Online First Article Phytase as a Feed Additive: Impact on Growth, Nutrient Digestibility and Carcass Composition of Catla catla Fingerlings Fed on Different Plant By-Products Based Diets Ambreen Sabir 1 , Muhammad Zubair ul Hassan Arsalan 1 , Tanveer Ahmed 1 , Syed Makhdoom Hussain 2 *, Shafaqat Ali 3,4 **, Aumme Adeeba Bano 2 , Muhammad Wasim Aslam 1 , Huma Naz 5 , Sadia Tabassum 2 , Muhammad Amjad 2 , Adan Naeem 2 and Eman Naeem 2 1 Department of Life Sciences, Khwaja Fareed University of Engineering and Information Technology, Rahim Yar Khan, Pakistan. 2 Fish Nutrition Laboratory, Department of Zoology, Government College University, Faisalabad, Pakistan 3 Department of Environmental Sciences, Government College University, Faisalabad, Pakistan 4 Department of Biological Sciences and Technology, China Medical University, Taichung 40402, Taiwan 5 Department of Zoology, Cholistan University of Veterinary and Animal Sciences, Bahawalpur, Pakistan Article Information Received 30 July 2024 Revised 15 September 2024 Accepted 23 September 2024 Available online 16 January 2025 (early access) Authors’ Contribution Writing-original draft: AS. Conceptualization, data curation, methodology, supervision: MZHA. Investigation, formal analysis, data curation: TA, SMH, SA. Writing- review and editing, data curation: AAB, MWA, HN, ST, MA. Data curation, writing-review and editing: AN, EN. All authors read and approved the final manuscript. Key words Plant by-products, Phytase, Catla catla, Nutrients digestibility, Carcass composition, Growth performance Owing to existence of anti-nutritional factors (ANFs) such as phytate in diets based on plant by-products, fish show poor growth and nutrition retention in their bodies because less minerals and nutrients are available to them. To overcome this problem, phytase was supplemented with different plant by-products based diets and impact was checked on digestibility of nutrients, growth performance and carcass composition of Catla catla fingerlings (average weight= 6.10±0.01 g), in a feeding trial lasting seventy days. The fingerlings were fed a variety of diets that were based on different plant by-products with phytase supplementation at levels of 0, 250 and 500 FTU Kg -1 , in three-factorial configurations with a completely random design in triplicates. Chromium oxide was added to test diets at a concentration of 1% as an inert marker. Diets were served twice a day, @ 4% of body weight of fingerlings. Feces from every fish tank were gathered twice a day to measure nutrients digestion. Phytase supplementation into the test diets considerably (p<0.05) enhanced the growth rate, nutrient digestibility (including crude protein, crude fat, and gross energy), and carcass composition of C. catla. The findings showed that the test diet with 50% MOLM and 500 FTU Kg -1 phytase exhibited the best growth performance, nutrient digestibility, and carcass composition. Additionally, it was discovered that phytase supplementation lowered the nutrient discharge via feces, hence reducing the amount of contaminants in the water. In conclusion, adding phytase at a level of 500 FTU Kg -1 was favorable for using with different plant by- products to create an inexpensive and ecologically friendly fish diet. * Corresponding author: drmakhdoomhussain@gcuf.edu.pk, shafaqataligill@gcuf.edu.pk 0030-9923/2025/0001-0001 $ 9.00/0 Copyright 2025 by the authors. Licensee Zoological Society of Pakistan. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). INTRODUCTION I t is estimated that by 2030, there will be almost 8.5 billion people on the earth and global food security and sustainable development will be considerably supported by aquaculture, which is estimated to supply roughly 53% of the seafood consumed worldwide (FAO, 2022; UN DESA, 2023). By 2025, the world’s aquaculture output of fed species is expected to be 58.9 million tons, and the demand for aqua feed is expected to reach 69.5 ABSTRACT Pakistan J. Zool., pp 1-9, 2025. DOI: https://dx.doi.org/10.17582/journal.pjz/20240730064447