DESIGN AND EVALUATION OF A SAMPLER FOR MEASURING 



THE NEAR-BOTTOM VERTICAL DISTRIBUTION OF 



PINK SHRIMP, PANDALUS JORDANI 



Alan J. Beardsley^ 



ABSTRACT 



A shrimp sampler was constructed as one portion of a research effort dealing with the 

 development of a fish-shrimp separator trawl. The sampler segregated shrimp caught 

 in a series of 1-ft high vertical openings positioned between the seabed and a height of 

 13 ft above the seabed. Knowledge of the vertical distribution of shrimp was considered 

 essential in the design of an efficient shrimp trawl. Results indicated that vertical dis- 

 tributions of shrimp vary, and the amount of light striking the seabed is suggested as 

 the triggering stimulus. Auxiliary investigations conducted with the sampler dealt with 

 evaluations of mesh size and tickler chain. Experiments indicated that mesh sizes smaller 

 than 2 inches restrict the passage of shrimp. The weight of shrimp caught was nearly 

 doubled when a tickler chain was used. The sampler may have application to both shrimp 

 biologists and commercial fishermen. 



Research was begun in our laboratory on trawls 

 capable of separating pink shrimp, Pandalus jor- 

 dani, from other marine organisms and debris 

 while the net is being towed over the seabed 

 (High, Ellis, and Lusz, 1969; Beardsley and 

 High, 1970). Knowledge of the near-bottom 

 vertical distribution of shrimp was considered 

 essential to the design of an effective shrimp 

 trawl since the vertical height of any bottom 

 trawl should approximate the off-bottom distri- 

 bution of the target species. 



Subsequently a multipurpose shrimp sampler 

 was designed to facilitate investigation of shrimp 

 distributions above the seabed by 1-ft intervals. 

 Auxiliary investigations conducted with the sam- 

 pler included evaluating the effects of: (1) diel 

 or circadian movements on the abundance of 

 shrimp near the seabed; (2) light on shrimp 

 vertical distribution; (3) mesh size on the re- 

 tention of shrimp and other marine organisms; 

 and (4) a tickler chain on shrimp catch rate and 

 vertical distribution. 



' Northwest Fisheries Center, National Marine Fish- 

 eries Service, NOAA, 2725 Montlake Boulevard East, 

 Seattle, WA 98102. 



Manuscript accepted July 1972. 



FISHERY BULLETIN: VOL. 71. NO. 1, 1973. 



METHODS AND MATERIALS 



The shrimp sampler was designed for towing 

 on the seabed either attached in the mouth of a 

 conventional 57-ft Gulf shrimp trawl or directly 

 to dandy lines without the net. All data pre- 

 sented in this paper resulted from tows without 

 an attached net. This fishing configuration made 

 the sampler easier to set and retrieve; it also 

 eliminated the variability in shrimp catches 

 caused by differences in fishing modes. Com- 

 mercial trawling conditions were simulated using 

 5- X 7-ft otterboards, 15-fm dandy lines, and a 

 towing speed of 21/? knots. 



The shrimp vertical distribution sampler 

 (Figure 1) consists of an aluminum frame par- 

 titioned into 18 openings each measuring 1 ft 

 vertically by 2 ft horizontally. The sampler 

 openings are positioned in six horizontal rows 

 (1 ft high) and three vertical columns (2 ft 

 wide), resulting in a triplicate series of vertical 

 samples. A vertical extension was bolted with 

 %-inch bolts behind the sampler, permitting 

 sampling as high as 12 to 13 ft above the seabed. 



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