MORPHOMETRIC COMPARISONS OF UPPER MISSOURI RIVER STURGEONS

Morphometric comparisons were made among three isolated populations of pallid sturgeon Scaphirhynchus albus and shovelnose sturgeon S. plato rynchus from the upper Missouri River. Six measurements were made on 8 9 pallid and 204 shovelnose sturgeons. Means of several morphometric c haracteristics were statistically different between populations of bot h species. Pallid sturgeon means showed proportional trends relative t o location on the river. Toward the head-waters, relative head and int errostral lengths were progressively shorter and outer barbel length r elatively longer. The only trend observed for the shovelnose sturgeon was that relative head length became longer upriver. This was the oppo site of the trend observed for the pallid sturgeon. Morphometric ratio s commonly used to differentiate the two species were useful measures for live fish from isolated populations of Missouri River sturgeon but not for the overall sturgeon population. The exclusivity of morphomet ric ratios currently used to distinguish between the two species did n ot hold for our larger fish and large sample size. A cumulative morpho metric characteristic index is described to aid managers in comparing individual fish within a composite sturgeon population, and the findin g of three possible hybrids is discussed.