Assessment of Zooplankton Size Fractionation for Monitoring Fry and Fingerling Culture Ponds
Ch.C. Mischke, P.V. Zimba-2001
North American Journal of Aquaculture, 63(4): 289-292
Abstract:
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.
(Thad Cochran National Warmwater Aquaculture Center, 127 Experiment Station Road, Post Office Box 197, Stoneville, Mississippi 38776, USA)