Universiti Malaysia Terengganu Journal of Undergraduate Research Volume 2 Number 1, January 2020: 15-20 Universiti Malaysia Terengganu Journal of Undergraduate Research Volume 2 Number 1, January 2020: 15-20 eISSN: 2637-1138 © Penerbit UMT THE EFFECT OF LOW PH LEVEL ON ZEBRAFISH (Danio rerio) REPRODUCTIVE PERFORMANCE IKRAM ISMAIL A AND SITI-ARIZA ARIPIN B* a,b Faculty of Fisheries and Food Science, Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia *Corresponding author: siti.ariza@umt.edu.my Danio rerio or commonly known as zebrafish are a very popular fish among scientists and also a well-known vertebrate model species widely used in research. Zebrafish, are also a popular species among aquarists and have been put in aquariums all around the world as ornamental fish. The acid rain phenomenon has lowered the pH level of the wild habitat of zebrafish by shifting it to a more acidic pH level. This study was carried out to observe the effect of low pH level on the reproductive performance of zebrafish. The zebrafish were quarantined for a week to make sure they were healthy to be used in the experiment. The zebrafish were reared continuously for 14 days in three different pH treatments T1 (pH 2-4), T2 (pH 4-6), T3 (pH 6-8)). T3 (pH 6-8) was used as the control treatment. Hydrochloric acid (HCl) was used to control the pH level of treatments T1 (pH 2-4), T2 (pH 4-6), T3 (pH 6-8) with three replicates of each treatment. The male chasing female frequency was significant (p: 0.0001) and the data showed the highest frequency (2568.000±140.6272) at treatment 3 (pH 6- 8). For the spawning frequency of zebrafish, treatment 3 (pH 6-8) showed the highest value (4.000±0.5774) followed by treatment 2 and treatment 1 and the data was significant (p: 0.0004). The fertilisation rate of the zebrafish was significant (p: 0.0001) and the highest was shown at T2 (pH 4-6) with 89.8018±0.3782, followed by T3 and treatment T1. For the hatching rate of the zebrafish, the data collected were significant (p: 0.0002) and the highest value of 2.9350±0.4070 was shown at T3 (pH 6-8), followed by T2 (pH 4-6) and T1 (pH 2-4). The overall result showed that pH 2-4 had the worst effect on the reproductive performance of zebrafish. Therefore, low pH has a significant effect on reducing the reproductive performance of zebrafish. The local fish population can be affected by the decrease of pH level due to acid rains and chemical waste pollution. Keywords: zebrafish, pH, spawning frequency, fertilisation rate, hatching rate Introduction The zebrafish, Danio rerio (Hamilton, 1822), are freshwater fish native to Southeast Asia. The zebrafish are in the same family with carps and minnows and found in areas in the Himalayan region, including Myanmar, Pakistan, India, and Nepal (Kinth et al., 2013). Danio rerio are also a good model species for vertebrate and have been used widely in research (Hoo et al., 2016). Research using Danio rerio have yielded advances in many fields such as biology, toxicology, reproductive studies (Hoo et al., 2016), genetics, and environmental science. This species has been a good biological model system that has numerous advantages for scientists (Graham et al., 2018). The pH drops in natural habitats usually happen due to acid rains and chemical waste pollution. Diluted sulphuric or nitric acids are falling everywhere on the globe. In chemical terms, acid rain has a pH of less than 5.6 (Rogers, 2018). In some countries, the pH of acid rain can be as low as 4.2. The more acidic the water is, a lesser number of embryos can survive and hatch (Poppe, 2017). According to Poppe (2017), the number of embryos that survive and hatch decreases as the pH decreases. The release of chemical waste into the environment has also decreased the water pH in wild habitats and affected the aquatic animals and human health. In Malaysia, the pollution of chemical waste reported at Sg Kim Kim, Johor Bharu