Executive summary

To understand, track, and compare fish welfare in commercial aquaculture production systems, the industry requires Operational Welfare Indicator (OWI) protocols that build on the full available scientific literature, provide transparency into the development of their indicators, and feature practical guidance on how to score fish welfare.

This protocol is the first such tool for assessing the welfare of farmed rainbow trout (Oncorhynchus mykiss) in seawater systems, including a scoring system based on a comprehensive species- and life-stage-specific literature review for every indicator.

This protocol serves farm personnel conducting routine welfare monitoring and other industry professionals, including certification bodies, auditors, and buyers. It is designed to align with the welfare monitoring and assessment requirements of key certifications, such as RSPCA Assured and Aquaculture Stewardship Council.

This protocol enables farm personnel to systematically monitor the welfare of their stock, identify potential issues early, and implement corrective actions. This proactive approach helps producers understand the relationship between welfare and production performance, resulting in reduced losses, improved regulatory compliance, strengthened consumer trust, and a stronger and more sustainable aquaculture industry.

Scope

Developed specifically for ongrowing rainbow trout in seawater, this protocol may be applied to juveniles or other production systems with careful interpretation.

While practical guidance on OWIs has been developed for other species, such as the Laksvel protocol for Atlantic salmon,1 and valuable documentation on rainbow trout welfare has been published by Nofima2 and APROMAR,3 existing resources lack comprehensive, standardized, and threshold-based assessment across a wide range of indicators for rainbow trout producers. This protocol addresses that gap.

This protocol includes:

  • 33 OWIs across three categories:
    • 14 individual-based indicators
    • 5 group-based indicators
    • 14 environmental indicators
  • Scoring system for each indicator, split into four tiers comprising levels 0–3
  • Reference images and scoring guidance for practical field application

  1. Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. “Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages.” Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.

  2. Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. “Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare.” Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

  3. APROMAR. “A Guide on Fish Welfare in Spanish Aquaculture – Volume 4: Welfare of Rainbow Trout.” Spanish Aquaculture Business Association, 2025. https://drive.google.com/file/d/1axrO7Z6CNXBZdFsOE0vC2ZqiYKIhuugZ/view.