Ignoring the vertical dimension: biased view on feeding dynamics of vertically migrating sprat (Sprattus sprattus)

Diel vertical migration (DVM) is a common behaviour and often relates with the diurnal feeding periodicity. Nonetheless, sprat feeding behaviour and daily ration (DR) estimation are usually based on daytime stomach contents from deeper layers. Our study provides a new approach for DR estimation, taking into consideration the DVM associated feeding periodicity. We analysed sprat DVM by hydroacoustics and collected stomach contents in the Central Baltic Sea at different depths. The main outcome is that feeding in the deep during the day represents only 16–39% of the DR. DRs were 1.4-times higher using our approach compared to the established approach. The underestimation by the established approach was mainly caused by ignoring the effect of higher temperatures from the upper layers on the gastric evacuation rate. Our results have major implications not only for DR estimates but also on the interpretation of prey selectivity. The study emphasizes the importance of adapting the sampling design to the vertical feeding dynamics to avoid a biased picture of predator prey interactions.