BEARDSLEY: DESIGN AND EVALUATION OF A SAMPLER 



NO SEPARATOR PANELS 



TWO- INCH MESH SEPARATOR 

 PANELS 



8 12 16 20 24 8 12 16 20 24 



CARAPACE LENGTH (mm) 



Figure 5. — Relation of length frequency of shrimp to 

 distance off bottom for shrimp sampler tows off Destruc- 

 tion Island, Wash. Four tows were made with no sep- 

 arator panels and four with 2-inch separator panels. 

 N is the total number of shrimp taken in the sampler 

 openings with the cumulative totals for four tows shown 

 at the top of the figxire. 



indicated the vertical distribution of shrimp is 

 not static but subject to dynamic changes, often 

 in a brief period of time. Representative mid- 

 morning tows for three different days are pre- 

 sented in Figure 6, Widely divergent shrimp 

 distributions are evident in this figure. The 

 most rapid alteration in shrimp distribution was 

 observed between midmorning tows on 22 and 

 23 June (Figure 6). Over the same time pe- 

 riod the weather changed from partly cloudy to 

 complete overcast with rain. Commercial shrimp 

 fishermen trawling on the same shrimp grounds 

 reported a reduction in shrimp catches accom- 

 panying the change in cloud cover. As commer- 

 cial shrimpers drag their trawls on bottom re- 

 gardless of weather, it is conceivable that their 

 nets were passing under large numbers of 

 shrimp. During sunny weather with relatively 

 clear water, shrimp were found concentrated 

 near the seabed as on 9 August in Figure 6. 



DIEL MOVEMENTS OF SHRIMP 



Diel variations in shrimp distribution were 

 investigated with the sampler during three day- 

 night cycles using the shrimp sampler. All tows 

 were 15 min in length and were made with 2- 

 inch separator panels on the sampler. During 

 any single diel cycle, all tows were made in the 

 same direction and in the same location as indi- 

 cated by the vessel's compass, depth sounder, and 

 loran. 



Vertical distributions for pink shrimp collected 

 during three diel cycles are presented in Figure 

 7. Tows on 24 March were conducted under a 

 cloudless sky with the greatest quantity of 

 shrimp taken early in the day. Greatly reduced 

 catches were taken at night. In general, shrimp 

 appeared to be higher above the seabed during 

 early morning and evening tows. 



The weather preceeding 28 March was over- 

 cast with rain. Tows during 28 March (Figure 

 7) indicated shrimp were not plentiful near the 



8-1 

 7- 

 6- 

 5- 

 4- 

 3- 

 2- 

 I- 

 



8- 

 7- 

 6- 

 5- 

 4- 

 3- 

 2- 

 I- 

 



Overcast (90% cloud cover) 



June 22, 1969 



off Grays Horbor, Wash. 



Overcost (100% cloud cover) 



June 23, 1969 



off Groys Horbor, Wo$h. 



T 1 1 



Clear (10% cloud cover) 

 August 9, 1969 

 off Destruction Island, 

 Wash. 



100 



200 



300 



400 



WEIGHT OF SHRIMP (g) 



Figure 6. — Examples of three widely divergent shrimp 

 vertical distribution patterns in the shrimp sampler on 

 three different tows. 



247 



