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2. Anadromous and catadroTnous species 



Anadromous species pass through the estuarine zone on their journey 

 from the sea to the freshwater environment where they spawn. Some 

 species, such as the Pacific salmon, die after spawning and others, such 

 as the striped bass, live to return to the estuarine zone and the sea. 

 The young of all anadromous species spend varying periods of time 

 in the freshwater areas where they were spawned, but all eventually 

 migrate to the estuaries and then the sea. 



There are few truly catadromous species that mature in the fresh 

 or brackish water environments, and then migrate to higher salinity 

 waters of the estuary of the adjacent sea to spawn. The American eel 

 and the blue crab are examples of this type. 



3. Migratory estuarine species 



The great majority of estuarine dependent species fall under this 

 classification. Some use the brackish and freshwater areas of the 

 estuarine zone for reproduction; some as a source of food; some for 

 shelter, either as adults or young ; and some for all these reasons. They 

 all have in common the basic need for both estuarine and ocean en- 

 vironments at some point in their life cycle. This group includes the 

 great majority of fish and shellfish of direct importance to man, such 

 as shrimp, menlhaden, flounder, and red drum. 



Estuarine wildlife can be classified into four categories: (1) fur- 

 bearing animals, (2) game waterfowl, (3) ornamental shore birds, 

 and (4) the common wildlife that can tolerate human presence. 



The primary fur bearers are the fur seal in Alaska, nutria in the 

 South Atlantic and Gulf States, the common eastern muskrat in New 

 Jersey, the Virginia muskrat in the Central Atlantic States, and the 

 Louisiana muskrat in Alabama, Mississippi, Louisiana, and Texas. 

 Secondary in importance are the raccoon, mink, and otter. 



The dependence of waterfowl on the estuarine zone is both complex 

 and incompletely understood. The primary sport species, such as mal- 

 lards and canvasbacks, have been successfully adapted to manmade 

 changes in their environment, particularly those changes not affecting 

 the nesting sites. 



The ornamental shore and sea birds are a particular aesthetic attrac- 

 tion among the national fauna. These birds are generally more de- 

 pendent upon estuarine conditions than the more mobile waterfowl 

 and, in addition, have demonstrated a considerably greater sensitivity 

 to the overall encroachment of man. These birds include whooping 

 cranes, pelicans, bald eagles, egrets, ibis, and many others. 



GOVERNING SUBDIVISIONS OF THE BIOPHYSICAL ENVIRONMENT 



Solar energy and gravitational energy are the basis for everything 

 that happens naturally in the estuarine zone. This discussion of the 

 biophysical environment has been concerned primarily with the en- 

 vironmental conditions surromiding the transformation of these 

 energies into forms useful in living processes and exploitable by man. 

 Three different sets of subdivisions of the biophysical environment 

 were used in this discussion. 



Differences in the external environment divided the estuarine zone 

 of the United States naturally into ten geographic regions, each sub- 



