Blue crab, continued 



tion of the blue crab vary with the life stage and sex of 

 the individual (Killam et al. 1 992). The eggs of the blue 

 crab are the most sensitive to change in environmental 

 conditions such as temperature and salinity, while 

 juveniles and adults have greater tolerances to 

 flucutations. Juveniles and adults are also more mo- 

 bile, and can avoid degraded areas if possible. 



Temperature - Eggs: Eggs have been successfully 

 hatched under laboratory conditions in temperatures 

 ranging from 19° to 29°C (Sandoz and Rogers 1944). 



Temperature - Larvae: Megalopal survival is highest at 

 temperatures between 21.5° and 34.5°C, but larval 

 development is fastest between 24° to 31 °C (Costlow 

 1967, Copeland and Bechtel 1974). 



Temperature - Juveniles and Adults: Blue crabs have 

 been collected at temperatures from 3° to 35°C 

 (Copeland and Bechtel 1 974). Adults cease feeding at 

 temperatures below 1 0.8°C, and burrow in mud at 5°C. 

 Mortalities of blue crabs have been related to extreme 

 cold and sudden drops in water temperature (Van 

 Engel 1982, Couch and Martin 1982). Tagatz (1969) 

 evaluated maximum and minimum median thermal 

 tolerance limits (48 hours) of juvenile and adult blue 

 crab from St. Johns River, Florida, and found them to 

 be 3°C and 37°C. However, thermal limits are highly 

 dependent on acclimation temperature and salinity. 

 Adult males are more tolerant of temperature extremes 

 than females and juveniles. Temperature apparently 

 plays a key role in molting (Copeland and Bechtel 

 1974). 



Salinity: This species is euryhaline and has been found 

 from freshwater to hypersaline lagoons (0-50% o ). Up- 

 per and lower lethal limits (LC-50s) determined for two 

 different Gulf of Mexico populations were 56%o and 

 67%o for the upper limits, and 0%o and 1 %o for the lower 

 limits (Guerin and Stickle 1990). 



Salinity - Eggs: Eggs have been observed to hatch 

 under laboratory conditions in salinities ranging from 

 1 0.3 to 32.6%o, but the optimum salinities ranged from 

 23%o to 28%o (Sandoz and Rogers 1944). 



Salinity - Larvae: Early zoeae are found at high 

 salinities, usually 20%o or greater (Dittel and Epifanio 

 1 982). Megalopae may be transported to lower salini- 

 ties, and have been found in waters as low as 5%o 

 (Costlow 1 967, Benson 1 982). Highest survival occurs 

 between 1 6 and 43%o, but larval development is fastest 

 from 11.5 to 35.5%o at 24° to 31 °C (Costlow 1967, 

 Copeland and Bechtel 1974). 



Salinity - Juveniles: Juvenile crabs are found in lower 

 salinity waters, typically 2-21 %o. Reported salinity 



values for juveniles vary, and specific salinities are not 

 critical to postlarval crabs. 



Salinity - Adults: Adult males are usually found at less 

 than 1 0%o. Egg-bearing females (sponge) are found in 

 23-33%o and 19-29°C waters (Millikin and Williams 

 1 984, Van Den Avyle and Fowler 1 984, Williams 1 984). 

 The interaction of salinity and temperature reveals the 

 blue crab to be less tolerant of low salinities at high 

 temperatures and high salinities at low temperatures 

 (McKenzie 1970). 



Dissolved Oxygen (DO): The blue crab is very sensi- 

 tive to low DO conditions. Survival times of 2 hours at 

 parts per million (ppm) DO (32°C and 15%o salinity) 

 and 4.3 hours at ppm DO (25°C and 15%o salinity) 

 were reported by Lowery and Tate (1986). The occur- 

 rence of dead crabs in traps is fairly common during 

 warmwaterconditions. The fishermen usually remedy 

 the problem by moving their traps into shallower water 

 to avoid any low DO water layers. Often the presence 

 or boundary of a low DO water mass can be inferred by 

 the placement of crab traps in any given area. Mass 

 mortalities have been reported to be associated with 

 low DO conditions (May 1973). 



Migration and Movements : Migrations within estuarine 

 systems are related to phases of life cycle, season, 

 and, to a lesser extent, the search for favorable envi- 

 ronmental conditions. Most crabs move to relatively 

 deeper, warmer waters during winter, but some juve- 

 niles will burrow in shallow water substrate for protec- 

 tion. Blue crab return to rivers, tidal creeks, salt 

 marshes and sounds when conditions become more 

 favorable. They also move out of waters with low DO 

 levels, and in some cases will actually leave the water 

 to escape anoxic conditions (Lowery 1 987, Killam et al. 

 1992). In Mobile Bay, large masses of migrating blue 

 crabs and other animals occasionally occur while at- 

 tempting to avoid low DO conditions, and such events 

 are referred to as "jubilees" (Lowery pers. comm.). 

 Blue crabs are recruited to Gulf estuaries as megalopae, 

 with molt to the first crab stage occurring in nearshore 

 waters (Thomas et al. 1990, Perry et al. 1995). 

 Oesterling and Evink (1977) proposed a larval dis- 

 persal mechanism for the northeastern Gulf in which 

 larvae could be transported 300 km or more. If such 

 mechanisms do exist, larvae produced by spawning 

 females in one estuary could be responsible for recruit- 

 ment in others. In the Gulf of Mexico, immature 

 females approaching their final molt during the spring, 

 move to lower salinities to mate, and then, typically, 

 migrate backtohighersalinity waters within theestuary 

 during June and July (Adkins 1972b, Millikin and Will- 

 iams 1984). In Florida, females may leave estuaries 

 after mating and move along the coast to specific 

 spawning areas near Apalachicola Bay (Oesterling 



100 



