Determination of acute toxicity of unionized ammonia in juvenile longfin yellowtail (Seriola rivoliana).

Vera, L., Aguilar Galarza, B., Reinoso, S., Bohorquez-Cruz, M., Sonnenholzner, S., & Argüello-Guevara, W.

This study provides an important assessment of unionized ammonia (NH3-N) and total ammonia nitrogen (TAN) toxicity for juvenile longfin yellowtail, Seriola rivoliana. The authors assessed the median lethal concentration (LC50) after a 96 h exposure of triplicate groups of fish to different ammonia concentrations: 0.55 ± 0.00; 0.94 ± 0.02; 1.18 ± 0.00; 1.72 ± 0.02, and 1.97 ± 0.09 NH3-N mg/L. A control group (0.00 ± 0.00 NH3-N mg/L) was also included and the 96 h LC50 for unionized ammonia was found to be 0.58 mg/L for S. rivoliana. Clinical toxicity was evident in fish exposed to different concentrations and appeared as specific lesions on gill tissues; characterized by hyperplasia, epithelial lifting, and secondary lamellae fusion. Irreversable damage was evident along with erratic behavioral signs that included swimming in circles and hyperventilation. The goal of this study was to provide a first-hand evaluation of the effects of unionized ammonia on S. rivoliana and establish parameters for proper water quality management. Their findings suggest that a concentration that does not exceed 0.06 mg/L or 1.68 mg/L for unionized ammonia or TAN, respectively, is safe for rearing juvenile S. rivoliana at a salinity of 32 g/L, pH8, and 25°C water temperature. This study provides valuable baseline water quality data for this marine species and will be important for intensive and semi-intensive culture of S. rivoliana.

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Prevalence and antibiotic resistance of Aeromonas schubertii causing internal white spot disease on snakehead fish, Channa striata, in the Mekong Delta, Vietnam

Pham Thi, H. H., Kim, D.-H., Quach Van, C. T., Nguyen, P. T., & Nguyen, T. L.

Snakehead, Channa striata, are a high-value and popular aquaculture species but have had a long history of invasion in the Mekong Delta and other regions of the world resulting in negative impacts on native aquatic species biodiversity. A bacterial disease known as internal white spot disease caused by Aeromonas schubertii is a major problem in the production of snakehead. This study was aimed at characterizing the prevalence of antibiotic resistance in bacterial isolates collected from farms in the Mekong Delta region due to the potential public health concern should multi-antibiotic resistant (MAR) bacteria spread to wild populations or to consumers. Isolates were collected from white nodules in affected fish and identified based on morphological and biochemical tests, combined with an analysis of housekeeping genes (16S rRNA, gyrB, and rpoD). Disease causing isolates were strongly identified as A. schubertii, and it was found that all isolates (n = 25) were phenotypically multidrug-resistant, with resistance to erythromycin (84%), rifampicin (84%), flumequine (88%), amoxicillin (96%), ciprofloxacin (92%), and florfenicol (92%). The MAR indices ranged from 0.33 to 0.92, with one isolate of A. schubertii showing resistance to 11 of the 12 antibiotics tested. Further hierarchical clustering analysis of antibiotic-resistant bacteria suggested the potential to spread between ponds/fish farms and the ability for bacteria to survive for up to 3 months in aquatic environments. These results suggest that the diversity of MAR A. schubertii in snakehead represents a potential risk to native aquatic organisms and consumers, and emphasize the need for judicial use of antibiotics in aquaculture to limit the development of MAR bacterial species.

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