Influence of the Physical Characteristics of Feed on the Digestive Processes of Atlantic Salmon, Salmo salar , Focusing on Gut Transit Time

). These influences can accelerate the transit of digesta, leading to reduced nutrient digestibility and uptake, whilst impacting feed intake, appetite and gut evacuation, which collectively may result in inefficient feed utilisation in commercial aquaculture. To address these issues, manipulating the physical properties of the diet, specifically pellet and raw material particle size, may present a solution via the modulation of the GTT in fish. This study aimed to evaluate the GTT of 360 pre-smolt Atlantic salmon (120.58 ± 0.25 g) fed one of six dietary treatments varying in raw material particle size (fine: 0-250 μm, medium: 250-500 μm or coarse: 500 μm-1000 μm) and subsequent pellet size (3 mm or 5 mm) using a factorial design. GTT was quantified by assessing the relative distribution of an inert feed marker (ballotini® beads), incorporated in the feed, in three defined digestive tract regions: the stomach (ST), mid-intestine (MI) and distal intestine (DI). It was found that the larger pellet size resulted in faster transit of digesta, but the relationship between raw material particle size (fine, medium or coarse) and transit rate was highly variable. However, the time taken for 50% evacuation of the digesta present in the GI tract is modulated by both pellet and raw material particle size. The insights gained from this research could guide the development of commercial diets designed to mitigate the negative impacts of an accelerated gut transit. Consequently, this approach has the potential to enhance feed efficiency from both nutritional and economic perspectives and reduce the environmental impact caused by excessive nitrogenous waste production in aquaculture.