Evaluation of Sampling Methods for Maturation Stage Determination in the European Eel Anguilla anguilla

Abstract

Monitoring data is important in ecological research, but differences between and within areas or species in data collection methods could introduce bias in the analyses. Standardizing data collection is particularly important when monitoring migratory species that have a distribution that crosses several national borders. The European Eel Anguilla anguilla is an extreme example of such a species since it constitutes one stock across the entire distribution area. One important variable collected for the European Eel is maturation stage. This data is needed to monitor silver eel escapement to assess population trends. To determine maturation, data on length, weight, diameter of the eyes, and pectoral fin length are used to calculate Pankhurst eye index and Durif's silver index. In this study, we investigated effects of precision and interobserver variability on data collection relevant for maturation stage determination according to Pankhurst and Durif's indices. We found that eye diameter differed in size between the left and right eyes; however, the mean difference (0.19 mm) is probably an artifact of the large sample size (n = 16,977) and can be regarded as being within the measurement precision. Meanwhile, there was no significant difference in pectoral fin length. These results suggest that either side of the eel could be used without losing precision. Visually determined maturation stage classifications differed from those calculated with Pankhurst and Durif's indices but could still provide useful information; hence, it is recommended to collect this variable. Measurements performed using computer software generated greater precision than using calipers, which increased interobserver variability. Since the difference was relatively small and since computer analysis of images may not always be an option, measuring method can be decided based on the level of precision needed in each case. These suggested implementations can reduce observation bias and streamline the data collection used for stock assessments of the European Eel.

Minimally Invasive Collection of Biometric Data Including Maturation Stage on European Eel Using Photography

“…This is corroborated by (Sundin et al. 2022), who stated that distortions by camera lenses are thought to be minimal if the focal organ is in the center of the picture.…”

“…A likely explanation for this finding is that the delimitation between eye and the surrounding skin may sometimes be difficult to judge on fresh eels, depending on the coloration of the individual (Sundin et al. 2022). The authors further reported that defrosted samples may be easier to measure, as the delimitation is often clearer after thawing.…”

“…Although the use of appropriate optics (least distortion at approximately 50 mm focal length) was impractical due to limitations in the maximum distance between camera and eel, care was taken to center eels in the image and a camera with good distortion control (i.e., built in software postprocessing) was used. The digital measurement procedure followed the description in Sundin et al (2022), using the open-source software ImageJ (version 1.53; Schindelin et al 2012Schindelin et al , 2015. The fresh, photo, and live body length measurements were taken in the field or after transporting the eels to the lab; the defrosted measurements were conducted in the lab.…”

“…The digital measurement procedure followed the description in Sundin et al. (2022), using the open‐source software ImageJ (version 1.53; Schindelin et al. 2012, 2015).…”

“…The precision of this method has previously been evaluated with respect to the body length of small-sized fish (≤50 cm) by Andrialovanirina et al (2020), attesting that accuracy can be very high. More recently, Sundin et al (2022) introduced the use of photography for measurements of eye size (but not pectoral fin length) of eel. Their analyses suggested that digital measurements from photographs could reduce the variability of measurements and the resulting maturation stages across different observers, compared with caliper measurements.…”