Introduction

Rainbow trout, like other teleost fish, are animals capable of experiencing pain and stress. Ensuring their welfare during production requires systematic monitoring through Operational Welfare Indicators (OWIs). The increasing focus on OWI implementation reflects priorities shared across the aquaculture value chain — from producers, certification bodies, and seafood buyers, to consumers, governments, and NGOs — and aligns with broader international standards for health monitoring (for example, see Article 4.1.9(2)(e) of the Aquatic Animal Health Code published by the World Organisation for Animal Health). By improving welfare outcomes, producers simultaneously achieve better growth, survival, robustness, and production efficiency, while meeting regulatory requirements and evolving consumer expectations.

This protocol aims to provide a standardized, systematic framework for real-world commercial use, built upon the strongest available scientific evidence to support both on-farm management and compliance with key certification requirements. It is intended as a strong monitoring foundation that complements — rather than replaces — broader welfare requirements set by certification schemes, legislation, or other standards at system level.

These guidelines were primarily developed for ongrowing rainbow trout in seawater. Users can apply this protocol to other life stages or production systems, though application in these contexts requires careful interpretation. The indicators and scoring levels in this protocol describe welfare risk rather than welfare state. Because the internal experience of the fish cannot be directly observed, and because the relationship between any single indicator and actual welfare depends on the production context and individual fish history and physiology, scores should be understood as structured assessments of the likelihood of welfare compromise as opposed to definitive welfare judgements.

This document is a living document that will be updated and refined over time as rainbow trout welfare science matures.