organic matter, including algae mats. Jones (1973) 

 found that 25- to 44-mm (1- to 1.7-in) shrimp ran- 

 domly ingest nearshore surface sediments and detritus 

 which is composed of decaying marsh plant vegetation 

 and animal feces. The detritus and sediment contain an 

 organically rich community of microorganisms that are 

 digested by juvenile shrimp. 



As the shrimp grow larger (45 to 64 mm or 1 .8 to 

 2.5 in), predation on benthic animals such as amphi- 

 pods and polychaetes becomes important, though the 

 shrimp continue to ingest detritus. The partial shift in 

 diet is associated with movement from the nearshore 

 environment to the deeper waters of the estuary (Jones 

 1973). 



Shrimp spawn in the Gulf of Mexico. Adult brown 

 shrimp (pver 135 mm or 5.3 in) spawn at depths of 46 

 to 110 m (150 to 360 ft), with a major peak from 

 September to December, and a minor peak from March 

 to May (Kutkuhn 1962, Renfro and Brusher 1963, 

 Temple and Fisher 1968, Cook and Lindner 1970). 



Adult white shrimp over 140 mm (5.5 in) spawn in 

 shallower water (8 to 31 m or 27 to 102 ft) than brown 

 shrimp. They exhibit a June peak in the April to August 

 spawning period (Lindner and Anderson 1956, Renfro 

 and Brusher 1963, Temple and Fisher 1968, Bryan and 

 Cody 1975). 



Factors that may threaten the shrimp resource 

 include the alteration of freshwater inflow into estuarine 

 water circulation patterns, temperature, and salinity 

 regimes, as well as reductions in the supply of marsh 

 plant detritus. Thus, marsh deterioration, land loss, 

 bulkheading, channelization, dredge spoil disposal, 

 leveeing, and modification of river discharge patterns 

 are all concerns of the shrimp industry and the renew- 

 able resource manager. 



The time and intensity of spring warming of the 

 estuaries is important in the initial growth and survival 

 of brown shrimp. Little or no growth of juvenile brown 

 shrimp occurs below 20° C (68° F). When temperature 

 exceeds this value, growth rates from 1 to 2 mm (0.04 

 to 0.08 in) per day are expected (St. Amant et al. 

 1965). 



Summer growth of juvenile white shrimp does not 

 appear to be temperature-limited, and proceeds at a 

 rate comparable to juvenile browns. However, during 

 the fall, rapidly decreasing temperatures associated 

 with passing cold fronts reduce growth rates. 



Perret et al.( 1971) reported that densities of brown 

 shrimp in estuaries are more related to temperature 

 than salinity. The average density was normally low at 

 temperatures less than 20° C (68° F). This supports the 

 observation of the Louisiana Department of Wildlife 

 and Fisheries (LDWF) that distribution of brown 

 shrimp is largely limited to salinities of 15%o or greater 

 when temperatures are below 20° C (68° F). When 

 water temperatures remain above 20° C (68° F), salinity 

 does not appear to limit the distribution of brown 

 shrimp. 



The density distribution of white shrimp, however, 

 is not correlated with temperature above 10°C (50° F) 

 (Perret et al. 1971). This lack of pattern is consistent 

 with the observation that catch of white shrimp is less 

 predictable than brown shrimp, and that white shrimp 

 have a greater tolerance than brown for salinities less 

 than 10%r. 



Gunter et al. (1964) found that salinity optima 

 vary from 5 to 20%o for young shrimp of commercial 

 varieties found in estuaries along the Gulf coast. 



A prime example of man's effect on shrimp pro- 

 duction occurred in Sabine Lake. Winter discharges 

 from the Toledo Bend Dam were retained in Sabine 

 Lake until mid-May at which time the water was 

 released. Instead of the natural occurrence of increasing 

 salinities in estuaries during spring and summer, a nearly 

 freshwater condition was created during late May and 

 continued throughout the summer. This was devastating 

 to the brown and white shrimp populations (Wliite and 

 Perret 1973). 



5.6.5 BLUE CRAB (Callinectes sapidus) 



In summer the adult female blue crab migrates 

 inland to mate in brackish water (less than 257co)- The 

 mating process usually lasts about two days (Leary 

 1967). After mating, the female moves back to higlier 

 salinity areas to spawn. 



How far offshore the female spawns is unclear but 

 it may be in shallow oceanic water or even in bays if 

 the salinity is high enough. Burke and Associates, Baton 

 Rouge (unpublished data), saw "berry" crabs taken by 

 dip net along the beaches of Grand Isle in the summer, 

 indicating that spawning takes place nearshore. Nichols 

 and Keney (1963) found the greatest numbers of larval 

 blue crabs 32 km (20 mi) from shore, which indicates 

 that spawning and subsequent hatching may also occur 

 offshore. 



Adult male and female crabs exliibit different 

 salinity preference. Adkins (1972) found large females 

 (120 mm or 4.8 in width) in deep water (salinity greater 

 than \1.2%c) on hard bottoms. Smaller crabs (60 to 80 

 mm or 2.3 to 3.1 in width), primarily female, were 

 found on soft bottoms in shallow water. Juveniles and 

 adult males prefer brackish water. 



Generally, blue crabs feed on whatever is available. 

 Gut analyses have shown some specific food items such 

 as rangia mussel, snails, fishes, plants, insect larvae, 

 amphipods, shrimp, barnacles, xanthid crabs such as 

 fiddlers, other blue crabs, and even human flesh 

 (Adkins 1972, Dugas unpublished manuscript). 



Low salinity is an important requirement for the 

 reproduction of blue crab. The female crab must leave 

 its usual habitat with high salinity and move inland to 

 areas of lower salinity (less than 257cr) to mate. The 

 sperm deposited during mating will serve to fertilize 

 eggs of the female for its lifetime (about 2 yr). The 

 female mates only once, while the male may mate sev- 

 eral times. The male seldom leaves areas of low salinity. 



264 



