JAN.- APR. 



a: 



o 



ti 'Oh 



3 4 5 6 7 8 9 10 II 12 13 14 



AUG.- DEC. 



Q- 30 - 



3 4 5 6 7 8 9 10 II 12 13 14 

 TOTAL LENGTH (MM.) 



Figure 10. — Length-frequency distributions of post- 

 larval brown shrimp (P. aztecus) taken in plankton 

 hauls, 1961. 



March. . ." Renfro and Brusher (see footnote 

 4), however, reported that, although brown 

 shrimp may spawn continually throughout the 

 year, major periods of spawning activity are in 

 April to June and September to November. 

 Furthermore, in the absence of large catches of 

 larval stages during January to March 1961, 

 the possibility arises that this spring group of 

 postlarvae originated from a large spawning 

 in the fall of 1960. If this is true, young brown 

 shrimp must remain offshore either as larvae or 

 postlarvae for a longer period than was previ- 

 ously suspected. 



If young brown shrimp do overwinter off- 

 shore, the developmental rate of these larvae or 



- "A behavioral < oinparison of postlavval Fenaeuti (tztccu^i and 

 I'- fietifcrus, witli regard to burrowing as a response to reduced 

 temperatures. " by David "\'. Aldrich, Carl E. Wood, and Kennetli 

 X. Baxter, Bureau of Commercial Fisheries Biological Laboratory, 

 Galveston, Tex., 3 figs. 



the growth rate of the postlarvae, or both, must 

 be slower than has been previously reported 

 for white shrimp. Cook (1966 b), while rearing 

 larval brown shrimp, observed retarded devel- 

 opmental rates at temperatures lower than 30- 

 C. Zein-Eldin and Aldrich (1965) reported 

 that postlarvae held in the laboratory under 

 controlled temperature had a maximum growth 

 rate of about 1.4 mm. per day at 32° C. and 

 1.1 mm. per day at 25° C. They also found that 

 growth of postlarvae held over a 30-day period 

 at 11° C. was practically nil, but that survival 

 was high. 



Additional support for the hypothesis that 

 brown shrimp may overwinter before entering 

 the nursery areas was provided by Aldrich, 

 Wood, and Baxter." They found that in the 

 laboratory postlarval brown shrimp would bur- 

 row in response to experimentally reduced tem- 

 peratui'es. This response usually occurred be- 

 tween 12^ to 17° C. It appears then that under 

 certain temperatures postlarval brown shrimp 

 may burrow and grow at a slow rate. 



Results of laboratory experiments on larval 

 development rates, postlarval growth, and post- 

 larval burrowing characteristics are of particu- 

 lar significance because temperatures similar to 

 those tested occur in the waters over the Con- 

 tinental Shelf of the northwestern Gulf of 

 Mexico (Harrington, 1965). Of even greater 

 significance is the fact that these temperatures 

 occur between the fall peak of larval abun- 

 dance (apparently associated with brown 

 shrimp spawning) and the peak occurrence of 

 postlarval brown shrimp in Galveston En- 

 trance. Additional field and laboratory work 

 is required, however, to substantiate the hy- 

 pothesis of overwintering brown shrimp larvae 

 or postlarvae, or both, in offshore waters, 



LITERATURE CITED 



Ahlstrom, Elbert H. 



1948. A record of pilchard eggs and larvae col- 

 lected during surveys made in 1939 to 1941. Pish 

 Wildl. Serv., Spec. Sci. Rep. 54, i-t-76 pp. 



Anderson, William W., Joseph E. King, and Milton 

 J. Lindner. 



1949. Early stages in the life history of the com- 

 mon marine shrimp, Penaeus setiferiis (Lin- 

 naeus). Biol. Bull. 96(2) : 168-172. 



U.S. FISH AND WILDLIFE SERVICE 



333 



