ICHTHYOPLANKTON ABUNDANCE AND VARIANCE IN A LARGE RIVER SYSTEM CONCERNS FOR LONG-TERM MONITORING

System-wide spatial patterns of ichthyoplankton abundance and variabil ity were assessed in the upper Mississippi and lower Illinois rivers t o address the experimental design and statistical confidence in densit y estimates. Ichthyoplankton was sampled from June to August 1989 in p rimary milieus (vegetated and non-vegated backwaters and impounded are as, main channels and main channel borders) in three navigation pools (8, 13 and 26) of the upper Mississippi River and in a downstream reac h of the Illinois River. Ichthyoplankton densities varied among statio ns of similar aquatic landscapes (milieus) more than among subsamples within a station. An analysis of sampling effort indicated that the co llection of single samples at many stations in a given milieu type is statistically and economically preferable to the collection of multipl e subsamples at fewer stations. Cluster analyses also revealed that st ations only generally grouped by their pre-assigned milieu types. Pilo t studies such as this can define station groupings and sources of var iation beyond an a priori habitat classification. Thus the minimum int ensity of sampling required to achieve a desired statistical confidenc e can be identified before implementing monitoring efforts.