Anadromous or Catadromous?When we think of Maine's marine environments, we tend to divide them into two categories: either fresh water or salt water. Actually, fresh and salt water are interrelated in many ways. The rain that falls into our streams and lakes or runs off the land may have started out as water that evaporated from the ocean. The rivers and streams that flow into the ocean help to regulate its salinity, and the estuaries where the two waters meet support animal life like oysters that can live comfortably only in brackish water. One of the best ways to understand the connection between fresh and salt water is to examine diadromous fishes, those who live the major part of their lives in one type of water, be it fresh or salt, yet must migrate into the opposite water type in order to spawn.
While at sea, salmon eat shrimp, crabs and other marine invertebrates, storing energy for the long migration and period of fasting while in the rivers. While most of the salmon die soon after spawning, as many as 15% return downstream to the ocean. Formerly plentiful throughout the northeastern United States, salmon experienced a sharp decline in overall population due to the destruction of habitats and pollution in the streams where the salmon bred. While they still have not returned to all previously known salmon rivers, remarkable headway has been made by the release of young salmon into reclaimed rivers and streams through hatchery programs. The building of "fishways" that help the fish pass where dams once blocked their upstream journey has aided their comeback in the New England states. It is too soon to tell if this beautiful fish can reestablish itself with viable breeding populations, since it takes four or more years for their return from the sea. Another interesting, though not often sought after, fish is the American eel (Anguilla rostrata). Eels are scavengers and eat virtually anything dead or alive. For many years scientists were baffled as to their breeding habits. It was known that females lived for the most part in freshwater and males in brackish or estuarine waters. It was surmised that they bred in the ocean but little more was known. In the early 1950's, scientists determined that a fish known as "leptocephalus" was not a new species at all but a larval stage of the eel. They then traced the eels' spawning grounds by finding smaller and smaller larvae until they reached an area called the Sargasso Sea. Evidence indicated that mature American eels (between the ages of five and twenty years) make the pilgrimage to the Sargasso Sea, spawn and die. The fact that a spent eel (an eel that has spawned) has never been found supports the theory that the Sargasso Sea is the last stop in the life of the American eel. After the eggs hatch, the clear leptocephalus drift with the Gulf Stream toward shore where eventually they change into elvers or "glass eels" and begin to swim into estuaries or further into lakes and streams depending upon their sex. The eel is termed catadromous because of its journey from freshwater to the depths of the ocean to spawn. The eel's life is the antithesis of the life of the salmon. Both fish are dependent upon both ocean and freshwater in order to survive. Because of their intricate involvement with both environs, they are particularly vulnerable to habitat alterations and pollution. While few species are as tied to both water sources as these two fishes, they serve as an excellent reminder that the Gulf of Maine is not independent from fresh water lakes. rivers and streams. References:
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The Atlantic salmon is a popular sport and food fish indigenous to Maine. Both its common name
and its scientific name, (Salmo salar) come from the Latin term "salio" meaning "to leap". Anyone
who has seen this fish try to ford a dam or struggle upstream understands how it earned its name.
The Atlantic salmon is anadromous in that it lives its life in the ocean, but breeds in fresh water.
Several other fishes spend their lives in a similar manner including the alewife, Atlantic smelt, and
striped bass. At various times of the year, depending upon the species, these fish leave the relative
safety of the open ocean and begin a perilous journey in search of the stream where they were
hatched several years earlier. In the case of the salmon this homing instinct is so acute that it returns
to the proper site after four or five years of living at sea.