Ultrasonography to assist with timing of spawning in European eel

- Indicators of oocyte maturation were identified by ultrasonography to correctly time the induction of ovulation.
- The swim bladder becomes smaller due to the increasing pressure of the expanding ovaries by the hydration response.
- The vertical diameter of the swim bladder is a more accurate indicator than body weight index and body girth index.
- Swim bladder ultrasound imaging is a useful tool to assist with DHP injection and thus the timing of spawning.

Ovulation in the European eel can be induced by injection of DHP (17α,20β-dihydroxy-4-pregnen-3-one). The timing of injection is based on the developmental stage of the oocytes. The oocyte stage is determined by invasive biopsy or through external indicators of the oocyte hydration response: body weight index (BWI) and body girth index (BGI). However, in European eel, BWI and BGI are inaccurate indicators because the individual hydration response is highly variable. The aim of this study was to identify indicators of oocyte maturation non-invasively by applying ultrasonography. Ultrasound parameters that were determined were the body wall thickness; diameter of the cloaca opening; grey scale median (GSM) of the ovary, and diameters (vertical and horizontal) and surface area of ovary, liver and swim bladder. Data of farmed and wild eels were combined for correlation analyses and principal component analyses (PCA). Ovary and liver parameters were not correlated with the average oocyte stage. Body wall thickness was negatively correlated to the average oocyte stage but variability was high. The swim bladder vertical diameter (Sbv) was negatively correlated with the average oocyte stage and with the oocyte diameter. Swim bladder area (Sba) was also negatively correlated to the average oocyte stage but less accurate. Results showed that during the final stages of oocyte maturation, the swim bladder becomes smaller. This is most probably due to the pressure that is caused by the increasing ovaries as a result of the oocyte hydration response. Females with an average oocyte stage between 4.5 and 7.0 and an average oocyte diameter higher than 800 μm, had Sbv values decreasing from 4.3 to 2.4 mm and Sba values from 52 down to 11 mm2. The correlation between Sbv and oocyte stage is more strict than for BWI or BGI which makes Sbv not only a parameter that can be determined non-invasively but also one that is more accurate in indicating the oocyte developmental stage. Therefore, ultrasonography, and in particular the ultrasound imaging of the swim bladder, represents a useful tool to assist with the timing of spawning as induced by injection of DHP.