Brown shrimp, continued 



Habitat 



Ty pe: Eggs occur offshore and are demersal. Larvae 

 occur offshore and begin to immigrate to estuaries as 

 postlarvae around 8 to 14 mm total length (TL) (Cook 

 and Lindner 1 970, Zein-Eldin pers. comm.). In estuar- 

 ies, postlarvae and small juveniles are associated with 

 shallow vegetated habitats, but are also found over 

 silty sand and non-vegetated mud bottoms. Juveniles 

 and subadults are found from secondary estuarine 

 channels out to the continental shelf, but prefer shallow 

 marsh areas and estuarine bays, showing a prefer- 

 ence for vegetated habitats. Adults occur in neritic Gulf 

 waters (Perez-Farfante 1969, Copeland and Bechtel 

 1974, Williams 1984, Minello et al. 1990, Zimmerman 

 etal. 1990). 



Substrate : Substrate suitable for burrowing activity 

 generally seems to be preferred (Minello et al. 1990). 

 Postlarvae and juveniles inhabit soft, muddy areas, 

 especially in association with plant-water interfaces. 

 Adults are associated with terrigenous silt, muddy 

 sand, and sandy substrates (Hildebrand 1 954, Ward et 

 al. 1980, Lassuy 1983, Williams 1984). 



Physical/Chemical Characteristics : 

 Temperature: Eggs will not hatch at temperatures 

 below 24°C (Cook and Lindner 1 970). Postlarvae have 

 been collected from temperatures of 12.6° to 30.6°C. 

 Aldrich et al. (1968) demonstrated postlarval burrow- 

 ing in temperatures below 18°C. Extended exposure 

 to temperatures below 20°C may be detrimental to 

 population survival (Zein-Eldin and Renaud 1986). 

 Brown shrimp greater than 75 mm tolerate tempera- 

 tures between 4° and 36°C, with a preferred range of 

 14.9° to 31.0°C (Ward et al. 1980, Copeland and 

 Bechtel 1974). Estuarine water temperature appears 

 to affect growth more than salinity does (Herke et al. 

 1987). Maximum growth, survival, and conversion 

 efficiency occurs at 26°C (Ward et al. 1980, Copeland 

 and Bechtel 1974). No growth occurs below 16°C and 

 growth is reduced above 32.2°C (Ward et al. 1980, 

 Lassuy 1983). 



Salinity: Brown shrimp are euryhaline to stenohaline 

 depending on life stage. Larvae tolerate salinities 

 ranging from 24.1 to 36%> (Cook and Murphy 1966). 

 Postlarvae have been collected from salinities of 0.1 to 

 69% , and have good growth at 2 to 40% o . Juvenile 

 brown shrimp are distributed over to 45%o, but have 

 been reported to prefer 10 to 20%o (Cook and Murphy 

 1966, Copeland and Bechtel 1974, Zimmerman et al. 

 1 990). Adults tolerate salinities of 0.8 to 45%o, but their 

 optimum range is 24 to 38.9%o (Cook and Murphy 

 1966). Salinity tolerance is significantly narrowed 

 below 20°C (Copeland and Bechtel 1 974). Salinity and 

 temperature effects are more conspicuous at either 

 extremes (Ward et al. 1980, Zein-Eldin and Renaud 



1 986). 



Dissolved Oxygen: In one field study, abundance lev- 

 els were lower in areas that had been altered by 

 development where dissolved oxygen content had 

 dropped below 3 ppm (Trent et al. 1976). Detailed 

 laboratory studies of brown shrimp oxygen consump- 

 tion and its interactions with temperature, salinity, and 

 body size are presented by Bishop et al. (1980). 



Turbidity: The effects of turbidity on shrimp distribution 

 and abundance are not well known (Kutkuhn 1966). 

 General observations indicate that turbid water areas 

 tend to have higher concentrations of young shrimp 

 than clear water areas. Water turbidity has also been 

 observed to strongly affect the brown shrimp's habitat 

 selection preference for structure in laboratory experi- 

 ments (Minello et al. 1990). Significant reductions in 

 abundance occurred in habitats with structure when 

 turbidity levels were high. 



Migrations and Movements : Brown shrimp postlarvae 

 (10-15 mm TL) move into estuaries from February to 

 April with the incoming tides and migrate to shallow and 

 often vegetated nursery areas (Copeland and Truitt 

 1 966, King 1 971 , Minello et al. 1 989b). In the northern 

 Gulf of Mexico, estuarine recruitment may occur all 

 year (Baxter and Renfro 1967). Rogers et al. (1993) 

 hypothesized that the estuarine recruitment is en- 

 hanced by downward migration of brown shrimp 

 postlarvae as northerly cold fronts force out estuarine 

 water, and upward migration into the tidal water column 

 as waters is forced back into the estuary. When 

 juveniles reach a size generally greater than 55-60 

 mm, they move out into open bays. The sub-adults 

 then migrate into the coastal waters (Minello et al. 

 1989b). Emigration to offshore spawning grounds 

 occurs from May through August, coinciding with full 

 moons and ebb tides (Copeland 1 965). Some tagging 

 studies in the northern Gulf indicate a west and south- 

 ward movement of the adults with the prevailing cur- 

 rents (Cook and Lindner 1970, Hollaway and Baxter 

 1981); but other studies do not indicate a net move- 

 ment in any direction when fishing effort is taken into 

 account (Sheridan et al. 1 989, Sheridan pers. comm.). 



Reproduction 



Mode : Brown shrimp reproduce sexually by external 

 fertilization in offshore Gulf of Mexico waters (Cook and 

 Lindner 1970, Lassuy 1983). This species has sepa- 

 rate male and female sexes (gonochoristic). 



Mating/Spawning : Mating probably occurs soon after 

 the female molts and before the exoskeleton hardens 

 (Cook and Lindner 1 970). A spermatophore is placed 

 inside the thelycum of the female by the male before 

 her eggs are spawned. Spawning occurs offshore 



57 



