Development and evaluation of a volumetric quantification method for fecal particle size classification in rainbow trout fed different diets

Authors: Welker, Thomas; Overturf, Kenneth - Ken; Barrows, Frederic

Submitted to: North American Journal of Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2019
Publication Date: 1/10/2020
Citation: Welker, T.L., Overturf, K.E., Barrows, F. 2020. Development and evaluation of a volumetric quantification method for fecal particle size classification in rainbow trout fed different diets. North American Journal of Aquaculture. 82(2):159-168. https://doi.org/10.1002/naaq.10138.
DOI: https://doi.org/10.1002/naaq.10138

Interpretive Summary: Replacement of fishmeal with plant-protein sources, such as soybean meal, can interfere with digestion and cause a diarrhea in rainbow trout characterized by very fine fecal particles. These fines do not settle out of rearing water and cannot be collected from raceway effluent. An increase in dissolved solids and other nutrients may result and negatively impact receiving waters. Despite these negative effects, research on the influence of plant-protein sources and feed processing on fecal quality is lacking. The prevailing method for examination of feces quality (laser diffraction) requires expensive equipment and is labor intensive. Methods to measure feces quality that are affordable and easy to implement in a research setting are needed. We developed a visual, volumetric method that can separate feces into three particle-size classes. Feces were collected from tanks, transferred to a volumetric vessel (Imhoff cone), and allowed to settle. Three distinct layers were visually identified and categorized into three particle size classes: > 1.2 mm (bottom layer or large particles), 1.2 to 0.6 mm (middle layer or mids), and = 0.5 mm (top layer or fines). The method was further tested by evaluating the effects of three diets (fishmeal, plant-based, and plant-based + guar gum) and feeding method (hand-fed to satiation and automated feeding) on fecal quality. Addition of guar gum significantly reduced the production of fines and increased large fecal particles, but feeding method had no effect on particle size distribution.

Technical Abstract: 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.