BEARDSLEY: DESIGN AND EVALUATION OF A SAMPLER 



Table 2. — Comparison of pink shrimp caught behind separator panels of differing mesh 

 size. Means and F values were computed from 24 samples (i.e., six vertical openings 

 over a four-tow repetition). 



F value 



Item 



Mesh size (inches) 1.0 



Mean weight of shrimp (g) 27.6 



Mean length of shrimp (mm) 14.7 



Mesh size (inches) 1.75 



Mean weight of shrimp (g) 166.8 



Mean length of shrimp (mm) 19.1 



Mesh size (inches) 2.5 



Mean weight of shrimp (g) 231.3 



Mean length of shrimp (mm) 19.4 



^ The F value at the 5% significance level is 3.32 with 2 and 30 degrees of freedom. 

 - The F value at fhe 5% significance level is 4.17 with 1 and 30 degrees of freedom. 

 3 Not significantly different at 5% significance level. 



vertical column as a control or index of the rel- 

 ative abundance of shrimp available.' The po- 

 sitions of the separator panels being evaluated 

 (port and starboard) were switched after each 

 set of two tows, and tow direction was altered 

 180° after each' haul. This was done to reduce 

 any sampling bias attributable to one side of the 

 sampler or the other. Two different mesh sizes 

 were compared over four consecutive tows. 



The relationship between mesh size and the 

 catch of shrimp as determined by tows with the 

 shrimp sampler is shown in Table 2. The mean 

 sample weight of shrimp catches increased di- 

 rectly with the mesh size, but the rate of increase 

 diminished in mesh sizes above 2 inches. Dif- 

 ferences between the total weight of catches 

 taken with 21/2-inch mesh and 3-inch mesh were 

 significant {F = 4.71), but difference in the 

 mean weights (231.3 and 260.7 g) for these mesh 

 sizes was less than the differences between the 

 means for 1-inch and 1 1/2-inch mesh (27.6 and 

 112.7 g) and for 134-inch and 2-inch mesh (166.8 

 and 243.7 g). These differences are also sug- 



* In the situation where small mesh sizes (i.e., 1% 

 and l'/2 inches) were being compared on the lateral vert- 

 ical openings of the sampler, there is the possibility that 

 shrimp catches in the center vertical opening (2-inch 

 mesh) would be proportionally greater than when larger 

 mesh sizes (i.e., .21/2 and 3 inches) were being compared. 

 The likelihood of shrimp avoiding the small mesh was 

 considered remote on the basis of diver observations of 

 fish pinned against the web in the separator panels and 

 the presence of large numbers of gilled shrimp in small- 

 mesh web on the separator panels at the conclusion of a 

 tow. 



gested by the computed F values in Table 2. Fig- 

 ure 8 shows this relation between mesh size and 

 catch from the data in Table 2. In this figure 

 mean weight and carapace length (means) of 

 shrimp catches for the six different mesh sizes 

 have been converted into a percentage of the 

 weight and length (means) taken by the center 

 vertical column (control) with its standard 2- 

 inch mesh separator panel. 



In contrast, the mean carapace length of 

 shrimp did not differ significantly with increas- 

 ing mesh sizes above II/2 inches (Table 2 and Fig- 

 ure 8). This indicates that the larger shrimp 

 are able to pass readily through the 1 1/2-inch 

 mesh if the meshes are open and held at right 

 angles to the current. Meshes in the interme- 

 diate and cod end of conventional west coast 

 shrimp trawls are II/2- to 1%-inch mesh. How- 

 ever, shrimp do not pass through these meshes 

 in large quantities because the meshes are nor- 

 mally partially closed and nearly parallel with 

 the current (High et al., 1969). 



EFFECT OF TICKLER CHAIN 

 ON SHRIMP CATCHES 



Commercial shrimp fishermen in the north- 

 eastern Pacific commonly employ a contrivance 

 called a "tickler chain" to excite shrimp off bot- 

 tom in an effort to increase their catches. This 

 chain is several feet shorter than the trawl foot- 

 rope with the ends attached to the ends of the 

 foot rope. In this configuration the tickler chain 



249 



