Alternate Day Feeding as a Cost-effective Strategy for Tank Culture of the Pacific Shortfin Eel Anguilla bicolor pacifica Frolan A. Aya 1,*, John Carlo L. Unida 1 , Maria Rowena R. Romana-Eguia 1,2 , and Nerissa D. Salayo 3 1 Aquaculture Department, Southeast Asian Fisheries Development Center, Binangonan Freshwater Station, Binangonan, Rizal 1940, Philippines 2 Biology Department, College of Science, De La Salle University, 2401 Taft Avenue, Malate, Manila 0922, Philippines 3 Aquaculture Department, Southeast Asian Fisheries Development Center, Tigbauan Main Station, Tigbauan, Iloilo 5021, Philippines * Author for correspondence; E-mail: faya@seafdec.org.ph; Tel./Fax: (63-2) 9277825 Received: November 21, 2022/ Revised: February 19, 2023/ Accepted: June 06, 2023 There is a growing interest in the aquaculture of tropical anguillid eels as an export commodity. However, studies on feeding strategies, and the present demand to reduce feed costs need to be addressed to ensure the economic viability of eel farming. In this study, the effects of daily (DF) and alternate day (ADF) feeding on growth, feed utilization, body composition, blood chemistry, liver and intestinal morphology, and economic viability in the Pacific shortfin eel Anguilla bicolor pacifica were examined. Each feeding group of 30 elvers (166.25 ± 16.23 g mean initial wt) were randomly stocked in triplicate 4 m 3 outdoor concrete tanks. These were fed for 155 d with formulated eel powder diet (49.77% crude protein; 10.21% crude lipid) made into a paste. Growth and survival were not significantly different between the two feeding groups. However, feed efficiency was improved in the ADF group, with significantly higher protein efficiency ratio (0.63) than in the DF group (0.39). Feed conversion ratio (FCR) was lower in ADF (3.85) than in the DF group (6.44), resulting in a 40% reduction in total feed consumption. Biometric indices, body proximate composition, and blood chemistry were not significantly affected. Likewise, liver and intestinal morphology showed no apparent alterations between groups. Partial costs-and-returns analysis showed that ADF yielded higher net profit and profit index. These results suggest that feeding on alternate days promote compensatory growth, better feed utilization, and normal physiological condition of A. bicolor pacifica, and reduce feed cost in the rearing process. Therefore, alternate day feeding should be promoted as a sound feed management strategy in the tank culture of tropical anguillid eels. Keywords: Blood chemistry; feeding regimes; growth metrics; proximate composition Abbreviations: ADF—alternate day feeding, DF—daily feeding, DFI—daily feed intake, DO—dissolved oxygen, DTI— digestive tract index, FCR—feed conversion ratio, HSI—hepatosomatic index, L/LBM—lipid to lean body mass ratio, PER—protein efficiency ratio, RGL—relative gut length, SEAFDEC/AQD—Southeast Asian Fisheries Development Center, Aquaculture Department, VSI—Viscerosomatic index INTRODUCTION The freshwater eels of the genus Anguilla are composed of 19 species/subspecies (Froese and Pauly 2022), but only five temperate species (American eel A. rostrata, European eel A. Anguilla, Japanese eel A. japonica, Australian shortfin eel A. australis, and Australian longfin eel A. dieffenbachii) are known to be cultured. In the Philippines, seven different anguillid eel species (A. japonica, A. celebesensis, A. marmorata, A. interioris, A. bicolor bicolor, A. bicolor pacifica, A. luzonensis) have been documented to date (Briones et al. 2007; Jamandre et al. 2007; Teng et al. 2009; Watanabe et al. 2009; Han et al. 2012; Aoyama et al. 2015). Anecdotal reports of the nascent farming of Philippine native eels are adopted mainly from the culture of Japanese and European eel species. Among the tropical eel species, the Pacific shortfin eel A. bicolor pacifica is the most preferred because of its tender and ISSN 0031-7454 PHILIPP AGRIC SCIENTIST Vol. 106 No. 3, 281-292 September 2023 hps://pas.cafs.uplb.edu.ph