Results 



Cane River does not flow through to the sea for 

 much of the year (January- August). As the larvae 

 of M. faustinwn must develop in the sea (Hunte 

 1980a, b), juveniles enter the river during a short 

 period each year. Consequently, length-frequency 

 histograms showed distinct modes which were useful 

 for growth and mortality analyses (Fig. 2). 



Juveniles entered the river between September 



1973 and January 1974, and again in September 



1974 (Fig. 2). The weighted mean time of entry of the 

 1973 cohort was in early December, and was 

 assumed to be early December for all cohorts. At this 

 time shrimps were about 3 mo of age. The peak of the 

 breeding season is about 3 mo earlier (see Fig. 4). 

 Larval development in the laboratory takes about 95 

 d (Hunte 1980a) after which the length of the 

 juveniles (Hunte 1980b) is similar to that of juveniles 

 entering the river. 



Monthly growth for males and females (Fig. 3) was 

 estimated from changes in mean length of cohorts 

 (Fig. 2). Sex could not be distinguished before about 

 10 mo, when all members of a cohort are longer than 

 20 mm. Males grew slightly faster than females 

 (Fig. 3). 



The wet weight/length relationship for males was 

 log W=3.15 log L - 2.02, and for females log 

 W= 3.23 log L — 2.20. The regression coefficients 

 do not differ significantly, but are significantly >3, 

 indicating allometric growth (b > 3; for males t — 

 2.64, P < 0.01; for females, t = 5.14, P < 0.001). 



Females composed 49% of adult shrimps caught 

 (i.e., those >20 mm). This was not significantly dif- 

 ferent from a 1:1 sex ratio (x 2 = 0.81, P > 0.25). 



No females smaller than 26 mm (about 9 mo old) 

 bore eggs. This is therefore an estimate of the mini- 

 mal size (age) at sexual maturity. Of the females in 

 the 26-28 mm size class, 39% were berried, com- 

 pared with 5 1 % for all mature classes combined. This 

 suggests that most females mature in the 26-28 mm 

 size range, with little variation in size at sexual ma- 

 turity. 



Eggs were oval, about 0.54 by 0.42 mm when laid, 

 and about 0.70 by 0.53 mm prior to hatching. Egg 

 size was independent of female length. Eggs were 

 counted on females carrying eggs in advanced devel- 

 opmental stages, and the fecundity/length relation- 

 ship was log F = 3.52 log L - 2.47. 



Monthly percentages of mature females carrying 

 eggs (berried) showed that spawning was continuous 

 but peaked between June and November (Fig. 4), 

 just before or during the months of heaviest rainfall in 

 Jamaica. 



Total mortality for the M. faustinum stock is es- 

 timated from the decline in monthly catch of the four 

 year classes sampled in this study (Fig. 5). The per- 

 centage contribution of each age- group to the fisher- 

 men's catch shows the age of first capture to be 16 mo 

 and full recruitment to the fishery to be at 24 mo (Fig. 

 4). There is a distinct increase in mortality at, or just 

 before, the age of complete recruitment. We estimate 

 the instantaneous rate of natural mortality (M) as 

 equal to the instantaneous rate of total mortality (Z) 

 for the prerecruits (ages 5-15 mo). Fishing mortality 

 (F) is taken as equal to Z-M for the fully re- 

 cruited age groups (25-34 mo). The estimates of M and 

 F are 0.13 and 0.15, respectively, and the former is 

 used in the estimation of Y/R. 



The information used to calculate the partial re- 

 cruitment values is given in Table 1. The ratios of pro- 

 portion in catch to proportion in the survey indicate 

 thatF increases steadily with age and size. However, 

 as there are relatively few individuals in the older 

 age- groups, we felt it more appropriate to consider 

 recruitment as complete at age 2 4 and to assign r,= 1 

 to all older age-groups. Therefore, all values of r, for i 

 <24 are relative to the catch survey ratio at age 

 24. 



The relationship between Y/R and F (Fig. 6) ap- 

 pears to be asymptotic; therefore, no F for maximum 

 yield could be computed. However, beyond an F of 

 about 0.5 the returns in terms of Y/R for increased 

 F are minimal. Both the mean weight of shrimp in 

 the catch and the index of catch per unit effort 

 change most rapidly at values of F less than about 

 0.5. 



The relationship between fishing mortality and the 

 number of ripe eggs produced per recruit shows that 



Table 1.— The calculation of partial recruitment to fishing 

 mortality at each age for Macrobrachium faustinum in Cane 

 River, Jamaica. 



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