Implications of rudd (Scardinius erythrophthalmus) herbivory on submerged macrophytes in a shallow eutrophic lake

The role of rudd (Scardinius erythrophthalmus) herbivory was studied in Kirkkojarvi, a shallow and turbid basin in Lake Hiidenvesi, Finland. The submerged species dominating in the rudd diets were Potamogeton obtusifolius, Ranunculus circinatus, Sparganitan emersum, bryophytes, and filamentous algae. Plant consumption estimated with bioenergetics modelling increased with fish age, being highest in late summer concomitant with the macrophyte biomass peak. Depending on the age structure, a rudd biomass of 20 kg ha(-1) consumes 18-23 kg of macrophytes ha(-1) a(-1), while a rudd biomass of 100 kg ha(-1) results in plant consumption of 92-115 kg ha(-1) a(-1). Although, rudd seemed to feed rather unselectively on suitable-sized and edible plants, some species abundant in the littoral, such as Myriophyllum verticillatum and pleustophytic Ceratophyllum demersum, were not found in rudd guts, indicating selective plant consumption. In Kirkkojarvi, selective grazing by rudd and increased turbidity and high nutrient levels partly caused by bottom dwelling cyprinid fish, may promote the inedible and pleustophytic macrophytes, which have increased in Kirkkojarvi during the past decades.