First published at the 18th ISFNF conference (International Symposium on Fish Nutrition and Feeding), in Las Palmas de Gran Canaria, Spain, from 3rd to 7th June 2018.
Global production of farmed tilapia in more than 140 countries exceeded 5.7 million t in 2017, an increase of almost 130 percent in ten years. This tremendous growth in production requires high-quality fish feeds. Dietary organic acids, and especially potassium diformate (KDF) – the most widely tested organic acid salt in aquaculture, are among the various alternatives spearheading the drive towards environmentally friendly and nutritionally-sustainable aquaculture, which improves performance and survival of fish. Dietary KDF has been used in tilapia aquaculture since 2005. Trials on its antimicrobial impact (e.g. against Vibrio spp. or Streptococcus agalactiae) were previously mainly carried out under research conditions only. This study is among the first to analyse the impact of the additive on juvenile tilapia in commercial cage culture under challenged grow-out conditions in Northern Malaysia. That area had been previously reported to be a location of mass mortalities of tilapia, caused by S. agalactiae.
Material and methods: A trial was carried out to observe the impact of KDF on fish mortality, caused by higher bacterial levels in the water, among them S. agalactiae, using Nile tilapia, Oreochromis niloticus, of approx. 450 g live weight. Water temperature of the lake was 30°C throughout the trial. Two cage systems, located next to each other (each HDPE cage with a volume of ~1885 m3 ) were used for the study. One system served as the negative control, whereas fish in the other system were fed the commercial diet (31% CP) which included 0.4% KDF. The duration of the trial was 60 days; monitoring of mortality (number of fish and biomass) started on day 27 after feeding the additive. Data were subjected to statistical analysis and a significance level of 0.05 was used in all tests.
Results and discussion: Tilapia fed with 0.4% KDF had significantly lower mortality numbers compared to the control (P=0.023) – as measured over a period of 33 days (Fig. 1). Furthermore, the overall biomass lost due to mortality was also significantly reduced (P=0.005) due to the inclusion of KDF (Fig. 2). The weight of the dead fish did not differ between control and treatment (P=0.78).
The results show that under commercial conditions in Malaysia, survival rates in tilapia fed with dietary potassium diformate (traded as Aquaform, ADDCON) are improved during bacterial challenge as measured by mortality numbers and biomass, confirming data from laboratory trials.