The mean lengths show little variation 

 and indicate that the sannpling technique 

 provided a representative index of the 

 sizes in the catch. 



Catches of nets from 10 tows of cruise 

 16 were examined to discover whether 

 they differed in respect to the average 

 size of the shrimp and to the sex ratio 

 (table 3). Carapace length differences of 

 male and female shrimp combined, from 

 port and starboard nets, did not exceed 

 1.4 mm. within individual hauls. Neither 

 net consistently caught the larger average 

 shrimp. The variation in sex ratio of 

 shrimp caught by the two nets was not 

 extreme. These results justified pooling 

 catches of the two nets before drawing 

 the samples, or drawing them from either 

 net. 



Effect of Trawl Mesh Size 



Ideally, the same trawl cod end should 

 have been used throughout the study; 

 however, trawl nets do not wear well and 

 such a procedure was not possible. The 

 nets used were bought as 2 -inch stretched - 

 mesh manufacturer's measure. Because 

 of variations resulting from the manu- 

 facture, preservation, use, and other fac- 

 tors, trawl cod ends can vary greatly in 

 average mesh size (Medico, 1958). This 

 was evidenced in the present study. Cod- 

 end meshes were measured at intervals 

 for each shrimp net (table 1 ). 



Graham (1956) found that the sizes of 

 meshes in trawl cod ends govern the 

 average size of the fish caught. This has 

 also been shown for shrimp (Regan g< aL, 

 1957) and it was considered necessary 

 to determine how the different sizes of 

 shrimp trawl cod ends might affect our 

 results. The average size of female 

 shrimp caught by the main nets was 

 compared with the average size of female 

 shrimp caught by try nets fishing sinnul- 

 taneously (fig. 4). At the top of the figure, 

 sizes of shrimp in catches by a 1 3/4- 

 inch mesh try net used for the first half 

 of the study were compared with catches 

 of two main net cod ends, one with 1 1/4- 

 inch mesh and one with 1 3/4-inch mesh. 

 The results suggest little influence by the 

 nneshes of these two nets on average 

 female shrimp sizes. 



In the lower part of figure 4, catches 

 by a 1 7/8 -inch mesh try net, used for 



22j0 260 30.0 340 3B0 42.0 460 



MESH SIZES-MAIN NETS 

 ICOD ENDSI 



O ' 1-7/8" a l-7/a LINEfi 

 A : 1-1/4" 



TRY NET MESH: 1-3/4 



TRT NET MESH--I-7/8" 



18.0 220 260 300 340 380 420 460 

 AVERAGE CARAPACE LENGTH (MM) FROM MAIN RIGS 



Figure 4. --Relationship of carapace length of pink shrimp 

 caught by main nets and try nets. 



most of the remaining time, are compared 

 with catches of four main net cod ends, 

 including the 1 3/4 -inch mesh net used in 

 the above analysis. The results indicate 

 that the try net (1 7/8-inch mesh) caught 

 a somewhat larger average size of shrimp 

 than the main nets, but that the size 

 selectivity of the nets did not materially 

 affect the average size of female shrimp 

 sampled. 



Shrimp Size Isopleths 



Several methods were considered for 

 the analysis and presentation of the data 

 describing the size distribution of shrimp 

 on the fishing grounds. However, the data 

 would not fully satisfy the requirements 

 or assumptions of several statistical tests 

 and the decision was therefore miade to 

 present the results graphically. Diagrams 

 have been constructed showing the areas 

 of the fishing grounds where shrimp of a 

 certain size range occurred. Isopleth 

 lines were drawn on charts of the fishing 

 grounds, joining interpolated points from 

 observed average size of female shrimp. 

 Standard isopleths of 2.5 mm. head -length 

 intervals were used, the smallest being 

 20.0 mm. Data from successive cruises 



