17 



mechanical energy input in this zone leads to a great variety of eco- 

 system typesj even within small geographic areas. 



The groupmg of ecosystems outlined here describes a limited range 

 of recurring variation of chemical and physical properties to which 

 certain forms of life have adapted and on which they are now 

 dependent. 



The basic environmental needs for all living plants and animals in 

 such zones are zones of salinity consistently fluctuating over a limited 

 range of concentration; solar energy; water temperature variation; 

 water quality and nutrients favorable to their propagation, growth, 

 and survival ; and, for some life forms, bottom conditions suitable to 

 their unique needs. 



The dependence of fish and shellfish on the estuarine zone is gov- 

 erned by particular environmental requirements for reproduction, pro- 

 tection, food supply, or a combination of these. Estuarine-dependent 

 species are of three types : 



1. Species restricted to estuaries 



Amon^ the relatively few species of fish and shellfish that coni- 

 plete their entire life cycle in the estuarine zone is the Atlantic 

 (American) oyster. It will die after long exposure to fresh water 

 although it can stand limited periods of such exposure and can 

 thrive in relatively high salinity water. The spotted sea trout 

 occupies the estuary for all its life purposes and only occasionally 

 leaves the estuary under unusual extremes of salinity and 

 temperature. 



2. Anadromous and catadromous 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 rea- 

 sons. They all have in common the basic need for both estuarine 

 and ocean environments at some point in their life cycle. This 

 group includes the great majority of fish and shellfish of direct 



