Otter diet in reservoirs is known to experience seasonal changes. We selected a reservoir with a large population of exclusively wintering great cormorants and seasonal changes in stored water volume to test the relative influence of abiotic and biotic factors on otter foraging ecology. DNA metabarcoding of otter spraints revealed a dietary change from autumn to winter. Otters had a diet dominated by the exotic goldfish in autumn, but predated intensively on the native northern straight-mouth nase in winter. This change was likely caused by predation of cormorants on goldfish and to fish biology. Secondly, macroscopic analysis of spraints revealed that otters shifted from a diet dominated by fish (in terms of biomass) to a diet dominated by red swamp crayfish during spring–summer, when the latter became overabundant. As revealed by modelling, this second shift was most likely influenced by the sudden increase in stored water volume in spring, but also by the cumulative effect of cormorant predation on fish during autumn–winter. Macroscopic analyses of otter spraints collected in a second reservoir with no cormorants revealed a lack of seasonality. Hence, the combined influence of both biotic and abiotic factors explained otter diet seasonality in a lentic-water novel ecosystem.
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Fish samples to be used as positive controls for molecular analysis were kindly provided by Carlos Antunes, except for O. mykiss and Atlantic salmon S. salar that were purchased at a local fish market. David Stanton generously provided us with four otter tissue samples used during DNA metabarcoding optimisation. EMALCSA kindly provided data on water-level variation and Xunta de Galicia (Consellería de Medio Ambiente e Ordenación do Territorio) allowed us to have a copy of an unpublished report with detailed information about the structure of the fish community at the study reservoir. The long-term time series on great cormorant counts at the study reservoir was supplied by Xunta de Galicia (Dirección de Patrimonio Natural). Pablo Sierra helped us during field work at Eiras reservoir. We are thankful to Ignacio Doadrio, Juan Jiménez, Daniel Oro, three editors, Martin Čech and two anonymous referees for their valuable comments and suggestions. This work received funding from Xunta de Galicia (Grants GRC2014/050 and ED431C 2018/57) and Universidade da Coruña. A Martínez-Abraín was supported by an Isidro Parga Pondal research contract by Xunta de Galicia during the period 2011–2016.
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Martínez-Abraín, A., Marí-Mena, N., Vizcaíno, A. et al. Determinants of Eurasian otter (Lutra lutra) diet in a seasonally changing reservoir. Hydrobiologia 847, 1803–1816 (2020). https://doi.org/10.1007/s10750-020-04208-y
- Diet shifts
- Interspecific competition
- Foraging plasticity
- DNA metabarcoding
- Changes in stored water volume