Ž . Aquaculture 194 2001 93–98 www.elsevier.nlrlocateraqua-online Acid-insoluble ash as an inert reference material for digestibility studies in tilapia, Oreochromis aureus J.S. Goddard ) , E. McLean Department of Marine Science and Fisheries, College of Agriculture, Sultan Qaboos UniÕersity, PO Box 34, Al-Khod 123, Muscat, Oman Received 28 April 2000; received in revised form 15 August 2000; accepted 15 August 2000 Abstract Ž . Acid-insoluble ash AIA was evaluated as an inert marker material for digestibility studies in Ž . tilapia. No significant differences were found in apparent digestibility coefficients ADC for nitrogen, dry matter or gross energy determined in a practical diet using naturally occurring AIA, Ž . or AIA supplemented with Celitee 0.5%, 1.0% of the diet , compared with values obtained using Ž . chromic oxide 0.5% . The use of supplementary Celite increased the accuracy of measurement of Ž . AIA in feed samples. Acid-washed sand 0.5% gave significantly lower ADC values than chromic oxide. As an alternative to using chromic oxide in ADC measurements, Celite supple- mented AIA is inexpensive, easy to measure and gave comparable results in the present study with tilapia. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Digestibility; Acid-insoluble ash; Celite; Oreochromis aureus 1. Introduction Ž . Measurement of apparent digestibility coefficients ADC of feed ingredients is an Ž important step in the evaluation and formulation of balanced feeds Cho and Kaushik, . 1990 . Digestibility is a measure of the quantity of ingested nutrients retained and is most commonly measured by indirect methods using inert marker materials. By adding Ž . an inert material external marker to the feed or measuring an inert natural component Ž . of the food internal marker , digestibility can be calculated by comparing the ratio of the marker in the food and faeces to a specific nutrient. To be effective, a marker must ) Corresponding author. Tel.: q 968-515-254; fax: q 968-513-418. Ž . E-mail address: sgoddard@squ.edu.om J.S. Goddard . 0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0044-8486 00 00499-3