Submitted to: Aquaculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/13/2000
Publication Date: 6/27/2000
Citation: SHOEMAKER, C.A., EVANS, J.J., KLESIUS, P.H. DENSITY AND DOSE: FACTORS AFFECTING MORTALITY OF STREPTOCOCCUS INIAE INFECTED TILAPIA (OREOCHROMIS NILOTICUS). AQUACULTURE. 188 (2000) 229-235 Interpretive Summary: Stocking cultured fish at high density is a common production practice, yet little is known about the impact of infectious disease in water reuse and pond production systems. The purpose of this study was to demonstrate that infectious diseases had the greatest impact at high stocking densities by either transmission of infection by water and diseased dead fish. Streptococcal diseases caused by a bacterium appears to rapidly spread in fish cultured systems. We found that stocking densities of tilapia above 0.1 lb./gallon of water significantly increased their mortality when these fish were exposed to Streptococcus infected water. We also demonstrated that fish stocked at a high density of 0.2 lb./gallon of water were easily infected with Streptococcus from infected dead fish. Fish densities used in our study were less than in current production systems in which densities range from 0.2 to 2 lbs./gallon. The study demonstrated that reducing fish densities will decrease the spread of infectious agents transmitted by water or infected dead fish. Prompt removal of dead fish and lowered stocking rates could reduce the worldwide economic losses from $150 to $50 million due to streptococcal disease.
Technical Abstract: Fish density and infectious dose have been suspected to affect the mortality rate of cultured fish exposed to Streptocuccus iniae. We determined the effect of S. iniae dose and tilapia density on streptococcal disease mortality. Density and dose were evaluated by stocking tilapia at low (5.6 g/L), medium (11.2 g/L) and high (22.4 g/L) density and administering 2.5 * 10**7, 5 * 10**7 and 1 * 10**8 colony forming units/ml of S. iniae by immersion. Mortality was monitored for 28 days post challenge. A significant difference (P is less than 0.05) was seen in mortality when comparing low (4.8%) and medium (28.4%) and low and high (25.6%) density treatments. No significant difference was observed when comparing medium and high density treatments. Two way analysis of variance demonstrated density had a significant effect on S. iniae mortality (P=0.0001). Doses had little effect on mortality, except at high density by dose which did show a significant interaction (P=0.001). We have demonstrated density has a significant effect on streptococcal disease mortality in tilapia exposed to S. iniae by immersion. We also evaluated infection of susceptible tilapia using dead/moribund S. iniae infected fish (i.e., cohabitation). No significant difference in mortality pattern was observed between immersion in 8.6 * 10**7 CFU / ml S. iniae (37.6 and 34.6%) and cohabitation with S. iniae infected tilapia (24.0%). Although, densities used were less than in most water-reuse production system (30 - 290 g/L), tilapia density of 11.2 g/L and above was an important factor in mortality of tilapia infected with S. iniae. A health management strategy would be to reduce fish density thus lowering streptococcal disease mortality.