Millennia ago, long-range migrations provided benefits for many species of fish. By spawning in freshwater areas, species such as shad and river herring lessened predation on their young. And, by migrating into the ocean as adults, they were able to take advantage of the greater food resources of the marine environment.
But strategies that worked in ages past are proving a disaster for many of these travelers in the modern era.
Rather than reaping benefits, the reliance on so many habitats maximizes their exposure to human impacts, from pollution in freshwater rivers and dams that block migration routes to a gantlet of fisheries in rivers, coastal regions and the open ocean.
The result is a staggering decline in the populations of these diadromous fishes that "represents one of the greatest corruptions of the ecological connections between North American and European watersheds and the North Atlantic ecosystem," a recent paper in the journal BioScience stated.
Although the paper limited its review to the North Atlantic, "if we had gone to the Pacific, it would be the same story pretty much," said Karin Limburg, an associate professor of environmental science at the State University of New York, Syracuse, who co-authored the paper with John Waldman, a professor of biology at City University of New York, Flushing.
Diadromous fish include anadromous species, such as shad, which spawn in freshwater but live most of their lives in the ocean, and catadromous species such as eels, which spawn in the ocean but live most of their lives in freshwater.
Good data is lacking for many of these species. But the researchers were able to assemble time series data for 35 North Atlantic stocks; the populations of 13 had plummeted more than 98 percent from historic levels, and another 11 had declined more than 90 percent.
Many river-spawning stocks along the coast have vanished. Around the Chesapeake, shad native to the Patuxent and Choptank rivers are thought to be extinct while those native to the James and Susquehanna rivers are either gone or exist at very low levels-stocking programs in those rivers largely rely on progeny produced from other rivers.
The Atlantic sturgeon is so depleted it may be headed to the endangered species list-the National Marine Fisheries Service is expected to decide whether to list it later this year. Eels were reviewed for a possible listing several years ago, but federal agencies decided against doing so. Scientists believe other listings could be considered in the future.
Not all diadromous fish are in peril. The exceptions in the Bay are striped bass, hickory shad and white perch. But none of those fish migrate as far upstream, and white perch and hickory shad are thought not to venture very far into the ocean, limiting their exposure to coastal fisheries. Striped bass, whose population collapsed in the 1980s, may be the most intensely managed species along the East Coast.
But other species such as alewives were once so numerous they were likened to "passenger pigeons of the sea." When they migrated from oceans to rivers to spawn, they are said to have turned waterways silver with their great numbers.
Before development, migrations of shad and river herring are thought to have played a major ecological role by moving energy from the ocean to freshwater streams. After spawning, many of the fish would die, providing nutrients that helped fuel production in those streams. Eagles and osprey nesting coincided with shad and herring runs so they could feast on the migrating fish to get the needed energy to produce eggs and rear young.
"If you read anecdotal accounts from the 1700s, folks on horseback were reluctant to ford streams during the herring run because there were so many fish that horses would walk on them and trip and fall," said Greg Garman, director of Virginia Commonwealth University's Center for Environmental Science. "Supposedly, you could smell the rotting herring for miles during the spawning run."
The list of contributing factors to the fishes' demise is long. Dams are a major problem. For shad, access to a third of their historical spawning areas has been blocked because of dam construction. Many have been greatly overfished in rivers. In the ocean, they continue to be caught in bycatches. Water pollution, especially in spawning rivers, has been a problem. Nonnative fish have greatly increased the number of predators in spawning rivers.
"The problem with these diadromous fishes is that they do go through several environments, and each has its own issues," said Eric Hilton, a fisheries scientist with the Virginia Institute of Marine Science. "In the end, there probably isn't a single definite cause that is preventing recovery."
Although these fish were once "staggering in their plenitude" and supported major fisheries, many populations have been depleted for so long, and so gradually, that it did not trigger a major alarm. Instead, the authors contend, a sort of societal amnesia has developed about many of these species.
"This downward spiral of events lacks a term, but we suggest that it is a kind of ecosocial anomie, a breakdown both of expectations of what species should be present in healthy populations, and societal loss of interest," they wrote.
Dave Secor, a fisheries scientist with the University of Maryland Center for Environmental Science, said part of the problem is "the term 'diadromous' just doesn't capture the public imagination very much."
"But," he added, "there's a lot that should interest the public about this group of species, because they are travelers. The shad an angler catches on the Susquehanna Flats may have been to the Bay of Fundy. A striped bass caught at Solomons may have originated from the Catskills."
Eel be seeing you upstream
Eels, such as these captured at the base of Conowingo Dam, have suffered dramatic population declines in recent decades.
Although many factors likely play a role, biologists believe dams and other obstructions that block or hinder migrations to headwater streams are part of the problem.
For the second year, biologists are capturing eels at the Conowingo Dam and transporting them to two Susquehanna River tributaries-Buffalo Creek and Pine Creek-where their movements will be tracked.
Biologists hope to eventually have eelways erected at dams on the river to once again allow upstream migration.
Chesapeake Bay Diadromous Species
- Sea Lamprey
- Atlantic Sturgeon
- Shortnose Sturgeon (no spawning populations, but visits)
- \American Eel
- Hickory Shad
- Blueback Herring
- American Shad
- Striped Bass
- White Perch