636 



Fishery Bulletin 92(3), 1994 



were significantly more abundant in samples taken 

 during flood tides than during ebb tides (P<0.001; 

 Fig. 3). Significantly more P. aztecus were collected 

 at night on flood tides ( 3c=228.7/l,000m 3 ) than dur- 

 ing daylight flood tides ( 3c = 147. 4/1, 000m 3 ; P<0.001), 

 whereas no significant difference between catches 

 made during day ( 3c=877.9/l,000m 3 ) versus night 

 (3c =610. 2/1, 000m 3 ) were noted for P. setiferus 

 (P=0.114; Fig. 3). No significant differences in catches 



were noted for either species when surface and bot- 

 tom collections were compared (P=0.595 for P. 

 aztecus; P=0.270 for P. setiferus; Fig. 3). 



Significant differences among catches made by 

 regular sampling grouped by lunar phase ( new moon, 

 first quarter, full moon, last quarter) were detected 

 for both P. aztecus (P<0.0001) and P. setiferus 

 (P<0.006). Fewest postlarvae of both species were 

 collected during full moon phases; P. aztecus were 

 more abundant during the last quarter than at other 

 times (Fig. 4), whereas P. setiferus was most abun- 

 dant during the first quarter. 



Results of correlation analysis revealed a low cor- 

 relation (r 2 =0.03; P=0.523) between annual indices 

 of abundance of postlarval P. aztecus and subsequent 

 landings. A much higher correlation was obtained 

 for P. setiferus (r 2 =0.79;P<0.001; Fig. 5). The regres- 

 sion equation for estimated number of P. setiferus 

 landed was Y= 40.4 + 119.0X(y=estimated number 

 landed, X=annual index of postlarval abundance). 



Discussion 



Our results are generally similar to observations on 

 Penaeus postlarvae made by others in the southeast- 

 ern United States (including the Gulf of Mexico). 

 Penaeus aztecus is most abundant in springtime at 

 water temperatures comparable to the range of tem- 

 peratures observed in this study (Bearden, 1961; 

 Williams and Deubler, 1968; Allen et al., 1980). It 

 has been postulated that the majority of brown 

 shrimp postlarvae recruited in the spring are pro- 

 duced by spawning from the previous fall (Temple 

 and Fischer, 1967; Aldrich et al., 1968; Anderson, 

 1970; Whitaker 3 ). Ingress begins after nearshore 

 water temperatures approach 12°C. In contrast, 

 Penaeus setiferus postlarvae are recruited in spring 

 and summer shortly after being spawned in adjacent 

 oceanic waters (Pearson, 1939; Lindner and Ander- 

 son, 1956). When compared to overall catches of P 

 setiferus, P. aztecus have often been collected in 

 greater abundance in South Carolina (Bearden, 1961; 

 Lunz, 1965; Olmi, 1986; Wenner and Beatty, in press) 

 and in Texas (Baxter and Renfro, 1967). 



Prior studies have shown thatP aztecus postlarvae 

 are often more abundant in nighttime collections and 

 on high (flood) tides (Caillouet et al., 1970; Duronslet 

 et al., 1972), similar to the pattern observed in this 

 study. This is probably due to both increased noctur- 

 nal activity and decreased gear avoidance at night 



3 Whitaker, J. D. 1982. A possible mechanism for brown shrimp 

 postlarval recruitment. Paper presented at South Carolina Fish- 

 eries Workers Assoc, annu. meet., Clemson, SC, 24-25 Feb. 1982. 

 Unpubl. manuscr., 3 p. 



