Biological control of streptococcal infection in Nile tilapia Oreochromis niloticus (Linnaeus, 1758) using filter-feeding bivalve mussel Pilsbryoconcha exilis (Lea, 1838)

Abstract

Summary Since bivalve mussels are able to graze heavily on bacteria, in this paper it is hypothesized that when mussels are cultured with fish, the filtering efficiency of the mussels will keep the bacterial population below a certain threshold and thus assist in reducing the risk of bacterial disease outbreaks. The ability of the filter-feeding bivalve mussel Pilsbryoconcha exilis to control Streptococcus agalactiae was tested in a laboratory-scale tilapia culture system. Juvenile Nile tilapia (Oreochromis niloticus), the bivalve mussel as well as the bacteria were cultured at different combinations using four treatments: treatment-1: mussel and bacteria but no fish, treatment-2: tilapia and mussel but no bacteria, treatment-3: tilapia and bacteria but no mussel, and treatment-4: tilapia, mussels, and bacteria. All treatments were run in three replicates; stocking rates were 10 tilapia juveniles; five mussels; and about 3.5 × 105 colony forming units (CFU) ml−1 of bacteria in 50-L aquaria with 40-L volume. The mussel reduced the bacterial population by 83.6–87.1% in a 3-week period whereas in the absence of the mussel, the bacterial counts increased by 31.5%. Oresence of the mussel also resulted in significantly higher growth and lower mortality of tilapia juveniles than when the mussel was absent. The results of this experiment suggest that the freshwater mussel P. exilis could control the population of S. agalactiae in a laboratory-scale tilapia culture system. Future studies should focus on the dynamic interactions among fish, mussels, and bacteria as well as on how input such as feed and other organic materials affect these interactions.