi Island they ascend 135 meters (442 feet) vertically, similar in scale to a human summiting Mount Everest.
Habitat destruction and reduced flows caused by climate change threaten this unusual fish.
5) Gilded catfish (Brachyplatystoma rousseauxii)
The gilded catfish of Amazonia makes the longest migrations of any fish species that remains within fresh waters. Though its biology is not well studied, scientists know the broad pattern of its movements. When ready to spawn, adult catfish move to the Amazon’s western headwaters at the foothills of the Andes, cuing to the high flows of the rainy season. Larvae and juveniles drift downriver to the estuarine waters of the eastern Amazon River. After a year or two they move upriver to feed and mature in the central reaches of the river. The entire circuit is some 8,000 kilometers (5,000 miles).
Although the young are sometimes sold in pet shops, these catfish truly are goliath — a common name for members of the genus Brachyplatystoma — reaching almost 3 meters (10 feet) in length and weighing as much as 225 kilograms (500 pounds). They are sought by commercial, artisanal and sport fishers and are sometimes caught using live piranhas as bait. The species suffers from overfishing, habitat destruction, pollution and, of late, an explosion in dam construction in the Amazon Basin.
6) Pacific bluefin tuna (Thunnus orientalis)
Pacific bluefin tuna range across the planet’s widest ocean. Though born in the Sea of Japan, many young tuna swim some 8,000 kilometers across the entire Pacific to the California and Mexico coasts, where they spend several years feeding and growing. Along the way they travel through cold northern waters, but they are aided by an adaptation: they generate heat while they swim. This heat is conserved in the fishes’ bodies by a counter-current circulatory system that aligns the warm veins with cooler arteries, raising body temperatures as much as 20 degrees Celsius (36 degrees Fahrenheit), and making the fish effectively warm blooded! They can also dive deep to feed, as far down as 500 meters (1,600 feet). As powerful adults, they can make the journey back to the Sea of Japan in as little as 55 days.
Pacific bluefin tuna can reach almost 3 meters in length and 450 kilograms (990 pounds) in weight. They have become extraordinarily valuable. In 2013, a 222-kilogram (489-pound) Pacific bluefin tuna caught off northeastern Japan sold in the first auction of the year at the Tsukiji fish market in Tokyo for a record $1.76 million. Not surprisingly then, this species has been heavily overfished, with the current population at only 3 percent of natural levels.
John Waldman is an aquatic conservation biologist with Queens College of the City University of New York and author of “Running Silver: Restoring Atlantic Rivers and their Great Fish Migrations.” Visit his website or follow him on Twitter: @DrJohnWaldman.
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