The Chesapeake Bay and its tributaries are like a vast highway system for fish. Traffic is heavy, especially in spring as fish make their way to spawning grounds to reproduce. Some fish are beginning a journey from the offshore Atlantic waters to fresh water. These fish are known as anadromous, a word with Greek origins that means running uphill. And these fish literally swim upriver against the flow of water.

What is amazing about these fish is that they usually return to the same river where they were born. How they do this is a mystery. Many scientists believe that this homing instinct may be due to an uncanny sense of smell and sensitivity to magnetic signals, polarized light and unique characteristics of the natal stream.

Probably most famous for its spring runs is the American shad, a fish in the herring family. Rising spring temperatures prompt shad to leave the ocean and return to the waters in which they were born. Biologists believe the fish find their natal streams through their uncanny sense of smell. Males arrive on the spawning grounds first, followed by egg-laden females. A female releases 100,000 to 600,000 eggs into the water to be fertilized by several males. Adult shad return to the ocean soon after spawning.

The transparent fertilized eggs are carried along by the current. The larvae hatch in four to 12 days. Juvenile shad spend their first summer in fresh water. In autumn, the young shad gather in schools and swim to the ocean. They will live in the ocean three to six years—until they are sexually mature—then return to fresh water to complete their life cycle.

Other herring, like alewife and blueback, are also migrating to spawning grounds in the spring. The alewife is silver with a bronze-green back, while the blueback herring, also silver, has a bluish back. Their silvery scales scatter light to reduce the herring’s visibility to predators. Both species share a single dark shoulder spot and vary in length from 12 to 15 inches when fully grown.

The spawning season for alewife generally runs from March through April. Blueback spawn from mid-April through late May. Alewife favor slow-moving sections of streams while blueback herring prefer to spawn in swift water. These species, collectively called river herring, have also experienced a population decline in the Chesapeake Bay.

Upon reaching the spawning ground, the males circle a lone female. As this mass of fish swirls around, the female releases her eggs and the males release their sperm. After spawning, the adults swim downstream and return to the ocean.

A female herring can lay several hundred thousand eggs. The eggs, about 1 millimeter in diameter, are sticky and adhere to rocks, gravel and debris. They hatch in three to seven days. Flowing water carries the larvae downstream toward saltier water. Juvenile alewife and blueback herring migrate from the Bay to coastal waters in the early fall and remain there until reaching sexual maturity in three to six years.

As with many fish, overharvesting and the degradation or loss of habitat have taken their toll on these once-abundant fish. The Bay states have imposed a fishing moratorium on hickory shad and American shad and limited the season in which other herring species can be taken. But these measures alone won’t restore these fish.

Many historical spawning areas are not accessible. Fish are cut off by culverts, pipes, dams and other obstacles. More than 2,500 manmade blockages in the Chesapeake Bay watershed have greatly reduced fish spawning for both resident and migratory fish.

Dams and other blockages also affect a waterway by fragmenting the river, changing hydrological characteristics and trapping large amounts of sediment. These changes affect the waterway and landscape downstream of the blockage, shrinking channels, deactivating the floodplains and impairing water quality.

After a dam is removed, there is often an increase in both the abundance and diversity of aquatic insects and fish. But the most significant effect of dam removal is the immediate opening of upstream spawning habitat for fish.

To help migratory fish reach their spawning grounds, the states of Maryland, Pennsylvania and Virginia and the District of Columbia agreed to remove dams or provide fish passages. Since 1988, 1,838 miles of habitat in the Chesapeake Bay watershed have been reopened to both migratory and resident fish.

These partners have also agreed to complete 100 fish passage or dam removal projects, opening 1,000 miles of fish spawning habitat by 2014.

One of the most recent projects was the removal of a small dam on the Octoraro Creek in Cecil County, MD. This creek is a small tributary of the Susquehanna River.

In addition to the shad and river herring that use Octoraro Creek, another species, the American eel, will also be helped by this removal. Eels are catadromous, meaning they are born in the ocean and spend most of their life in streams and rivers.

Other upcoming dam removal projects include North Branch Potomac Rubble Dam, near Cumberland, MD, and Raven Rock Dam near Hagerstown, MD.

For information about dam removals and other fish passage projects, contact U.S. Fish and Wildlife Service biologist David Sutherland at 410-573-4535 or david_sutherland@fws.gov.