Faecal characteristics such as size, structure and consistency are widely recognized as indicators of gastrointestinal health and digestive functioning in humans and terrestrial animals. The Bristol Stool Form Chart (BSFC) has long served as a reliable method for classifying stool types in humans. Similarly for various terrestrial animals, including cattle, pigs and pets as well, faecal scoring systems have been developed for assessing the prevalence of diarrhea and other digestive disorders (Larson et al., 1977; Heaton and Thompson, 1999; Meyer et al., 1999; Pons et al., 2025). However, the variation in faeces types in terms of characteristics such as colour, shape, texture, composition remain unknown for fish species.
Recent studies suggest that dietary factors, such as ingredient and nutrient composition of diet as well as environment factors impact the characteristics of fish faeces. However, variation in faeces types in terms of characteristics such as colour, shape, texture and composition remains unknown. This study extends the concept of faecal scoring system to aquaculture by proposing a classification to describe faeces types of saline water reared rainbow trout (Oncorhynchus mykiss) as an example. Faeces used in this study were collected overnight by sedimentation. Fish were fed with 2 diets varying in protein to energy ratios: namely high protein to energy ratio and low protein to energy ratio diet. Collected faeces were categorized into four distinct types based on physical characteristics such as size, structure, colour and consistency. Identified faeces types were categorized as “mucosal envelopes”, “mineral precipitates”, “intermediate faeces”, and “feed-related faeces” (Fig. 1). The mineral composition and ratios were found to be different in different faeces types (P < 0.001). Stereomicroscopic observation revealed that all faeces types had a double layered slimy coating on its surface. To understand the chemical nature of mucoidal coating, faecal pellets of varying morphology were randomly selected and fixed overnight at 4 ºC in Carnoy’s fixative (60% ethanol, 30% chloroform, 10% glacial acetic acid). Fixed samples were embedded in paraffin wax following standard histological procedures. Faecal pellets were positioned in longitudinal or transverse position and 6 µm thick faecal sections were cut using a microtome. Sections were deparaffinised with xylene and subsequently passed through a graded ethanol-water series beginning with anhydrous ethanol. Staining was done with Alcian blue, Periodic acid and Schiff’s reagent followed by Crossman’s trichrome (Mason) staining to visualise mucoidal covering over the faeces and the undigested matter trapped in the faecal matrix. Sections were viewed under Leica-DM6 inverted microscope using ×5 and ×10 objectives. Images of faecal pellets were captured with a DFC450C CCD colour camera (Leica Microsystems) and composite images were generated using datasets of 50 to 500 overlapping microscope views.
Based on the PAS-Alcian blue-Crossman staining of feed-related faeces, slimy coating were confirmed to be of mucoidal origin. The role of the mucoidal coating in determining the size, structure and consistency is discussed. Implications of the finding are discussed in the context of relevance to fish physiology and practical aquaculture.