Feeding selectivity of Calanus finmarchicus in the Trondheimsfjord

- Incubation experiments showed an omnivore feeding of Calanus finmarchicus.
- Ciliates were positively selected by C. finmarchicus at varying food concentrations.
- The degree of omnivory depended on the proportion of ciliates available.

The feeding selectivity of Calanus finmarchicus was studied by carrying out three incubation experiments; two experiments with natural seawater sampled during spring bloom (Exp. 1) and post-bloom conditions (Exp. 2) and a third experiment with cultured dinoflagellates and ciliates (Exp. 3). In the first two experiments a gradient in ciliate concentration was created to investigate the potential for prey density dependent selective feeding of C. finmarchicus. Results of microplankton counts indicated C. finmarchicus to be omnivorous. Diatoms contributed chiefly to the diet during spring bloom conditions. Despite the high microphytoplankton biomass during the spring bloom (Exp. 1), ciliates were selected positively by C. finmarchicus when the ciliate biomass exceeded 6.5 μg C L− 1. A selection in favor of large conic ciliates such as Laboea sp. and Strombidium conicum was indicated by positive selectivity indices. Ciliates were throughout positively selected by C. finmarchicus during Exp. 2, and selectivity indices indicated a negative selection of diatoms. The results from Exp. 3 showed that C. finmarchicus has the ability to switch from dinoflagellates to ciliates as sole food source, even if the dinoflagellate was offered in surplus. This suggests that other factors, such as nutrition may be of significance for the feeding selectivity of C. finmarchicus.