Assessment of Zooplankton Size Fractionation for Monitoring Fry and Fingerling Culture Ponds

Specific sizes and species diversity of zooplankton are desirable in fish nursery ponds. Prestock sampling can determine whether appropriate prey are available for fish fry. Current methods of zooplankton assessment can be tedious, and some rapid assessment methods generally do not provide enough information to make accurate stocking decisions. Screening water samples separates zooplankton into size or taxonomic groups that can be visually assessed. In this study, we evaluated zooplankton size groups sieved by six different nylon mesh sizes (55, 105, 250, 500, 750, and 1,000 μm). Samples were collected from five ponds and passed successively through the array of screens, starting with the largest mesh size. We serially fractionated and tallied living and preserved zooplankton samples and compared sizes retained by the screens. For living zooplankton, mean sizes retained by each mesh differed significantly ( P < 0.05); the preserved zooplankton produced poor separation. Although the mean sizes of live zooplankton reflected the size of the mesh on which they were retained, more than 50% of the organisms were larger or smaller than the range in sizes that mesh should have retained. Filtration failed to separate either living or preserved zooplankton into taxonomic groups. This technique does not appear to be an effective way to quantitatively separate different sizes of zooplankton, but it did provide slightly better information than visual techniques alone because screens eliminated most phytoplankton and suspended solids.