Limitations

While OWIs provide valuable welfare assessment capabilities, users should be aware of several significant limitations:

  1. Evidence base variability: The scientific foundation supporting individual indicators varies considerably. Some indicators are well-validated in ongrowing rainbow trout in seawater, while others rely on evidence from different salmonid species, alternative life stages, or laboratory studies that may not fully reflect commercial conditions. Users should interpret indicators with limited validation more cautiously. More information on specific indicators is provided in the section “Evidence quality” below.
  2. Indicator interpretation challenges: Some indicators may interact with each other, producing compounding effects on welfare that an isolated assessment of individual indicators cannot capture.
  3. Sampling limitations: Morphological assessments require the sampling of individuals, which may not accurately reflect the welfare status of the entire production group. In large-scale operations, localized welfare issues could be missed if sampling protocols are insufficient or if welfare problems are unevenly distributed throughout the population.
  4. Assessor subjectivity: Many morphological indicators rely on visual scoring instead of detailed measurements. While this approach enables rapid assessment, it introduces inter-observer variability and requires standardized training protocols to ensure consistency across evaluators. In addition, because OWIs are developed and interpreted from a human perspective, there may be an inherent anthropocentric bias that may not fully reflect the fish’s subjective experience.
  5. Practical limitations: OWI protocols must be pragmatic and flexible for implementation across different commercial settings, which inherently involves trade-offs with precision. Because most producers cannot monitor OWIs continuously, assessments are typically carried out at discrete time points, providing only snapshots of welfare. In addition, fish may physiologically adapt to chronic stress and appear outwardly normal, meaning that short-term observations can underestimate ongoing welfare issues.
  6. Equipment dependency: Environmental monitoring capabilities depend on the availability of sensor technologies and their placement. Producers with limited monitoring equipment may have reduced capacity to track key environmental parameters, potentially missing welfare-relevant changes in water quality or environmental conditions. Some environmental parameters, like oxygen saturation, may also vary significantly in both space and time (at least in sea cages), with the most critical levels easily going undetected.
  7. Routine assessment only: This protocol focuses on routine welfare assessment. Further, no concrete recommendations are provided for addressing identified welfare gaps. Producers should therefore ensure they have strong management plans in place that enable them to act upon identified welfare issues.

These limitations highlight the need for producers to routinely and systematically measure multiple indicators across different categories rather than focusing on single indicators or one-off assessments. In tandem, protocols such as this one must be regularly reviewed and updated to reflect emerging scientific evidence and evolving understanding of fish welfare.