Evidence quality

To address variability in the evidence base across indicators (for more information, see the “Limitations” section) and to ensure transparency, the table below summarizes the level of supporting evidence for each indicator. The table provides two considerations for each indicator:

• Rainbow trout-specific evidence: scored based on: (1) the quantity and quality of available studies, (2) appropriateness of study design for assessing welfare impacts (including sample size and duration), and (3) relevance to commercial seawater conditions. This column is about the strength of evidence that the metric is a valid indicator of welfare in rainbow trout.
  • Example scoring:
    • High: Multiple empirical studies (e.g., 4+) on rainbow trout, with some testing substantial welfare-relevant outcomes, usually with at least one testing rainbow trout in seawater (depending on how relevant it is for the indicator)
    • Moderate: Some empirical studies (e.g., 1–3) on rainbow trout testing substantial welfare-relevant outcomes
    • Low: Some rainbow trout-specific evidence, but substantial welfare-relevant outcomes are not tested (e.g., growth/production metrics only)
    • Very low: No empirical rainbow trout-specific evidence or the indicator is only mentioned incidentally, largely relying on assumptions, expert opinion, or extrapolation from other species
• Threshold validation: scored based on whether studies tested a wide range of realistic indicator levels that would be observed in commercial aquaculture against welfare parameters (such as mortality or cortisol), ideally within the same experiments. This column is about the strength of evidence that the proposed thresholds correspond to different levels of welfare risk.
  • Example scoring:
    • High: A wide range of realistic indicator levels have been empirically tested against welfare parameters
    • Moderate: Multiple realistic indicator levels covering what would be observed in commercial aquaculture settings have been empirically tested against welfare parameters, but some specific thresholds may be partly interpolated or based on expert judgment
    • Low: Thresholds mainly rely on non-empirical data (e.g., regulatory or industry benchmarks or guidance), or testing against production metrics rather than welfare parameters
    • Very low: Thresholds are essentially untested, and reliant on expert judgment

While some indicators are more strongly validated than others, producers are encouraged to measure as many indicators as practicable and relevant to their production system. Additionally, tracking correlations between indicators over time can strengthen understanding of how these indicators reflect welfare within production systems. This table underscores the gaps in rainbow trout welfare research and should inform the priorities of research scientists.

For further details on the exact evidence relied on for the development of the scoring guidance for each indicator, please consult the evidence sections within each indicator.

Applicability of salmon research to rainbow trout

Where direct research on seawater rainbow trout is limited, evidence from Atlantic salmon can provide additional context. The two species belong to the same family (Salmonidae) and are reared as post-smolts in broadly similar environmental conditions. However, some key differences must be considered when applying evidence from Atlantic salmon to rainbow trout, including:

• The two species evolved in different parts of the world (Atlantic salmon in the North Atlantic Ocean and rainbow trout in the Pacific Ocean) and accordingly have adapted to different environmental conditions, e.g., different temperatures and salinities;
• Domestication: Atlantic salmon have only been domesticated for approximately the last 50 years, while rainbow trout have been domesticated for more than a century;
• Atlantic salmon is predominantly anadromous, with the vast majority of wild populations migrating to sea, while rainbow trout is facultatively anadromous.

Applicability of freshwater rainbow trout research to seawater rainbow trout

Before being transferred to seawater from freshwater, rainbow trout need to develop the capability to tolerate the new salinity conditions. Some key changes that occur during this smoltification process include:

• Change in gill chloride cells and pumps to adapt to new seawater conditions.¹³
• Increase in ion intake, e.g., through increased drinking and increased absorption of ions by the intestine.¹⁴
• Altered metabolism to sustain the increased energy demands of smoltification, including of proteins, lipids, and carbohydrates.¹⁵
• A change in physical appearance, including a more silver body color.

While key physiological changes occur as part of the smoltification process, rainbow trout reared in freshwater and rainbow trout reared in seawater are fundamentally the same animal, and as long as different rearing conditions (e.g., the chemistry of freshwater vs. seawater) and physiological differences are accounted for, findings based on freshwater rainbow trout studies can inform how we think about welfare in seawater rainbow trout.