Evaluating bioelectrical impedance analysis (BIA) as a tool for assessing freshwater fish condition

The nutritional status, or condition, of fish is a key health proxy influencing vital life history processes including migration, reproduction, and recruitment. Numerous morphological, biochemical, and electronic metrics are used to assess fish condition, yet it remains unclear if these metrics are congruent with each other and across species. This study evaluates bioelectrical impedance analysis (BIA) as a tool for assessing fish condition by comparing it with morphological (e.g. condition index, hepatosomatic index) and biochemical (e.g. percentage and total-body mass compositions of lipid, mineral, protein and water content and bulk C:N ratios in muscle tissue) condition metrics in three native Australian fish species (Macquaria ambigua, Nematalosa erebi, and Percalates novemaculeata) and one invasive species (Cyprinus carpio). Using simple least-squares linear regressions and Principal Component Analysis (PCA), we uncovered that BIA parameters correlated strongly with total-body masses of biochemical components that covary with body size, but showed weaker correlations with percentage compositions of the same components. However, phase angle, a common BIA measure, showed some significant correlations with morphological condition and lipid content, although these relationships were generally weak and species dependent. These findings suggest that BIA can effectively estimate certain physiological parameters, but caution should be applied when interpreting BIA metrics, as their relationship with established indicators of condition may vary. The current study underscores the importance of combining multiple methods to provide a comprehensive understanding of fish condition in freshwater ecosystems, which can be used to provide mechanistic linkages with anthropogenic impacts.