In vivo – determination of the fat content in mirror carps (Cyprinus carpio) using ultrasound, microwave and linear measurements

Carps are the third largest species in aquaculture worldwide and belong to the fish family ‘cyprinids’, which make up the largest part of aquaculture production. The production of lean carp is one of the main goals in the carp farming business. Carps are usually traded alive; in order to ensure product quality and achieve a high consumer acceptance, it is important to estimate the fat content of the living fish. Therefore, during this study, a total of 250 living carps were examined using a mobile ultrasound device. Additionally, a microwave technology based Fish Fatmeter (Distell, Fauldhouse, Scotland, UK) was used to determine the fat content of the living fish. Further measurements included weight, lengths, height and circumferences. The ultrasound measurements were performed on non-sedated fish using narrow water-filled containers. Two sagittal images per fish were taken to measure the backfat thickness at defined locations. Subsequently, the fish were taken out of the water in order to determine the fat content using the Fish Fatmeter. Weight and linear measurements were taken. After the in vivo measurements, about 10 fish per pond, in total 51 fish, were slaughtered and the fillets were analysed chemically. Linear regression models were developed. The Fish Fatmeter turned out to be an accurate method to determine the fat content of the fillet in mirror carps (R² = 0.95). Regarding the ultrasound measurement of the backfat thickness, moderate correlations between ultrasound and Fish Fatmeter were achieved (R² = 0.33–0.45). Pearson's correlation for linear measurement and the Fish Fatmeter showed negative prediction. In order to evaluate the relative backfat thickness, the ultrasound measurement was divided by linear measurements. The best correlation was found using the ultrasound measurement point where the backfat thickness reached a constant thickness divided by the circumference around the thinnest part of the tail fin (R² = 0.74). In the next step, the carps were divided into groups of ten fish per pond. The mean Fish Fatmeter measurement and ultrasound measurement divided by circumference was calculated and correlated achieving an R² of 0.92. Based on these findings, it seems plausible to estimate the fat content using a small sample of ten fish with an ultrasound device and a measuring tape.