Manual fish length measurement accuracy for adult river fish using an acoustic camera (DIDSON)

Abstract

Total lengths (LT) of 50 free‐swimming fish in a tank, silver carp Hypophthalmichthys molitrix and rainbow trout Oncorhynchus mykiss, were measured using a DIDSON (Dual‐frequency IDentification SONar) camera. Using Sound Metrics software, multiple measurements of each fish (LT, side aspect angle and distance from the camera) at different times were analysed by two experienced operators while a subset of data was analysed by two inexperienced operators. The main result showed high variability in intra‐fish LT measurements. The number of measurements required to minimise errors and to obtain robust fish measurements (true LT ± 3 cm) was estimated by a bootstrap method. Three to five measurements per fish were recommended for fish surveys in rivers. In this experimental study, aiming to reproduce river conditions, no evidence of fish position (side aspect angle and distance from the camera) effect was detected, but an operator effect (partially explained by training) was observed. General linear mixed models also showed that lengths of the smallest fish (LT < 57 cm) were overestimated and lengths of the largest fish (LT > 57 cm) were underestimated in comparison with their true lengths. In conclusion, we highlight that this technology, like any monitoring methods, returns imperfect observations. We advise DIDSON users to ensure that measurements are carried out correctly in order to draw accurate conclusion from this new technology.

Length measurement accuracy of adaptive resolution imaging sonar and a predictive model to assess adult Atlantic salmon (Salmo salar) into two size categories with long‐range data in a river

“…Many previous studies have reported that there is no range dependency in the accuracy of the length estimates using high-frequency mode and a shorter range (Burwen et al, 2010;Daroux et al, 2019;Gurney et al, 2014;Hightower et al, 2013). Burwen et al (2007) found no range-dependent bias on free-swimming fish but found it on tethered fish, and had a lower measurement accuracy on tethered fish.…”

“…In the last 5-10 years, the use of automatic fish detection and measuring techniques has increased (Jing et al, 2017;Tušer et al, 2014;Zhang et al, 2014), but manual or semi-automatic counting and measuring is still the most common analysis method when using imaging sonar technology for monitoring fish populations (Atkinson et al, 2016;Lilja, Orell, et al, 2010;Lilja, Romakkaniemi, et al, 2010). However, in manual measurements, the consistency of human-generated lengths remains an issue that contributes to a measurement bias and interobserver error has been reported in many instances (Daroux et al, 2019;Hakkola, 2011;Keefer et al, 2017). Daroux et al (2019) also highlight a high intra-individual error and suggest that three to five measurements per fish on the best-quality frames will improve the length measurement accuracy.…”

“…However, in manual measurements, the consistency of human-generated lengths remains an issue that contributes to a measurement bias and interobserver error has been reported in many instances (Daroux et al, 2019;Hakkola, 2011;Keefer et al, 2017). Daroux et al (2019) also highlight a high intra-individual error and suggest that three to five measurements per fish on the best-quality frames will improve the length measurement accuracy.…”

In situ evaluation of European eel counts and length estimates accuracy from an acoustic camera (ARIS)

“…This emphasizes the need to specifically describe the difference between L T and L AV for each target species. Furthermore, Daroux et al (2019) detected an operator effect on the difference between L T and L AV .…”

“…This data can be useful to differentiate species (Becker and Suthers, 2014;Grote et al, 2014), adults from juveniles of the same species (Doehring et al, 2011) or in the case of European eels, males from females (Vollestad, 1992). Several studies estimated the reliability of length estimates based on acoustic videos by comparing the known size (usually total length L T ) of different species to the length estimate from acoustic video (L AV ) in the laboratory (Hightower et al, 2013;Zhang et al, 2014;Cook et al, 2019;Daroux et al, 2019) or in the field (Burwen et al, 2010). These studies concluded that L AV could be either over estimated or under estimated compared to L T depending on the species (Hightower et al, 2013), its size (Daroux et al, 2019), the orientation of the fish compared to the camera and the detection range (Cook et al, 2019).…”