Analysis of refractometry for determining total plasma protein in hybrid striped bass (Morone chrysops × M. saxatilis) at various salinities

Total plasma protein (TPP) is a broad clinical indicator of health, stress, and well being. A simple and rapid technique for determining TPP is refractometry, which measures the refractive index of all dissolved materials in solution. It was hypothesized that plasma dissolved solids in fish held at increasing salinity levels would result in plasma samples with a higher refractive index and affect true TPP readings. Efficacy of refractometry over a range of salinities was evaluated by comparing TPP values obtained by refractometry with those obtained by a dye-binding assay (Biuret method) specific to proteins. Blood analytes were evaluated in hybrid striped bass Morone chrysops × M. saxatilis (HSB) acclimated to nominal salinities of 0, 10, 20, and 30 g L− 1. Hybrid striped bass acclimated to hypertonic environments exhibited higher (P < 0.05) electrolytes, TPP, and osmolality than those acclimated to low salinity, but were still within the normal reference interval for HSB. Although salinity affected TPP determined by both methods, the observed increase in TPP values over the range of salinities was significantly greater in samples analyzed by refractometry. Total plasma protein concentrations measured by refractometry were always higher and the difference in absolute values determined by the two methods appears to be caused by a factor not measured in this investigation. It is concluded that refractometry is not well suited for analyzing accurate TPP in HSB or determining relative changes in TPP across a range of salinities.