Spatial and temporal distribution of fingernail clam (Musculium transversum) populations in the upper Mississippi River

Abstract

I analyzed temporal trends in populations of the fingernail clam Musculium transversum to validate recent anecdotal observations of reductions in their abundance in the upper Mississippi River. Sufficient historical data (1973-1990) on densities of fingernail clams were obtained from regional scientists and published literature for eight navigational pools. Historical data were augmented with field data collected in 1991. Significant declines in fingernail clam populations were evident in six of the eight pools examined (Pools 2, 5, 7, 8, 9 and 19; Mann-Kendall test; p< 0.1). The greatest decline in abundance occurred in Pool 19, which had the longest historic record of data on fingernail clam abundance. Densities in Pool 19 averaged 32,000/m2 in 1985 but declined to near zero by 1988; recovery was not evident in 1990. Combined data from all eight pools showed a significant decline in abundance. Declines in populations of fingernail clams, an important food source for lesser scaup Aythya affinis, gizzard shad Dorosoma cepedianum, and common carp Cyprinus carpio, may adversely affect fish and waterfowl. Moreover, these decreases may signal a deterioration in the health of the riverine ecosystem.