Spatial and temporal variation in brook trout and brown trout proportional stock densities in Wisconsin streams

Abstract  Proper interpretation of measures used to describe fish populations requires knowledge of the measure's inherent spatial and temporal variation. Proportional stock density (PSD), the ratio of 'quality-length' fish to 'stock-length' fish multiplied by 100, is commonly used as a measure of population size structure; PSD values range from 0 to 100. Spatial and temporal variation in brook trout, Salvelinus fontinalis (Mitchill), and brown trout Salmo trutta L., PSD scores in Wisconsin are described and tested to determine if variation differed by stream order and ecoregion. Neither stream order nor ecoregion significantly affected variation of PSD scores. The mean standard deviation of PSD scores over time at a site was 12.49 for brook trout populations and 12.95 for brown trout populations. The mean standard deviation of PSD scores between sites in the same stream was 15.07 for brook trout populations and 12.50 for brown trout populations. Sampling frequency required to characterise a PSD score of a single population of trout in Wisconsin streams with a degree of precision equal to the amount of observed temporal variation is approximately 14 sites for brook trout and 20 sites for brown trout.