Figure 10. — Subenvironments In Florida Bay. 



sampling gear. It was learned that trawls 

 and stationary traps with directive leads had 

 limitations that made them unsatisfactory. 

 We then began developing equipment to ade- 

 quately sample shrimp from the substrate 

 enclosed under a metal container. 



In the first device (fig. 11), the release of a 

 chennical repellant within the container drove 

 the enclosed shrimp and associated organisms 

 into an attached nonreturn trap. This sampler 

 was effective in obtaining a quantitative sam- 

 ple of shrimp except when very small shrimp 

 were present. Shrimp less than about 20 mm. 

 (0.8 in.) total length did not avoid the repellant 

 as did larger individuals. 



We also developed a suction dredge. This 

 dredge operates on the venturi principle. Water 

 is pumped at 35 lb. per sq. in. pressure 

 through a pressure nozzle into a venturi tube. 

 A vacuum is created, and strong suction 

 forms in the attached sampling hose. Water 

 is then discharged into a collecting screen. 



To determine the most efficient use of the 

 3-in. sampling hose, we tested several methods 

 of enclosing the area to be sampled. En- 

 closures tested were: (1) a trough, 1/12 m.^ 

 (0.9 ft.^), with sliding top through which the 

 suction head is inserted; (2) a cylinder, 

 l/lO m.2 (1.1 ft. 2), from which all water can 

 be exhausted; and (3) a hood, l/lO m.2 (1.1 

 ft. ), attached to the suction head. With the 

 first two enclosures the suction head captured 



Figure 11. — Sampler designed for use with 

 chemical repellant. 



all shrimp enclosed, but the sampler could be 

 easily used only in shallow water. With the 

 hood attached, the gear functioned satis- 

 factorily in deeper waters but did not capture 

 all of the shrimp enclosed. 



To elinninate problems encountered in the 

 use of enclosures, the suction head was at- 

 tached to a specially constructed sled (fig. 12). 

 Two parallel keels or guides were affixed on 

 the underside of the sled and the suction head 

 was mounted between the guides at right 

 angles to the substrate. These guides limit 

 the width of the swath "vacuumed" to 80 mm. 

 (3.1 in.). A hand winch is used to draw the 

 sled at a standard speed. Pump, venturi 

 tube, intake, and discharge collection screen 

 are supported by a specially designed raft 

 that is attached to and drawn behind the sled 

 by the sampling hose. This unit can be quickly 

 taken apart and put together and is readily 

 transported on a 20-ft., shallow-draft boat. 



In comparative trials with a beam trawl 

 in 60 cm. (23.6 in.) of water, the average 

 catch by suction sampler was 13.5 Penaeus 

 shrimp per square meter vs. 1.4 shrimp per 

 square meter by the beam trawl. In addition 

 to taking penaeid shrimp, the suction sampler 

 collected caridean shrimp, am phi pods, 

 isopods, crabs, moUusks, worms, and fish. 

 We are still studying the efficiency of the 

 suction sannpler in capturing all or a known 

 percentage of the shrimp lying in its path. 



16 



