Method

OWI selection process

This protocol selected OWIs through a comprehensive analysis of existing OWI frameworks and materials, a complete list of which is in the Appendix under ‘OWI selection’.

The selection process involved:

• Systematic review of indicators used in established protocols (e.g., Laksvel for Atlantic salmon and Nofima’s ‘Welfare indicators for farmed rainbow trout: tools for assessing fish welfare’).¹⁶
• Review of other materials considering operational welfare indicators, including those published by industry.
• Analysis of certification requirements from major schemes (e.g., ASC, RSPCA Assured).
• Consideration of commercial need for the indicator to be included, scientific validity, practical feasibility, and welfare relevance for rainbow trout.

Threshold development process

Scoring thresholds were aligned with Laksvel as many rainbow trout producers are already using the Laksvel system, making it easier for producers to adopt this protocol. Threshold recommendations for each OWI were developed through an indicator-specific systematic review of the current scientific literature, prioritizing evidence from ongrowing rainbow trout in commercial seawater conditions. Where such evidence was limited (which was often the case), the analysis incorporated evidence from different production systems, laboratory settings, alternative life stages, or other salmonids, as well as expert opinions.

Each paper was evaluated based on the following criteria:

• Quality: Study design, sample size, peer review status, etc.
• Indicator relevance: How directly the study investigated the impact of a specific indicator on welfare.
• Context: How closely the study context and findings apply to ongrowing rainbow trout in commercial seawater production, considering laboratory/commercial conditions, life stage, environmental conditions (e.g., stocking densities), and species.

Papers were qualitatively graded as high, moderate, low, or very low according to the quality and relevance of each study (considering factors such as study design, sample size, duration, and applicability to commercial seawater trout) with stronger and more relevant evidence given greater weight in setting the threshold recommendations. This approach ensures the recommendations are grounded in the best available science while providing practical guidance where specific knowledge gaps exist.