Does pellet size influence the digestive process in salmon?

In modern aquaculture, success lies not only in what the fish eats but in how it processes it. Gastrointestinal Transit Time (GTT) is the “hidden stopwatch” that determines whether nutrients are converted into biomass or end up as waste on the seabed. Excessively rapid transit prevents enzymes from acting, wasting costly ingredients such as fishmeal.

A team of researchers from Deakin University and Ridley AgriProducts has published a comprehensive study in Aquaculture Nutrition regarding how raw material particle size and final pellet diameter modulate the digestive system of Atlantic salmon (Salmo salar).

Key Points

• Effect of pellet size: Larger pellets (5 mm) accelerate intestinal transit compared to 3 mm pellets.
• Strategic retention: The diet featuring fine particles and small pellets (3 mm) remains in the digestive tract the longest (42.5 h for 50% evacuation).
• Enhanced digestibility: Utilizing finely ground raw materials significantly increases the digestibility of dry matter and gross energy.
• Environmental sustainability: Adjusting transit velocity allows for the reduction of nitrogen and phosphorus waste in marine ecosystems.

The Engineering Behind the Experimental Diet

The study utilized 360 pre-smolt salmon (approx. 120 g) distributed across 18 tanks in a Recirculating Aquaculture System (RAS), with water temperatures maintained at 15°C. Six diets were formulated using a factorial design combining:

1. Raw material particle size: Fine (0-250 µm), Medium (250-500 µm), and Coarse (500-1000 µm).
2. Pellet size: 3 mm and 5 mm.

To track feed movement, scientists incorporated glass microspheres (ballotini beads) of 425-600 µm as an inert marker, allowing for the precise quantification of digesta flow in the stomach (ST), mid-gut (MI), and distal gut (DI) via microscopic analysis.

The Digestive “Tipping Point”

• The Large Pellet Paradox: Contrary to expectations regarding the hydration difficulty of larger pieces, the study found that the 5 mm pellet size resulted in faster transit. At 16 hours post-ingestion, marker density in the stomach was significantly lower in fish fed with 5 mm pellets compared to those fed 3 mm.
• $T_{50\%}$: The Efficiency Indicator: Researchers defined $T_{50\%}$ as the time required to evacuate 50% of the total intake. The results showed a notable spectrum:
  • 5 mm Coarse Diet: The fastest, reaching $T_{50\%}$ in just 22.81 hours.
  • 3 mm Fine Diet: The slowest, retaining feed for up to 42.50 hours.
• Digestibility and Grinding: Although fish growth did not vary significantly over the 35-day period, gross energy digestibility was higher in fine and medium-particle diets compared to coarse ones. This suggests that a larger surface area allows for more effective enzymatic action.

Implications for Precision Aquaculture

The relationship between pellet hardness and particle size is crucial. It was observed that hardness increased as grinding size decreased. This creates a “braking” effect in the digestive system: harder, denser pellets (typically 3 mm with fine grain) evacuate more slowly, providing more time for nutrient absorption.

Global Impact: Climate and Environment

This finding is vital in the context of thermal stress caused by climate change. High temperatures naturally accelerate salmon metabolism and intestinal transit. By formulating feed with specific physical characteristics (such as finer grinds), the industry can mechanically counteract this effect, ensuring the fish utilizes the feed even under suboptimal conditions. Furthermore, optimized transit means less phosphorus and nitrogen are released into the environment, mitigating issues such as eutrophication and the deterioration of benthic zones near farms.

Redesigning the Future of Aquafeed

The study by Miles et al. demonstrates that the physical design of feed is as powerful a management tool as its chemical composition. The ability to modulate GTT by controlling pellet size and raw material granulometry opens the door to customized diets for different growth stages and environmental conditions. In this sense, manipulating pellet physics is not merely a matter of factory engineering; it is a biological strategy to maximize salmon health and producer profitability.

Contact
David S. Francis
Nutrition and Seafood Laboratory, Deakin University
Queenscliff, Victoria, Australia
Email: d.francis@deakin.edu.au

Reference (open access)
Miles, P. C., Mock, T. S., Jago, M. K., Salini, M. J., Smullen, R. P., & Francis, D. S. (2026). Influence of the Physical Characteristics of Feed on the Digestive Processes of Atlantic Salmon, Salmo salar, Focusing on Gut Transit Time. Aquaculture Nutrition, 2026, Article ID 3269414. https://doi.org/10.1155/anu/3269414.

Milthon Lujan

Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.