Non-invasive ultrasound measurements for determination of hepato-somatic index in Greenland halibut (Reinhardtius hippoglossoides).

Hepato-somatic index (HSI) is widely used to indicate fish body condition and level of energy reserves related to feeding/physiology and reproduction. HSI is traditionally obtained by dissection and measurement of liver mass versus body mass, but ultrasound technology may provide a minor invasive method to obtain liver dimensions to estimate the HSI of live fish. Images of Greenland halibut (Reinhardtius hippoglossoides) livers (n = 37) were obtained using ultrasonography, then subsequently dissected, weighed and measured using conventional methods. Four liver measurements (width n = 1 and depth n = 3) were obtained from ultrasound images and compared with corresponding physical width/depth measurements. The relationship between the derived ultrasound width/depth metrics and liver mass was then used to estimate total liver mass (LM_U) and LM_U values divided by the total body weight of each fish to derive the HSI values (HSI_U). The LM_U and HSI_U values were not significantly different from the original liver mass and associated HSI values. Ultrasound liver depth was the most accurate metric for estimating HSI_U values, but a degree of variability was observed. Derived HSI_U values varied between maturity stages matching predictions but did not vary by sex or capture location. This non-invasive method can be conducted quickly with minimal stress to fish, with applications to studies measuring HSI in commercially important to endangered species. The framework presented for measuring and estimating ultrasound-derived HSI in live fish provides a baseline that can be improved with further validation work.