Mortality remains a major challenge in farmed Nile tilapia production, with rates in Brazil ranging from 5% to 10%. Resin acids are substances secreted by coniferous trees at sites of mechanical injury to prevent the invasion of pathogenic bacteria (Kettunen et al., 2015). These resins contain compounds such as abietic and neoabietic acids, which form a diverse group of hydrophobic diterpene carboxylic acids with antimicrobial properties. The aim of this study was to evaluate the effect of resin acids in Nile tilapia feed on survival rate, biomass growth, and real-time PCR analysis of ISKNV/RSIV (Infectious Spleen and Kidney Necrosis Virus/Red Sea bream IridoVirus ) and Streptococcus agalactiae in spleen, liver, and kidney tissues.
For this trial, evaluations were conducted at the experimental facilities of a Nile tilapia farm in Brazil in March 2025. The study involved two experimental treatments: CON – fish fed a commercial diet containing 50% crude protein; RAC – fish fed the same diet as the CON group, supplemented with 2 kg/t of Progres® (9% Resin Acid feed additive, AB Vista, UK). A total of 160,000 fish were used per treatment. Fish were distributed across two tanks, and the experimental period lasted 20 days. The tanks were located in an area confirmed positive for Streptococcus agalactiae. Performance parameters, including biomass weight gain, survival rate, feed consumption, and real-time PCR for ISKNV/RSIV and S. agalactiae, were evaluated. For the PCR analysis, 30 fish per treatment were collected on day 20. The fish were necropsied, and spleen, kidney, and liver tissues were homogenized in pools of 5 fish each, resulting in six evaluations per treatment. Table 1 presents the survival and weight gain results of the animals. It can be observed that the survival rate in the RAC group was 3.1% higher than in the CON group. Another observation is that the animals in the RAC group started the study with a biomass weight 25% lower than the CON group; however, after 20 days, the difference in the RAC group was reduced to only 10.7%. Table 2 presents the molecular biology data across treatments.
Fish that received RAC showed a 25% lower positivity for ISKNV/RSIV compared to the CON group. Similarly, with respect to S. agalactiae, animals that consumed RAC showed a 20% lower positivity compared to the CON group
In conclusion, the use of resin acids serve as tool to improve biomass weight and survival rate, while reducing the positivity rates for ISKNV/RSIV and S. agalactiae PCR in Nile tilapia.