nal catches of postlarv-ae moving into the greater 

 Barataria Bay, La., area, whereas Copeland and 

 Truitt (1966) captured most of the postlarvae at 

 night at Aransas Pass, Tex., inlet, and Baxter 

 and Furr (1964) estimated that 70 percent of the 

 postlan-ae in their samples at the entrance of Gal- 

 veston Bay, Tex., were caught between 9 p.m. and 

 6 a.m. Copeland and Truitt (1966) also reported 

 that during the day no significant difference could 

 be detected between surface and bottom samples 

 and suggested that "Perhaps, penaeid postlarvae 

 actively pursue the upper layers (even during the 

 day) to take advantage of faster currents on the 

 surface." Rearing experiments by Cook (1965) in- 

 dicated that larvae are positively jjliototropic, for 

 when reared under an overhead light they swim to 

 the surface. 



The effects of phases of the moon on the be- 

 havior of brown shrimp have not been well estab- 

 lished. Hildebrand (1954) stated that some 

 shrimpers maintain that fishing for brown shrimp 

 is most successful during the full moon, but that 

 others believe that catches do not change sub- 

 stantially at any moon phase. Copeland (1965) 

 found that peak seaward migrations of brown 

 shrimp through Aransas Pass, Tex., inlet occurred 

 at times of full moon. 



MOVEMENTS 



The larvae of P. a. aztefnis move from the high- 

 salinity waters of the spawning grounds toward 

 brackish waters where they arrive as postlarvae. 



It was generally believed that larvae and post- 

 larvae moved toward estuaries and that the latter 

 entered them directly. Temple and Fischer 

 (1967), however, have gathered evidence that in 

 the northwestern Gulf of Mexico "brown shrimp 

 larvae and postlarvae, or both, overwinter in wa- 

 ters of the Continental Shelf." 



Postlarvae enter the nursery grounds at 8 to 14 

 mm. t.l. (Baxter and Renfro, 1967). They move 

 into the estuaries on flood tides (St. Amant et al., 

 1966; Copeland and Tniitt, 1966). Major post- 

 larval brown shrimp influx into the nursery areas 

 occurs earlier — in late winter or early spring — 

 than do those of the sympatric Pen-aeus. Studies 

 by Baxter and Renfro (1967) showed that post- 

 larvae were scarce in the entrance of Galveston 

 Bay and in Galveston Island Beach, Tex., through- 

 out most of the winter. Their number increased 

 rapidly and reached a peak between mid-March 

 and mid-April. After the spring peak few post- 

 larvae were caught until mid-June when their 



number increased again and reached a second peak 

 in August and September. Copeland and Truitt 

 (1966) in Aransas Pass, Tex., inlet observed two 

 similar peaks of postlarval abimdance. St. Amant 

 et al. (1966) reported that in Barataria Bay, La., 

 peak catches of postlarvae occurred in April in 

 1962 and 1963, in February in 1964, and in March 

 in 1965. Christmas et al. (1966) first took post- 

 larvae of brown shrimp in Louisiana in Febru- 

 ary. They observed that the period of inshore 

 movement lasted through September and that few 

 individuals were present during October and 

 November. 



Studies by Villalobos et al. (1967) in Laguna de 

 Alvarado, Veracruz, Mexico, showed that post- 

 larval stages of P. a. aztecnis were abundant in 

 March (dry season) ; in contrast, none were found 

 during August (rainy season) . In South Carolina, 

 Bearden (1961) found the greatest abundance of 

 postlarvae in February and March. Williams 

 (1955a, 1965) collected postlarvae in the sounds 

 of North Carolina from October to May and re- 

 ported the peak of abundance from late March to 

 early April. 



The postlarvae molt to the juvenile stage 4 to 6 

 weeks after they arrive in estuaries (Parker, 

 1966). A number of investigators have studied the 

 occurrence of juveniles and subadults in the nurs- 

 ery grounds of tlie United States adjacent to the 

 Gulf of Mexico (Gunter, 1950; Ingle, 1957; Chin, 

 1960; St. Amant et al., 1966; Trent, 1967). Brown 

 shrimp juveniles begin to appear in small num- 

 bers in the estuarine waters of Texas and Louisi- 

 ana late in March or early in April. Their num- 

 bers increase to a maximum in May and June. 

 They are abundant in July and August, become 

 increasingly scarce from September to the end of 

 October, and are almost absent during late fall 

 and winter. Seaward migration begins and 

 rapidly intensifies in May or June through August. 

 Tabb, Dubrow, and Jones (1962) found that P. a. 

 aefecus is extremely rare in the estuaries adjacent 

 to Florida Bay, because only a few yoimg speci- 

 mens wei'e taken during their extensive sampling 

 from April through August. 



Several investigations have also been conducted 

 on the seasonal abundance of juveniles in the nurs- 

 ery grounds of the eastern United States. In St. 

 Lucie estuary, eastern Florida, Gunter and Hall 

 (1963) found the smallest specimens of brown 

 shrimp in January, February, May, and October 

 and suggested that summer and fall were the 



WESTERN ATLANTIC SHRIMPS OF GENUS PENAEUS 



543 



