the Mississippi Sound in salinities between 2°/oo and 25°/oo; but 

 they are most common in salinities below 15°/oo, with the greatest 

 abundance in 2°/oo to 5°/ooS (Perry et al. 1974). However, it has 

 also been observed that within this low concentration range brown 

 shrimp are unevenly distributed. Food availability is suggested 

 as one of the reasons for the unevenness but it does not seem to be 

 the exclusive limiting factor. Therefore, it is necessary to deter- 

 mine if the deviation in ion ratios of the coastal waters has any 

 significant effect on the physiological responses and pattern of 

 natural distribution. The effects of ion ratio changes were also 

 studied on the behavior and survival of Penaeus aztecus. 



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