BEARDSLEY: DESIGN AND EVALUATION OF A SAMPLER 



was no significant difference between sample 

 means for shrimp carapace lengths in catches 

 made with or without the tickler chain (Table 5) . 



Table 5. — Comparison of mean carapace length of 

 shrimp (computed from 200-g samples) taken in the 

 shrimp vertical distribution sampler with ( + ) or with- 

 out ( — ) the tickler chain. These data were computed 

 from the 22 tows presented in Table 3. The F value at 

 the 10% significance level is 2.79 with 1 and 64 degrees 

 of freedom. 



DISCUSSION 



Confidence in the shrimp sampler as a research 

 tool capable of indicating the height of shrimp 

 near the seabed resulted from diver observa- 

 tions, trial tows in Dabob Bay, and statistical 

 evaluations of catches made on offshore shrimp 

 grounds. Neither daylight distributions of 

 shrimp off the seabed nor abrupt changes in ver- 

 tical distributions were expected. The param- 

 eters responsible for changes in distribution 

 were not investigated, but other papers (Schaef- 

 ers and Johnson, 1957; Schaefers and Powell 

 1958; Alverson, McNeely, and Johnson, 1960; 

 Pearcy, 1970) indicate pink shrimp follow diel 

 or circadian movements which may be triggered 

 by changes in light levels. Similar differences 

 in illumination during daylight hours may be the 

 cause of more subtle alterations in shrimp dis- 

 tribution near the seabed. Tows with the shrimp 

 sampler indicated shrimp were further from the 

 bottom during daytime tows under overcast 

 skies. Commercial fishermen using bottom 

 trawls commonly report reductions in shrimp 

 catches when they encounter turbid water and 

 overcast weather, or both. 



This evidence suggests changes in catch rates 

 for pink shrimp are not due entirely to endog- 

 enous factors as their distribution appears to 

 change in direct response to light intensity re- 

 gardless of the time of day. Exogenous rhyth- 



micity in crustaceans has been demonstrated 

 previously by Skud (1968) where during a total 

 eclipse of the sun over Maine in July 1963, cru- 

 staceans exhibited a variety of responses from 

 no response to movements toward or away from 

 the surface at totality. Data collected with the 

 shrimp sampler suggest pink shrimp will move 

 off bottom during the day (probably to feed) if 

 the light intensity is reduced enough to present 

 some protection from predators. These move- 

 ments may also be directly associated with mi- 

 grations of prey. One explanation for relatively 

 dense concentrations of shrimp near the seabed 

 at night during overcast weather is that these 

 shrimp were able to come off the bottom during 

 the day to feed and returned to the seabed when 

 they became satiated. 



The shrimp sampler indicated the highest 

 proportion of small shrimp occur near the sea- 

 bed. Commercial fishermen could avoid these 

 shrimp by "flying" their nets 2 ft off bottom, but 

 this strategy would also eliminate significant 

 numbers of large shrimp from the catch. Sep- 

 aration and release of small shrimp once they 

 enter the trawl is a difficult if not impossible task. 

 Experiments with the shrimp sampler indicated 

 that the mesh size necessary to segregate shrimp 

 by size is extremely critical even in fully opened 

 meshes at right angles to the current (Figure 8) . 

 The problem is compounded in a trawl where few 

 meshes are fully opened and their orientation 

 to the current varies. Moreover large numbers 

 of shrimp are swept in mass into the trawl cod 

 end and may never encounter trawl meshes. 

 These factors reveal the futility of shrimp trawl 

 mesh size restrictions when the design of the 

 trawl is not considered. 



Mesh size comparisons indicate that the op- 

 timum mesh size for separator panels in shrimp 

 separator trawls should be 2 inches (Figure 8). 

 Smaller mesh would reduce the catch of shrimp, 

 and a larger mesh size would not appreciably 

 increase the size or catch of shrimp captured 

 but could introduce more contaminating organ- 

 isms into the catch. These results compare fav- 

 orably with fishing trials of shrimp separator 

 trawls with different mesh sizes in the separator 

 panel. Two-inch separator panels are now stan- 

 dard in this gear. 



251 



