no 



or drift towards the Florida mainland about 100 miles distant 

 (fig. IV.1.3S). 



Movement to the estuary probably takes from 3 to 5 weeks and, de- 

 spite the large numbers of postlarvae entering the estuary, only an 

 estimated five out of every hundred eggs produce shrimp that survive 

 to this stage. 



By the time the estuary is entered, the postlarvae have developed 

 from planktonic to benthic feeders and have developed a wide tolerance 

 to varying salinity and temperature conditions. From aibout 2 to 4 

 months the juvenile ehrimp grow rapidly from perhaps one-half inch 

 in length to commercial size before returning to the sea and completing 

 the life cycle. 



The life cycle of the three primary commercial species are similar 

 but the species differ in their penetration of the estuary and their util- 

 ization of the estuarine environment after the adult stage is attained. 

 The brown shrimp spawns in waters 150 to 230 feet in depth and re- 

 mains <a relatively short time in the estuary. The white shrimp rarely 

 is found in waters deeper than 100 feet and possesses a greater affinity 

 for fresh water than do the others. 



The estuary fulfills two primary functions: (1) Provision of ade- 

 quate nourishment during a period of rapid physical growth and (2) 

 protection from predators. A large proportion of the slirimp's diet ap- 

 pears to consists of small, invertebrate animals, such as worms, mollusk 

 larvae, and small crustaceans, as well as fish larvae and nematodes. 



Shrimp is a primary food item for various estuarine animals, includ- 

 ing red drum, spotted seatrout, snook, and gray snapper ; but the estu- 

 ary undoubtedly provides more vegetation and debris for protection 

 than open waters, and sufficient alternative foods exist in the estuaries 

 to move some of the pressure from the shrimp. 



MENHADEN 



Spawning occurs at sea along the continental shelf, and the eggs 

 natch in the ocean after about 2 days. Larvae move into the estuaries 

 as far as the freshwater interface. A transformation of physical char- 

 acteristics accompanies the entrance into the estuaries as larvae grow 

 and shift from bem^ selective, particulate feeders to being nonselective, 

 filter- feeding juvenile menhaden which can tolerate wide variations in 

 both salinity and temperature. 



The menhaden population of a particular estuarine system seems 

 10 be determined by the number of larvae entering the waters, food, 

 oxygen, competition, and predators. Because they are primary con- 

 sumers, feedmg directly upon the natural vegetation, menhaden rep- 

 resent the base of the food chain for many predators, such as the 

 bluefish, striped bass, and sharks. 



SALMON 



There are today only token runs of Atlantic salmon into a few rivers 

 in Maine to spawn, although in colonial times this species was ex- 

 tremely abundant from the Housatonic River to the St. Croix River. 



In the shallow estuarine areas of the Bay of Fundy and the coastal 

 bays and sounds of Maine they are frequently caught in herring weirs 



