to 40 p.p.t. ; at temperatures below 15° C, however, 

 their tolerance to low salinities (below 10 p.p.t.) 

 decreased, and near 35° C. survival was reduced 

 regardless of salinity. The authors suggested that 

 the combined effect of low temperature and low 

 salinity could be partly responsible for the time 

 of postlarval influx into estuarine waters because 

 they do not seem to enter estuaries until the tem- 

 perature has increased to a degree that low salini- 

 ties are not harmful. They further suggested that 

 temperature may have a decisive effect on sur- 

 vival of postlarvae in estuaries because if the tem- 

 perature becomes lower in an environment of inter- 

 mediate temperature (18° C.) and low salinity, the 

 survival rate will be adversely affected. 



Experiments by Aldrich et al. (1967) showed 

 that postlarval P. a. aztecus regularly burrowed 

 into a silty clay substrate as temperature fell to 

 12° to 17° C, and emerged as temperature rose, to 

 18° to 22° C. The postlarvae of brown shrimp, 

 thus, may "hibernate" in burrows for a portion of 

 the winter. 



Williams (1960) investigated the influence of 

 temperature on osmotic regulation in young brown 

 and pink shrimps. He found that the brown 

 shrimp have a less efficient osmoregulatory mech- 

 anism in low temperatures than do pink shrimp ; 

 at temperatures of 8.7° to 8.8° C. the brown 

 shrimp's ability to regulate is impaired and its 

 blood tends toward isotonicity. This difference may 

 explain, in part, why brown shrimp do not 

 winter in the estuaries of North Carolina whereas 

 pink .shrimp do. On the other hand, the bi'own 

 shrimp seems to have a greater tolerance to lower 

 temperatures than does the white shrimp. P. a. 

 azteciu'i ranges farther north ; intensive spawning 

 seems to begin in late winter or early spring in 

 the cold waters of North Carolina, at least 2 months 

 earlier than that of P. setiferus; and postlarval 

 movement into inshore waters begins earlier and 

 ends later than that of P. set) ferns. 



Temperature also has a pronounced effect on 

 growth. The studies by Zein-Eldin and Aldrich 

 ( 1965) indicated that postlarvae were able to grow 

 in a wide range of salinity, but that growth was ar- 

 rested at low temperatures. Postlarvae survived 

 but did not grow at 11° C. and 15 p.p.t. salinity, 

 whereas growth rate increased with rises in tem- 

 perature, the maximum occurred between 18° and 

 25° C. Experiments by Zein-Eldin and Griffith 

 (1965) indicated that temperature affects growth 

 of postlarvae of brown shrimp, as it does those of 



white shrimp, by hastening molts rather than by 

 inci'easing the increment per molt. Cook (1965) 

 determined that temperature also greatly affects 

 the growth of larvae; the optimum range was 

 28° to 30° C. (see also under Growth). 



Although the jDrecise effects of extreme tempera- 

 tures are largely unknown, shrimp exposed to 30° 

 C. and above become flaccid and perish rapidly 

 when handled. Zein-Eldin and Aldrich (1965) esti- 

 mated that the maximum tolerable temperature for 

 postlarvae is probably only slightly above 35° C. 

 The lowest temperature that they can tolerate is 

 not known, but Gunter and Hildebrand (1951) re- 

 ported a mass narcosis of the young at 4.4° C. and 

 below. 



EFFECTS OF SALINITY 



Field observations indicate that juvenile and 

 subadult brown shrimp, like pink shrimp and 

 white shrimp, are able to withstand a wide range of 

 salinity. Gunter and Hall (1963) reported that 

 small juveniles (28-38 mm. t.l.) were taken in St. 

 Lucie estuary, Fla., at salinities of 0.22 p.p.t. and 

 0.36 p.p.t.; Loesch {In Gunter et al., 1964), found 

 P. a. aztecm in Mobile Bay, Ala., at 0.0 to 1.00 

 i:).p.t., the lowest range of salinity at which this 

 subspecies has been reported. At the other extreme, 

 Simmons (1957) in Laguna Madre, Tex., collected 

 specimens in salinity of 69 p.p.t. Experiments by 

 Zein-Eldin (1963a) showed that at constant tem- 

 peratures of 24.5° to 26° C, postlarval P. a. aztecus 

 survived and grew over a range of salinities of 27 

 to 40 p.p.t. Later, Zein-Eldin and Aldrich (1965) 

 demonstrated that postlarval brown shrimp with- 

 stand a wide range of salinity-temperature com- 

 binations except at extreme temperatures (see 

 above). 



Although juveniles of P. a. aMecm tolerate a 

 wide range of salinity, their optimum seems to be 

 higher than that of P. setiferus. In North Carolina, 

 Williams (1955b) found only small populations 

 of brown shrimp juveniles on low-salinity nursery 

 grounds, where P. setiferus was most abundant. 

 Gunter et al. (1964) stated that in the bays of 

 Texas, young brown shrimp "are found in the 

 greatest abundance within the salinity range 10 to 

 30, and with considerably higher concentrations at 

 salinities above 20 than at salinities below 10. This 

 is in considerable contrast to the white shrimp in 

 the same bodies of water, which are found at the 

 greatest abundance below 10." Joyce (1965) con- 

 cluded that although in northeast Florida the nui-s- 



WESTERN ATLANTIC SHRIMPS OF GENUS PENAEUS 



5i5 



