Table 4. — Comparison of rostral spine counts of M. 

 vollenhovenii from the lower St. Paul River, Liberia, 

 and the Catumbella River, Angola 



Number of 

 spines 



St. Paul River 



Catumbella River 

 (from Man, 1904) 



Shrimp 



% of 



total 



examined 



Shrimp 



% of 



total 



examined 





No. 



Percent 



No. 



Percent 



Dorsal-ventrJ 

 11-3 



il: 

 2 



0.9 







0.0 



12-3 



23 



10.6 



11 



25.0 



13-3 



11 



5.1 



2 



4.5 



14-3 



3 



1.4 



1 



2.3 



11-4 



4 



1.8 



1 



2.3 



12-4 



41 



18.8 



12 



27.3 



13-4 



61 



28.0 



7 



15.9 



14-4 



30 



13.8 



5 



11.4 



15-4 



3 



1.4 



1 



2.3 



12-5 



4 



1.8 







0.0 



13-5 



10 



4.6 



1 



2.3 



14-5 



16 



7.3 







0.0 



15-5 



6 



2.8 







0.0 



18-5 



0.0 



0.0 



2.3 



12-6 



1 



0.5 



1 



2.3 



13-6 







0.0 







0.0 



14-6 







0.0 



1 



2.3 



15-6 



1 



0.5 







0.0 



13-7 



1 



0.5 







0.0 



13-8 



1 



0.5 







0.0 



sal: 



11 



6 



2.8 



1 



2.3 



12 



69 



31.7 



24 



54.5 



13 



84 



38.5 



10 



22.7 



14 



49 



22.5 



7 



15.9 



15 



10 



4.6 



1 



2.3 



2.3 





o 

 o 



39 





17.9 





14 



31.8 





4 



139 





63.8 





26 



59.1 





5 



36 





16.5 





2 



4.5 





6 



2 





0.9 





2 



4.5 





7 



1 





0.5 









0.0 





8 



1 





0.5 









0.0 



I 



sampk 



;d in 



the 



1952 



and 



1953 



seasons, 



grouped monthly in 5-mm. intervals. 



The trap catch was selective: few juveniles 

 were caught before they reached the minimum 

 size of maturity (45-50 mm.) . Trap selectivity 



was possibly due to a dietary difference be- 

 tween the juveniles and adults. I found a sim- 

 ilar selectivity in the trap catch for the western 

 North American fresh-water crawfish, Pacif- 

 astacus leniusculus (Dana) (Miller, 1960). 



Knowledge of the shrimps' biology and the 

 length distributions of the catch led me to 

 speculate that the commercial fishery was sup- 

 ported primarily by a single age group. Fe- 

 male shrimp with eggs in advanced stage of 

 development were found from May to January, 

 which would explain the wide range of the 

 length distributions (fig. 5). 



The modal length increased each month 

 through November during the commercial 

 seasons (May to January) of 1952 and 1953. 

 In the 1952 season, the modal length declined 

 in December and January (no data for these 

 months in the 1953 season). This type of dis- 

 tribution indicates that the fishery was sup- 

 ported by a single age group, which was re- 

 placed late in the season by the incoming zero 

 age group. Growth rate was rapid, and age 

 group zero reached a modal length of 75 to 

 80 mm. in 9 months. Growth of the mature 

 shrimp was slow — the shrimp reached a 

 modal length of 75 to 80 mm. in January, 

 85 to 90 mm. in May, and 100 to 105 mm. in 

 November. 



The rapid growth of juveniles, and slow 

 growth of adults given for M. vollenhovenii is 

 similar to the growth rate of M. rosenbergii 

 reported by Ling (1967a). The females and 

 males of M. rosenbergii reached a length of 

 140 mm. in 5^4 months, and the females grew 

 only slightly beyond 150 mm.; the male growth, 

 however, continued to 200 mm. 



Two other possible explanations for the 

 length distributions were examined and then 

 rejected: (1) the unimodal distribution con- 

 sisted of more than two age groups, the same 

 as other trap caught crustaceans; and (2) the 

 large shrimp, being less salinity tolerant than 

 small shrimp, were the last to enter the estuary 

 at the beginning of the season and the first 

 to leave the estuary at the end of the season. 



The first explanation was rejected because 

 the modal length increased each month. I 

 found in trap-caught western North American 

 fresh-water crawfish, Pacifastacus Bott, that 

 aging by length distributions was nearly 



