Temperature Effects on Bioelectrical Impedance Analysis (BIA) Used to Estimate Dry Weight as a Condition Proxy in Coastal Bluefish

Abstract

The highly migratory nature of bluefish Pomatomus saltatrix makes comprehensive study of their populations and their potential responses to factors such as competition, habitat degradation, and climate change difficult. Body composition is an important ecological reference point for fish; however, estimating body composition in fish has been limited by analytical and logistical costs. We applied bioelectrical impedance analysis (BIA) to estimate one body composition component (percent dry weight) as a proxy of condition in bluefish. We used a tetra polar Quantum II BIA analyzer and measured electrical properties in the muscles of bluefish at two locations per fish (dorsal and ventral). In total, 96 bluefish ranging from 193 to 875 mm total length were used in model development and testing. On 59 of these fish BIA measures were taken at both 15 • C and 27 • C. Temperature had a significant negative effect on resistance and reactance. A subsample of these fish was then analyzed for dry weight as a percentage of their whole body weight (PDW), which is a good indicator of condition because it is highly correlated with fat content in fish. The BIA models predicting PDW inclusive of all lengths of bluefish were highly predictive for 15 • C (stepwise regression) and 27 • C. Regression (R 2 pred ) values that estimate future predictive power suggest that both models were robust. Strong relationships between PDW and other body composition components, coupled with the BIA models presented here, provide the tools needed to quantitatively assess bluefish body composition across spatial and temporal scales for which assessment was previously impossible.The growth of fish is believed to be an integrated measure of well-being that is linked to reproductive success, survival, habitat quality, and competition (Brandt et al. 1992;Roy et al. 2004;Amara et al. 2009;Vehanen et al. 2009). In aquaculture and other applications, such as those employing fish bioenergetics models, growth is often determined by measuring differences in the total weight of fish over time. However, fish are 60-90% water, and they often compensate for loss of fat by replacing it with water, making the use of total weight to measure growth and condition problematic (Shearer 1994;Breck 2008;Hartman and Margraf 2008). To fully evaluate growth in weight of fish requires knowledge of the percent dry mass of the fish. Dry mass can be measured on an individual by oven drying or by freeze drying but, in addition to being lethal, this process can be cumbersome for large individuals or impossible for rare taxa.Bioelectrical impedance analysis (BIA) has been used to determine water mass in human subjects since the 1970s and is now widely used in health clubs to assess human body condition. 307 308 HARTMAN ET AL.

Reliability of non-lethal assessment methods of body composition and energetic status exemplified by applications to eel (Anguilla anguilla) and carp (Cyprinus carpio)

“…However, Rasmussen et al (2012) used brook trout as a model species and calibration of BIA for relative body constituents might be species-specific. Although our study did not support the usefulness of simple BIA assessments to predict dry mass in eel or carp, it cannot be ruled out that combined dorsal and ventral BIA measurements, as conducted by Hartman et al (2011), and/or application to different species can be a way to substantially improve the performance of BIA applications.…”

“…In this context, in our study BIA measurements did not work to a degree that renders the device of use in the field. This finding contrasts with the results presented by Hartman et al (2011) andRasmussen et al (2012). Hartman et al (2011) combined dorsal and ventral BIA measurements in their analyses, and they used several electrical equations to calculate a broad range of candidate predictor variables for explaining relative dry mass of the fish.…”

“…This finding contrasts with the results presented by Hartman et al (2011) andRasmussen et al (2012). Hartman et al (2011) combined dorsal and ventral BIA measurements in their analyses, and they used several electrical equations to calculate a broad range of candidate predictor variables for explaining relative dry mass of the fish. This approach led to up to ten variables within the same model, most of them relying on the same two measurements of resistance and reactance.…”

“…This approach led to up to ten variables within the same model, most of them relying on the same two measurements of resistance and reactance. Further, variables included in the regression models of Hartman et al (2011) strongly differed between different size classes of the fish and with different temperatures at measurement. Thus, calibration results created by Hartman et al (2011) cannot easily be compared with other studies without additional calibration work for the size classes of interest at a specific temperature.…”

“…Further, variables included in the regression models of Hartman et al (2011) strongly differed between different size classes of the fish and with different temperatures at measurement. Thus, calibration results created by Hartman et al (2011) cannot easily be compared with other studies without additional calibration work for the size classes of interest at a specific temperature. By contrast, measurement techniques between our study and Rasmussen et al (2012) did not differ.…”