Effect of Size-Selective Predation by Muskrats (Ondatra zebithicus) on a Population of Unionid Clams (Anodonta grandis simpsoniana)

Abstract

SUMMARY (1) The effect of size-selective predation by muskrats on the population size, sizestructure, and growth of clams was evaluated in a small lake in central Alberta. Clam shells were collected at regular intervals over 1 year from muskrat middens found along the shoreline of the lake. The number, biomass, size distribution, and age distribution of clams eaten were compared with the number, biomass, and age and size distributions of clams in the lake. (2) Muskrats ate almost 37000 clams, or 3% of the population in the study area in I year. When expressed as biomass, muskrats ate 677 kg of clams, which was equivalent to 5 8% of the biomass and 31% of the annual production of clams in the study area. (3) Muskrat predation was highly size-selective-86% of the clams eaten were > 55 mm long compared to 37% for clams in the lake-and had a strong effect on the size structure of the clam population. When all age-classes were combined, the biomass of clams > 70 mm eaten exceeded the annual production by an amount equivalent to 9 5% of the estimated biomass in the lake. Muskrats ate the fastest growing clams between 3 and 9 years old. Mean length at annulus did not increase for clams older than age-8 in the lake. The biomass of large clams in the lake will therefore decline if the observed predation continues. (4) Muskrats ate a large fraction of the reproductive output (glochidia) of the largest clams. Only 6% of the glochidia of clams 70 mm long was eaten. Genetic selection for slow growth has not occurred.