Development and Evaluation of a Volumetric Quantification Method for Fecal Particle Size Classification in Rainbow Trout Fed Different Diets

Alternative plant protein sources, such as soybean meal, can interfere with digestion due to the presence of anti-nutritional factors that cause a diarrhea-like condition in Rainbow Trout Oncorhynchus mykiss, resulting in very fine fecal particles that can adversely affect aquaculture systems and the environment. More research is needed to examine the impacts of feed formulation changes on fecal quality. The prevailing method (laser diffraction) used to measure fecal particle size (FPS) requires expensive equipment and is labor intensive. Methods for FPS measurement that are affordable and easy to implement are required. We developed a visual, volumetric method that can separate feces into three particle size-classes. Feces were collected from tanks fitted with radial flow separators, transferred to Imhoff cones, and allowed to settle. Three distinct layers were visually identified, and microscopic analysis was used to categorize them into three particle size-classes: larger than 1.2 mm (bottom layer or large particles), 0.6-1.2 mm (middle layer or mid-sized particles), and 0.5 mm or smaller (top layer or fines). Independent observations confirmed that the method is reproducible and without observer bias. In a second study, the method was validated and further tested during a 4-week trial evaluating the effects of three diets (fish meal, plant based, or plant based + guar gum) and two feeding methods (hand feeding to satiation or automated belt feeding) on FPS production. Diets were isonitrogenous and isoenergetic, containing protein at 40 g/kg and lipid at 20 g/kg. Addition of guar gum significantly reduced the production of fines and increased the production of large fecal particles in Rainbow Trout (150 g/fish) that were fed the plant-based diet, but feeding method had no effect on FPS.