109 



matter of photosynthesis, are nowhere near as productive as the 

 oyster reefs and marshlands of the temperate zone^ where particulate 

 organic foods as well as solar energy are converted mto plant and ani- 

 mal tissue for use by animals higher in the food chain. 



DEPENDENCE OF FISH AND SHELLFISH ON THE ESTUARINE ZONE 



DependencjT is governed by particular environmental requirements 

 for reproduction, protection, 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 complete 

 their entire life cycle in the estuarine zone is the Atlantic oyster. It 

 will die after long exposure to freshwater although it can stand lim- 

 ited periods of such exposure and can thrive in relatively high salinity 

 water. Tlie spotted sea trout occupies the estuary for all its life pur- 

 poses 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 jour- 

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

 Some species, such as the Pacific salmon, die after spawing 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 esimarine species 



The great majority of estuarine dependent species fall under this 

 classification. Some use the brackish and freshwater areas of the estu- 

 arine zone for reproduction ; some as a source of food ; some for shel- 

 ter, either as adults or young; and some for all these reasons. 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 importance to man, 

 such as shrimp, menhaden, flounders, and red drum. 



Various types of dependency are illustrated by several examples. 



SHRIMP 



The commercially important shrimp are of three kinds: brown, 

 white, and pink. These species are concentrated along the South Atlan- 

 tic and Gulf coast of the United States. The pink shrimp spawns in 

 offshore waters at depths of 100 to 150 feet, salinity between 3.61 and 

 3.77 percent, and temperatures between 64° and 77° F. After 13 or 14 

 hours, the eggs hatch and the larval shrimp begin to pass through 

 a series of developmenta;l stages, at the same time beginning to move 



