temperatures may also contribute to a low level of 

 swamp pond production. Young crayfish grow best at 

 temperatures of 24° to 27° C (75° to 80° F). 



Water levels may control crayfish distribution, 

 productivity, and harvest. If the amount of rainfall is 

 low from September through November, then the sea- 

 son of peak crayfish harvest the next spring will be 

 later than normal (LaCaze 1970). 



Crayfish are preyed upon by insects, fishes, am- 

 phibians, reptiles, birds, and mammals (LaCaze 1970, 

 White 1970. Gary 1974, H. R. and J. J. Hebrard unpub- 

 lished). They are also susceptible to a bacterial infec- 

 tion ("burned spot" disease). Bacteria invade abraded 

 areas of the shell and feed upon chitin, a component of 

 the shell. The early stages of the infection cause dark 

 discolorations of the exoskeleton. Advance stages of 

 bacterial infection weaken the crayfish (Amborski et 

 al. 1975). The effect is most apparent in older crayfish 

 that molt slowly. The rapid molt of young crayfish 

 prevents the formation of deep lesions. 



The bulk of the crayfish crop is collected east of 

 the Chenier Plain study area in the Atchafalaya Basin. 

 Sixty percent of the total commercial catch is from 

 natural habitats and the remaining 40^ is from pond 

 aquaculture. The remaining crayfishing area of signifi- 

 cance in the Texas coastal zone is the lower Trinity 

 River, which includes parts of Liberty and Chambers 

 counties (C. D. Studzenbaker, unpublished). 



5.6.4 BROWN SHRIMP (Penaeus aztecus) and 

 WHITE SHRIMP {Penaeus setiferus) 



Adult brown and white shrimp spawn offshore in 

 Gulf waters at different depths and peak times. Fertile 

 eggs hatch into planktonic larvae, which then develop 

 through a series of molts into postlarvae. The postlarvae 

 (8 to 14 mm or 0.3 to 0.5 in) are a transitional stage 

 and at this point normally enter the estuary (recruit- 

 ment). 



Postlarvae of the brown shrimp usually enter the 

 estuary between February and May (Copeland and 

 Truitt 1966, Ford and St. Amant 1971). Though re- 

 cruitment is greatest on incoming tides (St. Amant et 

 al. 1965, King 1971), it may not be a passive phenome- 

 non. An overwintering of postlarval brown shrimp in 

 the shallow Gulf has been postulated, with recruitment 

 correlated to the warming of estuarine waters (Comp- 

 ton 1965, Temple 1968. and King 1971). 



The initial seasonal distribution of postlarvae in 

 estuaries is believed to be governed by circulation pat- 

 terns and the intensity of wind-driven tides. During 

 months of peak recruitment, strong north winds fol- 

 lowed by strong south winds cause a flushing-filling 

 action in the estuary which transports larval shrimp to 

 the critical marsh-water interface. Here they adopt a 

 benthic existence, continue to feed, and grow into sub- 

 adults. 



Brown shrimp emigrate from Louisiana's estuaries 

 in two stages. The first consists of 60- to 70-mm (2.3- 

 to 2.7-in) shrimp that move from fringing marshes to 



open bays. This movement normally begins in May. 

 The open bays serve as a "staging area" for the second 

 offshore emigration (90- to 110-mm or 3.5- to 4.3-in 

 shrimp), which begins in late May and peaks in June 

 or July (Gaidry and White 1973). The spring and sum- 

 mer peaks in emigration are strongly correlated with 

 the tides of the full and new moon (Blackmon 1974). 

 Once juvenile brown shrimp begin to emigrate from the 

 open bays, they move steadily to the deep waters of 

 the Gulf (37 to 92 m or 120 to 300 ft), where they 

 mature and spawn. 



White shrimp follow a similar movement pattern. 

 The postlarvae enter the estuary with peak recruitment 

 from June to September (Copeland and Truitt 1966). 

 Jn September and October when the shrimp attain a 

 length of 145 to 160 mm (5.66 to 6.24 in), they begin 

 their emigration offshore (Gaidry and White 1973). 

 Cold fronts and rapidly cooling waters force the 

 youngest white shrimp to migrate offshore in October, 

 November, and December. By January, most shrimp 

 have left the estuaries. White shrimp remain in shallow 

 nearshore Gulf waters (0 to 27.5 m or to 90 ft), and 

 may reenter the estuaries periodically in the spring and 

 fall. 



The portion of the shrimp's life cycle spent in 

 estuaries represents a crucial phase. Environmental 

 conditions (e.g., temperature, salinity, protection from 

 predation, and adequate food supply) critically influ- 

 ence populations. 



Saline, brackish, and intermediate marshes should 

 be considered prime shrimp nursery grounds. The 

 marsh-water interface is an extremely important habi- 

 tat for juvenile shrimp (Chapman 1966, White and 

 Boudreaux 1977). Mock (1966) noted that more than 

 90% of the shrimp caught in shallow estuarine areas of 

 Galveston Bay, Texas, were near salt marsh habitats. 

 Primary reasons may be an abundance of detritus and 

 protection from predators (Trent 1967). 



When an estuary is altered by the construction of 

 bulkheads, dredge spoil disposal, etc., a reduction in 

 the carrying capacity of the estuary can be expected 

 (Mock 1966). In this regard, Williams (1958) observed 

 a preference of young brown and white shrimps for 

 soft mud or fibrous peat (natural substrate of nearshore 

 environment), and an avoidance of bare clay or shell 

 bars (types of environments associated with spoU dis- 

 posal). 



Offshore, brown shrimp are found at depths down 

 to 108 m (360 ft), with adults being most abundant at 

 27 to 55 m (90 to 180 ft). White shrimp are found pri- 

 marily at depths less than 90 m (300 ft). 



Adults of both species prefei mud and silt bottoms 

 and are found, to a lesser extent, on mud and shell, or 

 mud and sand substrate (Christmas and Etzold 1977b). 



Larval shrimp in the Gulf feed on plankton and 

 suspended detrital material (Christmas and Etzold 

 1977b). During the estuarine phase of their life cycle, 

 juvenile shrimp are opportunistic omnivores. They feed 

 mainly at the marsh-water interface on a variety of 



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