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Better knowledge of capelin movement and acoustic properties in the Iceland-East Greenland-Jan Mayen area is needed, as fish movement and physiology can impact the echoes returned and measured by echosounders which consequently affects biomass estimations. Changes in life history traits of capelin were observed after the 2000s, driven by changes in environmental conditions, which can consequently influence target strength (TS). This study aimed to evaluate the effects of movement and physiology on capelin in situ TS measurements. Acoustic and biological data collected during capelin assessment surveys from autumn and winter seasons of 2018–2022 were used. These represent the late feeding period and the main spawning migration route, which follows a clockwise path from northwest to the southeast of Iceland along the continental slope, respectively. Additionally, new in situ TS measurements data were collected from a submersible echosounder (WBT-TUBE) positioned above capelin schools (~ 20 m above schools), in September 2021 and January 2022. Generalized linear mixed-effect models (GLMMs) were used to evaluate the effects of vertical movement (depth in the water column) and physiology on capelin in situ TS. Capelin displays diel vertical migration during both seasons, staying in shallow waters at night and migrating to deeper waters (~ 200–250 m) during the day. TS of capelin between 9 and 20 cm was found to be on average − 55.6 dB and − 53.7 dB in narrowband (for hull mounted and WBT-TUBE, respectively) and − 57.2 dB in broadband. Capelin TS was found to decrease with depth. GLMMs indicate that length, estimated swim bladder size, relative condition of capelin, and depth influence capelin TS. Further collection of in situ TS data of capelin at depth is necessary, as are additional studies on the influence of swimming behaviour and swim bladder size with depth on capelin TS and its effects in biomass estimation are recommended.
Silva et al. (Mon,) studied this question.