Comparison of acid insoluble ash with yttrium oxide for determining apparent nutrient digestibility in rainbow trout and hybrid striped bass with feces collection by stripping

Authors: Liu, Keshun; Sealey, Wendy; Rawles, Steven; Welker, Thomas; GAYLORD, GIBSON - Department Of Fish And Wildlife

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2026
Publication Date: 1/20/2026
Citation: Liu, K., Sealey, W.M., Rawles, S.D., Welker, T.L., Gaylord, G. 2026. Comparison of acid insoluble ash with yttrium oxide for determining apparent nutrient digestibility in rainbow trout and hybrid striped bass with feces collection by stripping. Aquaculture. 615, 743620. https://doi.org/10.1016/j.aquaculture.2026.743620.
DOI: https://doi.org/10.1016/j.aquaculture.2026.743620

Interpretive Summary: Digestibility determination of feed ingredients is very important for developing cost-effective diet formulations, optimizing animal growth, and limiting waste excretion into the environment. In aquaculture, a marker retention method, because it allows measurement of single feed and fecal samples, is preferred over a total collection method. Acid insoluble ash (AIA) is a fraction of ash in a biological material, consisting mainly of silica. AIA, either naturally present in feedstuffs or by supplementation with commercial silica products, has been widely used as an internal and/or external marker in nutritional studies with various animals, including terrestrial and farmed fish. However, conflicting results have been found among digestibility studies with various fish species, when using AIA as a marker. Unreliable analysis of AIA has been suggested as a possible cause. A few years ago, USDA-ARS scientists at Aberdeen, Idaho, developed a novel centrifugation-based method for measuring AIA, which is more efficient, sensitive, and repeatable than reported methods, Recently with this new method, the same group of ARS scientists compared AIA with yttrium oxide as markers for determining apparent nutrient digestibility in several protein ingredients fed to both rainbow trout and hybrid striped bass. For the first time, they found that the combination of using yttrium oxide as a marker and stripping for fecal collection was effective for apparent digestibility coefficient determination, but the combination of AIA as a marker and stripping for fecal collection was not, even with improved AIA analysis. The study pointed out that the slower movement of AIA than nutrient digesta in the fish digestive tract, and its aggravation by using stripping for fecal collection are likely the causes. Therefore, the study recommends that in determining apparent nutrient digestibility in fish, one needs to consider the effects of inert markers and fecal collection methods and their interactions.

Technical Abstract: Acid insoluble ash (AIA) has been used as a marker for nutrient digestibility determination in various animals with conflicting results. Lack of reliability for AIA analysis has been hypothesized as a cause. For validation, the present study compared AIA with yttrium oxide for determining apparent digestibility coefficients (ADCs) of nutrients in rainbow trout and hybrid striped bass, using a newly developed method for AIA measurement. Twelve diets were made by spiking a reference mixture with Y2O3 at 0.1% and Celite at 0, 0.3 and 0.6%, and mixing each with soybean meal, distillers' dried grains with solubles, or fishmeal at a 70:30 ratio, respectively. Trout and bass were grown in 36 tanks, three tanks per diet. Feces were collected by stripping. With both fish species, ADCs of dry matter, protein, fat, energy and phosphorus, calculated by yttrium content, were consistent and realistic. However, ADCs by AIA were significantly lower and variable, with many being negative, even though the issue with AIA analysis had been addressed. Further investigation revealed that yttrium content as % total ash in feces was higher than that in diets, but AIA as % total ash in feces was lower than that in diets. The observed low or negative ADCs by AIA likely resulted from slower AIA movement than nutrient digesta through fish gastrointestinal tracts, which was compounded by using stripping for fecal collection. Therefore, further studies incorporating methods of collecting naturally defecated feces are needed before definitive conclusions on AIA reliability as a marker can be made.