Effects of Static versus Flowing Water on Aquatic Plant Preferences of Triploid Grass Carp

Triploid grass carp Ctenopharyngodon idella were presented with three aquatic plant species (sago pondweed Potamogeton pectinatus, Eurasian watermilfoil Myriophyllum spicatum, and American pondweed P. nodosus) in outdoor canals with static and flowing water in winter, spring, and summer. Plant consumption by triploid grass carp in winter was low but increased dramatically in spring and summer. Based on plant shoot lengths, triploid grass carp preferences in spring for static water were sago pondweed = American pondweed, American pondweed = Eurasian watermilfoil, sago pondweed > Eurasian watermilfoil; for flowing water, sago pondweed = Eurasian watermilfoil > American pondweed. Summer preferences for static water were sago pondweed = Eurasian watermilfoil = American pondweed; for flowing water, sago pondweed = Eurasian watermilfoil = American pondweed. Plants of all three species produced longer shoots in canals with flowing water than with static water. The differences in shoot length may have altered the triploid grass carpˈs consumption rate and preference. Flowing conditions also had varying effects on nutritional content of plants, as shown in proximate analyses of dry matter and percent of fat, ash, protein, crude fiber, nitrogen free extract, and acid detergent fiber. Ash content was consistently higher in plants of all three species from canals with flowing water. This may reflect a morphological response to flow by the plants. None of the variables of the proximate analysis of plants correlated statistically with preference. This suggests that accessibility and ease of mastication were more important in determining preference than nutritional quality of the plants. Algae consumption by triploid grass carp, however, made it difficult to discern correlations between nutritional factors and vascular plant consumption.