Some yeast strains have been proposed as probiotics to improve the health of cultured fish. Cobia is a tropical benthopelagic fish species with potential for marine aquaculture; however, one of the main limitations to its large-scale production is the high mortality of fish larvae. In this study, we evaluated the probiotic potential of autochthonous yeasts from the intestines of cobia. Thirty-nine yeast isolates were recovered from the intestinal mucosa of 37 adult healthy cobia by culture methods. Yeasts were identified by sequencing of the ITS and D1/D2 regions of the 28S rRNA gene and typed by RAPD-PCR using the M13 primer. Yeast strains with unique RAPD patterns were characterized in terms of their cell biomass production ability; anti-Vibrio, enzymatic, and hemolytic activity; biofilm production; hydrophobicity; autoaggregation; polyamine production; safety; and protection of cobia larvae against saline stress. Candida haemuloni C27 and Debaryomyces hansenii C10 and C28 were selected as potential probiotics. They did not affect the survival of larvae and showed biomass production >1 g L⁻¹, hydrophobicity >41.47%, hemolytic activity γ, and activity in more than 8 hydrolytic enzymes. The results suggest that the selected yeast strains could be considered as potential probiotic candidates and should be evaluated in cobia larvae.

The bacterial community of the intestinal microbiota influences many host functions, and similar effects have been recently reported for the fungal community (mycobiota). Cobia is a tropical fish that has been studied for its potential in marine aquaculture. However, the study of its bacterial community has been underreported and the mycobiota has not been investigated. We analyzed the gut bacterial and fungal profile present in the intestinal mucosa of reared adult cobias fed two diets (frozen fish pieces (FFPs) and formulated feed (FF)) for 4 months by sequencing the 16S rRNA (V3-V4) and internal transcribed spacer-2 (ITS2) regions using Illumina NovaSeq 6000. No significant differences in the alpha diversity of the bacterial community were observed, which was dominated by the phyla Proteobacteria (~96%) and Firmicutes (~1%). Cobia fed FF showed higher abundance of 10 genera, mainly UCG-002 (Family Oscillospiraceae) and Faecalibacterium, compared to cobia fed FFPs, which showed higher abundance of 7 genera, mainly Methylobacterium-Methylorubrum and Cutibacterium. The inferred bacterial functions were related to metabolism, environmental information processing and cellular processes; and no differences were found between diets. In mycobiota, no differences were observed in the diversity and composition of cobia fed the two diets. The mycobiota was dominated by the phyla Ascomycota (~88%) and Basidiomycota (~11%). This is the first study to describe the gut bacterial and fungal communities in cobia reared under captive conditions and fed on different diets and to identify the genus Ascobulus as a new member of the core fish mycobiota.

The median lethal concentration (LC₅₀) of unionized ammonia (NH₃‐N) in longfin yellowtail, Seriola rivoliana juveniles was assessed after 96 h of exposure using a semistatic water system. Experimental fish were exposed in triplicate 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. Additionally, a control group (0.00 ± 0.00 NH₃‐N mg/L) was included. The 96 h LC₅₀ of unionized ammonia in S. rivoliana was 0.58 mg/L. Fish exposed to different ammonia concentrations for 96 h displayed several lesions on gill tissues, that is, hyperplasia, epithelial lifting, fusion of the secondary lamellae, and some irreversible damage. Erratic behavior, such as swimming in circles and hyperventilation was observed before the fish died. The safe concentration of unionized ammonia in this study was estimated at 0.06 and 1.68 mg/L of total ammonia nitrogen (TAN). The outcomes observed in this study provide valuable insights into unionized ammonia effects for better water quality management in semi‐intensive and intensive systems for S. rivoliana culture.

Aquaculture is a productive activity in economic growth due to market productivity which supports many Ecuadorian communities. However, exotic species has been the basis of this activity without considering the endemic and mega-diverse component of Ecuador. On the other hand, current aquaculture systems use a lot of water, generating discharge of nitrogen and phosphorus, causing a negative impact on natural watersheds. The present investigation focused on the diversification of aquaculture production and environmental care, through the polyculture of tilapia and river prawn, in a Biofloc system, which allows reducing the use of water, space, and food, taking advantage of all the nutrients of the aquatic environment without generating pollutant residues, from a carbon-nitrogen ratio of 20:1. The research project was developed under laboratory conditions at 2.728 amsl, for 8 weeks in pre-breading stages. Two factors were established in the study: type of culture (monoculture and polyculture) and production systems (traditional and biofloc), with three replicates. In the Biofloc system, the environmental variables such as temperature, pH, and oxygen did not show any significant differences between treatments (p≤0.05), while conductivity and total suspended solids increased during the test, and the nitrification processes stabilized at day 28 with values of 0.06±0.01 and 0.35±0.09 ppm in ammonium and nitrite respectively. The bacterial load was higher in the Biofloc system (113.66x103 CFU/ml) with predominance of Bacillus sp. and Pseudomonas sp. The bioflocs showed greater volume in polyculture (17.62 ml/L) and 26% of protein. Under these considerations, the best productive performance was detected in the Biofloc polyculture with a total productivity of 142.86g/m2 (p≤0.05). Therefore, diversification of species, types and culture systems are viable for aquaculture in Ecuador, generating an interaction between the chemical and biological components, allowing productive processes to be sustainable.