Syndromes associated with low salinity water in Arizona

By D.V. Lightner, Hui Gong, Josh Wilkenfeld, and Craig Collins. Shrimp farming in low salinity groundwater in the desert southwest has become a reality since USMSFP-funded work at the University of Arizona first demonstrated its technical feasibility in 1990-1992. In 2003, three farms produced crops of low salinity tolerant Pacific white shrimp, Litopenaeus vannamei. However, development in Arizona has been hindered by some specific disease problems that may be linked to the low salinity of the waters used, and the farmers in southwest Arizona have complained about several diseases during the culture cycle. (Table 1)

Figure 1. An aeration tower at Arizona Mariculture Associates. The tower contains multiple layers of screened plates which de-gas nitrogen from the supersaturated pumped groundwater, while increasing its oxygen content, as it falls through the screens.

Of significance has been the absence in Arizona of those diseases and their agents (eg. WSSV, TSV, YHV, IHHNV, NHP, and others) which are listed internationally or nationally by the OIE and USMSFP, respectively. The disease syndromes that have occurred in Arizona shrimp farms seem to be somewhat unique and are very likely related to such factors as high water temperature and deficiencies and imbalances of minerals such as potassium, magnesium and calcium.
Among these have been diseases like cramped muscle syndrome, various forms of focal cuticular, systemic and especially enteric vibriosis, hemocytic enteritis, and perhaps two other syndromes due directly to high temperatures and low salinity. Particularly troubling to the fledgling industry has been the onset and progressively increasing mortality during molting in large size (i.e., ~ >18g) L. vannamei. The relatively high frequency of this disease syndrome in 2001 and 2002 caused the farms to harvest some ponds earlier than would have been optimum for production of larger sized (and therefore higher value) shrimp just to prevent anticipated losses of large size shrimp during peaks of molting (usually coinciding with full and new moon). 

Osmotic failure was identified as a possible cause of the high mortality rates observed in these large juvenile shrimp at the Arizona farms. Studies on the osmolarity capacity of large juveniles showed them to be less capable of maintaining their internal ionic balance than younger juveniles. Collaborative work carried out with Arizona Mariculture Associates and Desert Sweet Shrimp Farms showed that some of the low salinity diseases could be managed. This was accomplished by either supplementing the feed with the addition of certain trace minerals (K and Mg) and certain lipids, or by the use of fertilizers containing K and Mg applied directly to the ponds. Either method reduced the incidence and severity of cramped muscle syndrome, white muscle syndrome, and molting mortality syndrome and reduced production losses associated with these diseases. Refinement of the trace mineral content of feeds for inland low salinity shrimp culture, as well as well as better defining methods of pond fertilization to improve the trace mineral content of shrimp ponds, should further improve the
management of low salinity disease syndromes.
D.V. Lightner, Department of Veterinary Science & Microbiology, University of Arizona, Tucson, AZ 85721; Hui Gong and Josh Wilkenfeld, Arizona Mariculture Associates, Hyder, AZ; Craig Collins, Desert Sweet Shrimp Farm, Gila Bend, AZ