24 



Fishery Bulletin 



1990 



Q. 



E 



n 



E 



z 



> 

 o 



z 



UJ 



Z) 



o 



111 



COHORT 1 



60 

 40 



08. 06.82 

 n=267 



COHORT 2 COHORT 1 



8° ^ 01.18.83 



V \ n=117 



I1.I1III1IIIIII1I.I,. 



5 10 15 20 25 30 35 



09.18.82 

 n=84 



...l..,l,.hllll.i,i. 



5 10 IS 20 25 30 35 



80 

 60 

 40 

 20 



5 10 15 20 25 30 35 



11.30.82 

 n=197 



.lilli.ll 



5 10 15 20 25 30 35 



60 

 40 

 20 



 



\ 



\ 



.\ 



\ 



.ll.....l.lllMI<....„... 



5 10 15 20 25 30 35 

 \ 

 \ 



5 10 15 20 25 30 35 

 1 



04.12.83 

 n=113 



5 10 15 20 25 30 35 



COHORT 3 



80 

 60 

 40 

 20 







\ 



07.06.83 

 n = 339 



I 



ill 



5 10 15 20 25 30 35 



COHORT 3 COHORT 2 



80 \ I 



80 

 60 

 40 

 20 

 



80 

 60 

 40 

 20 

 



80 

 60 

 40 

 20 

 O 



08.24.83 

 n = 233 



ll.lll.l.l.ll.,... 



5 10 15 20 25 30 35 



mill.... 



5 10 IS 20 25 30 35 



\ 1 



\ \ 



\ 1 



, , 12 04 83 



I 1 n=258 



\ I 



\ 1 



\ 1 



\ 



..mini 



llllllll.. 



5 10 15 20 25 30 35 

 \ 

 \ 



\ 



1 01.19.84 



' n=248 



.lllllllllllllli 



5 10 15 20 25 30 35 



CARAPACE LENGTH (mm) 



Figure 3 



Length-frequency histograms for three cohorts ofPenaeus indtcus in the St. Lucia system between 6 August 

 1982 and 19 January 1984. Dashed Mnes indicate positions of fitted cohort means. 



three cohorts overwintered in the estuary and emi- 

 grated during December-March (summer-autumn). 



PenaeuH ivdicu^i began to leave the system upon 

 attaining approximately 18 mm CL with most emigra- 

 tion occurring between 20 and 25 mm CL (Fig. 6). 

 Declines in the proportions of 20-25 mm CL shrimp 

 in both the beam trawl (Fig. 3) and bait fishery samples 

 (Fig. 7) supported the December-May (summer- 

 autumn) emigration hypothesis. Emigrating size 

 classes declined sharply in the baitfishery samples over 

 April-May 1983 and January- April 1985 (Fig. 7). 



Emigration did not appear to be related to salinity 

 fluctuations. The wet and dry seasons are not well 



defined in St. Lucia, which receives 62% of its annual 

 precipitation (1971-81 data for the estuary mouth, 

 Natal I'arks Board, unpubl.) during summer and 

 autumn when most emigration appears to occur. Dur- 

 ing our study period, salinities in the lower Narrows 

 showed little fluctuation during the emigration of the 

 first cohort, but did decline prior to and during the 

 emigration of the second (Fig. 4). 



A more convincing relationship exists between 

 emigration and declining water temperatures. Water 

 temperatures decreased during the emigration of 

 cohort 1 (Figs. 4 and 7) and may have begun to cool 

 by the onset of emigration in the second cohort. The 



