Benfield et al  Growth and emigration of Penaeus indicus 



23 



l)eam trawl catch was related to prior pulses of postlar- 

 vae in the plankton. Over this interval, the gi'owth rate 

 was estimated by relating the mean size of postlarvae 

 to the mean size of the subsequent cohort of juveniles 

 in the beam trawl catch. 



Length-frequency histograms were constructed from 

 the sum of all shrimp caught over the entire sampling 

 period for the beam trawl and bait fishery samples. 

 These two histograms were used to estimate the ap- 

 proximate size of shrimp at the onset of emigration. 

 Declines in the frequency of shrimp within the emi- 

 grating size classes over successive sampling intervals 

 were used to identify possible periods of emigration. 



Results 



Distribution and abundance 



Shrimp were captured throughout the Narrows and in 

 limited numbers along the western shore of South 

 Lake. Five specimens were taken along the eastern 

 shore (station 11). Catches were highest in the middle 

 Narrows (stations 4-6) and declined in either direction 

 from that section (Fig. 1). 



Shrimp concentrated in the middle Narrows for most 

 of the year with some seaward shift in their distribu- 

 tion evident December-February (summer) (Fig. 2). 

 The apparent shift towards the lower Narrows during 

 June- August (winter) is a consequence of the capture 

 of large numbers of 7-12 mm CL shrimp at station 1 

 in August 1983 (Fig. 2). 



Juvenile shrimp were present throughout the year, 

 declining over January- February (late summer) toward 

 a March-May (autumn) minimum and increasing dur- 

 ing June-November (winter and spring) (Fig. 3). 



A substantial proportion of the P. indicus population 

 was presumed to have been washed out of the system 

 during the January 1984 cyclone. During the months 

 following the cyclone, shrimp numbers increased slow- 

 ly, and by July 1984 shrimp were sufficiently abundant 

 for the bait fishery to resume operation. Beam trawl 

 sample sizes remained too low to adequately define the 

 population structure, and growth estimates could not 

 be provided for the post-cyclone samples. 



Gro\A/th 



Growth rates were estimated over the size range 1.72- 

 20.12 mm CL for three cohorts that entered the system 

 prior to the cyclone (Fig. 4). These estimates ranges 

 from 0.0005 to 0.1634 mm CL per day. Cohort 1 ex- 

 hibited approximately linear growth up to 13.92 mm 

 CL (Fig. 4) and had a mean growth rate of 0.058 mm 

 CL per day over the size range 7.25-13.92 mm CL. 

 Cohort 2 displayed a pronounced seasonal fluctuation 



SPRING (SEP-NOV) 25 



20 



SUMMER IDEC-FEB) 



1 2 3 4 5 6 7 8 9 101112 



123456789 101112 



AUTUMN (MAR-MAY) 30 



WINTER (JUN-AUa) 



123456789 101112 

 SAMPLING STATION 



1 2 3 4 5 6 7 8 9 101112 

 SAMPLING STATION 



Figure 2 



Mean seasonal distribution of Penaeus indicus in the St. Lucia 

 system. See Figure 1 for sampling station locations 



in its growth pattern (Fig. 4). The mean growth rate 

 of this cohort was 0.035 mm CL per day over the size 

 range 8.99-17.66 mm CL. Cohort 3 also displayed 

 seasonal fluctuation in its growth. The mean growth 

 of this cohort was 0.040 mm CL per day over the size 

 range 8.83-14.81 mm CL. 



For the size range 7.25-17.66 mm CL, growth was 

 significantly correlated with the mean water tempera- 

 ture of each sampling interval (Spearman's coefficient 

 of rank correlation r, = 0.746, t = 3.361, 9 df, jtXO.Ol) 

 (Fig. 5). This response was particularly evident in 

 cohorts 2 and 3 which showed a pronounced decline in 

 their growth during the cooler winter months (Fig. 4). 

 Cohort 1 also overwintered and probably followed a 

 similar pattern wiiile it was too small to be effectively 

 sampled by our gear. 



Emigration 



The first cohort entered the system during early May 

 1982 and emigrated over January-March 1983. A 

 second cohort which may have recruited during early 

 summer 1982 emigrated over December 1983 and 

 January 1984. Emigration of the third cohort, which 

 entered in April 1983, may have been prematurely 

 initiated by the cyclone at the end of January 1984. Two 

 cohorts thus entered the system during April-May 

 (autumn) with possible recruitment during November- 

 December (early summer) by the remaining cohort. All 



