Filter-foraging strategies of captive American paddlefish (Polyodon spathula) in relation to food type, density and distribution in the water column

The American paddlefish (Polyodon spathula) forages using ram filter-feeding to feed on (mainly) zooplankton throughout the water column. It makes an ontogenetic transition from buccal pumping to ram ventilation feeding in the first year. In wild conditions plankton is often distributed unevenly in time and space, however this does not always apply to captive conditions. In the study presented here two paddlefish were fed on a range of diets and supply methods to see if they would alter their foraging strategy. Insight in influencing foraging strategies could be beneficial for zoos and aquariua for display purposes or for improving welfare conditions. The paddlefish were studied during foraging and foraging strategy, bout length and usage of the aquarium space were recorded. Three foraging strategies can be distinguished: foraging while swimming in straight lines, foraging while swimming in circulatory paths and particulate foraging. Although these strategies appear to not have a fixed switch-point, a significant difference can be found between the foraging strategies for starting a certain strategy and ending it in relation to food density (p=<0.003). Mean time spent on a strategy differs in duration from 1 to 60 seconds. When negative buoyant food sources (e.g. Mysis) were used, the paddlefish foraged in the lower parts of the aquarium. Neutrally buoyant food sources (e.g. Daphnia pulex) caused the paddlefish to forage throughout different depths of the aquarium. (p=<0.001).