Evaluation of the effects of repeated fecal collection by manual stripping on the plasma cortisol levels, TNF-α gene expression, and digestibility and availability of nutrients from hydrolyzed poultry and egg meal by rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract

The determination of apparent nutrient digestibility and availability coefficients (ADC and AAC) of ingredients is an important process in the formulation of cost efficient diets for fish. Collection of feces by repeated manual stripping is commonly practiced. However, due to the stress and possible physical damage imposed on fish during repeated stripping, this practice may influence ADC and AAC values. Our study evaluates the effects of repeated stripping three times over a period of five days on the ADCs and AACs of nutrients from hydrolyzed whole poultry egg meal for rainbow trout. We also evaluated the effects of repeated stripping on the stress status (plasma cortisol), innate immune response (tumor necrosis factor-α (TNF-α) gene expression) and physical damage of the distal section of the digestive tract (histological examination) of rainbow trout (mean weight 308 g). Plasma cortisol levels were significantly elevated at the completion of each stripping event, indicating the procedure was stressful to rainbow trout. While the gene expression of TNF-α indicated a compounding chronic effect of the repeated stripping procedure on the innate immune response of rainbow trout. However, microscopic histological examination of the distal section of the digestive tract indicated no apparent sign of physical damage due to the repeated fecal stripping procedure. Dietary ADCs and AACs for dry matter, organic matter, protein, amino acids and energy were unaffected by repeated stripping. While ingredient ADCs and AACs for dry matter, organic matter, protein, amino acids and lipid were also unaffected by repeated stripping. However, dietary lipid and ingredient energy ADCs were significantly affected by repeated stripping. Additionally, the repeated stripping significantly (P<0.05) affected the dietary and ingredient AACs for minerals, suggesting that although the collection of feces by repeated manual stripping may be a useful method for the determination of protein ADCs and amino acid AACs, for lipid, energy ADCs and mineral AACs single stripping is a more accurate alternative.