World Aquaculture December 2018

48 DECEMBER 2018 • WORLD AQUACULTURE • WWW.WA S.ORG ponds that have plagued shrimp farmers in Alabama and Florida over the last several years. The results of commercial- scale pond-level studies are often difficult to interpret due to the large number of uncontrollable factors that can influence and confound study findings. This trial was no exception, with survival and yield variation among ponds irrespective of treatment. Some of the variation can be explained by factors unrelated to the probiotic treatment, such as high pH early in the season. Observed mortality late in the season, particularly at GAS, appeared to be caused by algal toxicity unrelated to probiotic treatment. Hence, it is highly likely that strong negative drivers (high pH and potential algal toxicity) superseded and obscured any measurable effects of probiotic application. Inclusion of probiotics in feed is an alternative strategy for probiotics application. This was not explored in this study but is a documented approach used by commercial fish and shrimp farmers. Sapcharoen and Rengpipat (2013) reported higher survival of shrimp fed a Bacillus probiotic compared to a control group. They also reported lower mortality in probiotic-fed shrimp when both treatment groups were challenged with Vibrio harveyi . Wang (2007) reported growth performance in Pacific white shrimp fed diets supplemented with probiotics compared to a basal diet. Hai et al. (2009) reported positive benefits in the use of feed supplemented with probiotic in western king prawn Melicertus latisulcatus . The application of a probiotic at the larval and post-larval stages of production is another avenue that has been explored (Zhou et al. 2009, Nimrat et al. 2012, Silva et al. 2013, Xiong et al. 2016). Commercial hatcheries are often located long distances from commercial shrimp farms and post-larval shrimp thus undergo significant stress during transport. In West Alabama farms that use an inland low salinity water source, further stress occurs during the acclimation phase from high to low salinity. Alternative strategies in the use and administration of probiotics merit further evaluation. During the last three years there have been high levels of Oscillatoria present during “late-term” mortality events at GPA, while mortality events at FS have often been associated with high levels of Prymnesium . In this study, increased density of Oscillatoria , a known producer of toxins, over the course of the production cycle, coupled with histological evidence of acute algal toxicity at GAS, suggests that toxic algae merits further investigation as a cause of mortalities that occur late in the production season. Mortalities from toxic algae have been reported by catfish farmers located near the shrimp farms in this study that also use inland low salinity groundwater as a water source. Quantification of known toxins and correlation with algal biomass would be beneficial information for commercial producers as would effective control measures. in shrimp histology. Pathologies observed varied in shrimp collected from the three farms and there were no differences between treatment groups. Histological examination of shrimp tissues from GPA in August and October revealed light bacterial necrosis with the presence of vermiform bodies in the hepatopancreas, regardless of experimental treatment (Fig. 13). Vermiform bodies resemble gregarines but are the result of sloughing of epithelial cells from hepatopancreatic tubules and are thought to be associated with vibrios (Sriurairatana et al. 2014). Shrimp from FS had light to moderate bacterial necrosis regardless of treatment in August while shrimp collected in October from probiotic- treated ponds had increased levels of bacterial necrosis compared to shrimp from control ponds. Shrimp evaluated in early August from GAS had only light bacterial necrosis, regardless of treatment, despite GAS reporting shrimp die-offs in numerous study ponds in late August. Samples collected in late August were characterized by an increased number of hemocytes and lipid vacuoles in the hepatopancreas, indicating possible issues with algal toxicity. Shrimp sampled in late September had an increased number of lipid vacuoles, damaged columnar epithelial cells in the intestinal tract and increased hemocytes, suggesting that high reported mortalities was likely due to an acute algal toxicity issue at the farm. Unfortunately, algal communities of GAS ponds were not evaluated in this study. In summary, histopathology results varied among farms and pond to pond variability was as prevalent as variability between treatments. Addition of probiotics to the pond had no impact on shrimp pathology. Probiotic Efficacy and Future Challenges The probiotic was effective at decreasing the total numbers of vibrios in pond water and decreasing the proportion of pathogenic bacteria in pond water, despite no apparent effects on vibrios in shrimp muscle. Laboratory screening determined that the probiotic was effective in preventing the growth of two Vibrio species (Fig. 9). These results are consistent with other published studies that have used a Bacillus probiotic. In a study with shrimp larvae and postlarvae, Vibrio counts were reduced in culture water and shrimp (Silva et al. 2013). Likewise, Vibrio counts were reduced in Bacillus - probiotic treated shrimp ponds compared to control ponds (Moriarty 1998). In this study on commercial shrimp farms in the southeastern US, use of a Bacillus probiotic had no discernable effects on selected water quality variables or pond algae populations. Probiotic treatment of ponds also had no measurable effect on shrimp survival or production. Probiotic addition does not appear to be the “magic bullet” to resolve late-term survival problems in semi-intensive Probiotic treatment of ponds had no measurable effect on shrimp survival or production and had no discernable effects on selected water quality variables or pond algae populations. The probiotic was effective at decreasing the total numbers of vibrios in pond water and decreasing the proportion of pathogenic bacteria in pond water. Although vibrios may not have been directly responsible for late-term mortality, higher Vibrio levels in ponds and shrimp may have compromised the shrimp immune system, causing shrimp to be more susceptible to other stressors. Observed mortality late in the season appeared to be caused by algal toxicity unrelated to probiotic treatment. Toxic algae merits further investigation as a cause of mortalities that occur late in the production season. Probiotic addition does not appear to be the “magic bullet” to resolve late-term survival problems in semi-intensive ponds.