between 11° C. and 25° C, and the maximum in- 

 crease was 1.4 mm. per day at 32° C. and 1.1 mm. 

 per day at 25° C. Almost no growth occurred at 

 11° C. 



Ringo (1965), using the difference in size 

 between the largest shrimp in successive collec- 

 tions, calculated that in Galveston Bay, Tex., 

 young brown shrimp grew an average of 0.1 mm. 

 t.l. per day from March to early April ; as tempera- 

 ture rose the rate of growth increased to an aver- 

 age of 1.7 mm. per day and reached a maxiinum of 

 3.3 mm. per day during late May. This growth rate 

 is the higliest reported for any Penaeus from the 

 Gulf of Mexico and the western Atlantic. 



Loesch (1965) estimated that in Mobile Bay, 

 Ala., very young shrimp (20 mm. mean t.l. at 

 the beginning of the investigation) grew at an 

 average rate of as much as 50 mm. t.l. per month 

 in spring, and juveniles and subadults increased 24 

 to 43 mm. t.l. per month during summer and 12 to 

 35 mm. t.l. in winter. St. Amant et al. (1966) 

 studied the growth of P. a. azteciui in the Bara- 

 taria Bay, La., area during March, x'^pril, and May 

 from 1962 to 1965. Growth varied, the range of 

 variation was from no growth to 2.5 mm. per day. 

 They stated that "There appeared to be a trend, 

 though not completely evident in these data, for 

 the growth of brown shrimp to be less than 1.0 

 mm per day when the water temperature was below 

 20C and less than 1.5 mm per day when the water 

 temperature was below 25C. Little or no meas- 

 urable growth was noted at cumulative average 

 water temperatures below 16C." 



Klima and Benigno (1965), through a mark- 

 recapture experiment in Mississippi, estimated 

 that during a 4- week period in summer, nearshore, 

 male brown shrimp grew from 104 mm. in total 

 length to 132 mm., and females from 104 mm. to 

 135 mm. — monthly increases of about 28 mm. and 

 31 mm., respectively. Chin (1960), on the basis of 

 length-weight relation, assumed that in Galveston 

 Bay, Tex., the growth rate for the sexes is almost 

 identical. 



Williams (1955a), taking the upper extremes of 

 size distribution, estimated that in North Carolina 

 the mean length of juveniles increases at a rate of 

 45.7 mm. per month from April to June. This 

 growth is slightly less than that of P. d. d^Mrarum, 

 but still extremely rapid. Joyce (1965) estimated 

 a similar growth rate of 45 mm. per month for 

 brown shrimp in northeast Florida. 



SEX DIFFERENCES IN SIZE 



The largest female on record, 236 mm. t.l., was 

 reported by Renfro (unpublished data) from 

 south of Morgan City, La., in 60 fm. The largest 

 males, also rejDorted by Renfro, were 195 mm. 

 t.l. — one from off Galveston, Tex., in 60 fm. and 

 one from southeast of Morgan City, La., in 45 fm. 

 The survey of the northern Gulf of Mexico popula- 

 tion by Renfro and Brusher (1964) indicated that 

 size of shrimjD varies with depth and, to some ex- 

 tent, distance from shore. Regardless of time of 

 the year, the mean length of brown shrimp in- 

 creased with increasing depth. 



Brown shrimp exhibit a sex-size difference (fe- 

 males are larger than males) which becomes 

 evident upon attainment of a length of about 100 

 mm. (Williams, 1955a). Burkenroad (1939) at- 

 tributed the difference in size in offshore 

 individuals to a longer life among females. 



Ecology 



FOOD 



Brown shrimp are omnivorous, feeding upon 

 different plants and animals as well as organic 

 debris. (See also under Food in the discussion of 

 Penaeus setiferus.) 

 SUBSTRATE 



Juveniles of P. a. aztecus prefer muddy or peaty 

 bottoms, but they also live on sand, silt, or clay fre- 

 quently mixed with fragments of shells and rocks. 

 They usually abound in areas covered with vege- 

 tation and plant debris (Williams. 1959). The 

 adults prefer a substrate softer than the sympatric 

 P. d. duoranim (Hildebrand, 1954; Kutkuhn, 

 1962). They are abundant on mud or silt bottoms 

 and, occasionally, where the bottom consists of 

 mud, sand, and shell. 

 DIEL CYCLE 



Adult brown shrimp are mostly nocturnal and 

 are usually taken at night, but, according to 

 Springer and BuUis ( 1952) , "The difference in the 

 catch rate between day and night fishing is not so 

 well marked in deeper parts of the range." Juve- 

 niles are more active diurnally than adults and can 

 be highly active during the daytime — a fact clearly 

 established by the sampling carried on by Joyce 

 (1965), who took about 71 percent of his P. a. 

 aztecus juveniles by day. 



Information on the diel activity of postlarvae in 

 approaching inshore waters appeai-s to be some- 

 what conflicting. St. Amant et al. (1966) found no 

 discernible difference between nocturnal and diur- 



542 



U.S. FISH AND WILDLIFE SERVICE 



