Two size classes of 0+ year perch: is phenotypic plasticity based on food resources?

Perch Perca fluviatilis of age 0+ years were caught in a gravel pit lake in June (unimodal size distribution) and in July (bimodal size distribution) to analyse morphological differences between the two growth cohorts. Independent of size, 0+ year perch developed a deeper body and perch of the large size cohort had an even deeper body than perch of the small size cohort. This might have been adaptations to either piscivory or planktivory, but might also be a hint that 0+ year perch of the small size cohort were undernourished and that they developed on a different trajectory than individuals of the large size cohort. In a second step the study was extended with a mesocosm experiment. This part of the study was designed to provide preliminary evidence for the extent to which morphological variations may be due to the consumption of different food resources when other factors such as habitat use could be neglected. Two groups of 0+ year perch in four mesocosms were fed for 40 days with the same biomass of either plankton or cyprinids. Although the experimental groups at the end of the experiment did not differ in size, they differed in morphology. The mouth of the piscivorous 0+ year perch became larger, the pectoral fins and the centre of mass of the posterior abdomen were shifted backwards. These results provide further evidence that the type of food is important and might lead to further functional adaptations in morphology.