FISHERY BULLETIN: VOL. 71, NO. 1 



I40r 



MESH SIZE (inches) 



Figure 8. — Relationship between mesh size of separator 

 panel and the means of shrimp length and total weight. 

 The values for the mean carapace length and mean sam- 

 ple weight are compared with the shrimp catches 

 (means) taken behind the 2-inch mesh separator panels 

 (control). 



The tickler chain caused a significant increase 

 in the number of shrimp entering the lower por- 

 tion of the shrimp sampler. This phenomenon 

 suggests that the tickler chain either excites 

 shrimp into the water column that normally 

 would pass under the sampler or confuses shrimp 

 so that they are unable to avoid the sampler. 

 The fact that pink shrimp can be readily taken 

 in a plankton sampler with an opening 15 cm. 

 in diameter indicates that avoidance may not be 

 an important consideration. The major effect of 

 the tickler chain is probably to move shrimp ver- 

 tically where they are vulnerable to capture by 

 the trawl. 



The utility of the shrimp sampler in describing 

 the vertical distribution of shrimp suggests that 

 it would be a valuable tool for providing sup- 

 portive evidence necessary for sound manage- 

 ment decisions regarding shrimp resources. For 

 example, in California, trawl surveys of the 

 shrimp beds are used to estimate the quantity 

 and quality of shrimp available and hence estab- 

 lish the quota size ( Abramson, 1968) . These an- 

 alytical estimations of the standing stock of a 

 shrimp bed are based on the assumption that 

 catch per unit of effort is a function of stock 



density in the area being surveyed and that 

 changes in catch per effort are directly propor- 

 tional to changes in density (Ricker, 1958; Gul- 

 land, 1964). Equations relating population size 

 to catch per effort for trawl gear (Alverson and 

 Pereyra, 1969) require some knowledge of the 

 vulnerability of shrimp within the influence of 

 the trawl and the proportion of the total shrimp 

 stock in the water column sampled by the trawl. 

 Estimates for vulnerability of shrimp have tra- 

 ditionally been placed near 1 largely because 

 of a lack of knowledge regarding the behavior 

 of shrimp to trawls. With consideration of the 

 size of the trawl opening and the erratic escape 

 movements of shrimp, a vulnerability coefficient 

 of 1 may be relatively accurate. However, use 

 of the shrimp sampler has shown that the co- 

 efficient of catchability for a shrimp trawl which 

 is towed a constant distance off bottom may vary 

 dramatically from day to day. Often the catch 

 coefficient would not approach 1 for a trawl 

 having a 4-ft vertical opening. Thus the over- 

 all coefficient, which is a product of the vulner- 

 ability and catchability coefficient, may at times 

 be considerably less than 1 if shrimp are being 

 sampled with a conventional shrimp trawl. 

 Moreover, the value for the overall coefficient 

 would vary from day to day and reduce the ac- 

 curacy of population size estimates. The tickler 

 chain must also have an important effect on the 

 vulnerability coefficient, but this relation has not 

 been explored. 



The greatest utility of the shrimp sampler in 

 providing estimates of standing stock may be 

 realized when the sampler is towed alternately 

 with a standard trawl. This approach has been 

 taken by scientists at the Auke Bay Fisheries 

 Laboratory, National Marine Fisheries Service, 

 NOAA, Auke Bay, Alaska (James Olson, pers. 

 comm.) where estimates were being made of 

 the standing stock of shrimp in Kachemak Bay, 

 Alaska. In this instance the sampler was used 

 to determine the catchability coefficient of the 

 standardized shrimp trawl used in the standing 

 stock estimates. 



The sampler has also been towed alternately 

 with trawls in an experiment to measure the 

 fishing power of four dissimilar shrimp trawls 

 near Kodiak, Alaska (Lael L. Ronholt, North- 



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