Assessment of zooplankton size fractionation for monitoring fry and fingerling culture ponds

Specific sizes and species diversity of zooplankton are desirable in fish n ursery ponds. Prestock sampling can determine whether appropriate prey are available for fish fry. Current methods of zooplankton assessment can be te dious, and some rapid assessment methods generally do not provide enough in formation to make accurate stocking decisions. Screening water samples sepa rates zooplankton into size or taxonomic groups that can be visually assess ed. In this study, we evaluated zooplankton size groups sieved by six diffe rent nylon mesh sizes (55, 105, 250, 500, 750, and 1,000 mum). Samples were collected from five ponds and passed successively through the array of scr eens, starting with the largest mesh size. We serially fractionated and tal lied living and preserved zooplankton samples and compared sizes retained b y the screens. For living zooplankton, mean sizes retained by each mesh dif fered significantly (P < 0.05); the preserved zooplankton produced poor sep aration. 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 l arger or smaller than the ran-e in sizes that mesh should have retained. Fi ltration failed to separate either living or preserved zooplankton into tax onomic groups. This technique does not appear to be an effective way to qua ntitatively separate different sizes of zooplankton, but it did provide sli ghtly better information than visual techniques alone because screens elimi nated most phytoplankton and suspended solids.