Bright, R.C., C. Gatenby, R. Heisler, E. Plummer, K. Stramer, and W. Ostlie. 1995. A survey of the mussels of the Pomme de Terre and Chippewa rivers, Minnesota, 1990. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 131 pp.
Nearly 60 species of freshwater mussels are endangered with another dozen species in decline that are harvested for their shell material and exported to Asia for the cultured pearl business. The causes for these declines vary from habitat destruction to industrial pollution. More recently, the invasion of the Zebra mussel (Dreissena polymorpha) has the potential to extirpate most of our native mussel fauna. These factors prompted the Minnesota Department of Natural Resources (MNDNR) to fund systematic mussel surveys in several rivers in Minnesota. The goals of the surveys were to gain information on the current status of Minnesota's mussel fauna and to better understand the natural history of these animals.
Mussels were examined and voucher specimens collected at 56 sites on the Pomme de Terre and Chippewa River systems. Twenty-four sites were surveyed in the Pomme de Terre River system and 32 in the Chippewa River system. Quantitative (density, shell dimensions, age) and qualitative (diversity, substrate type, and reproductive status) data were gathered using two methods. A grid method where 30 to 40 1/8 m2 quadrats were sampled to a depth of 12 cm. This was followed by a random one hour timed search method that involved three people searching out prime mussel habitat and collecting all live or dead animals.
The number of live specimens collected in the Pomme de Terre River system was 1688 and 4090 in the Chippewa River system. Live mussel densities ranged from 0 to 4 mussels/m2 and averaged 0.8 mussels/m2 in the Pomme de Terre system. Live mussel densities in the Chippewa River system ranged from 0 to 11.3 mussels/m2 and averaged 3.3 mussels/m2. Timed searches in the Pomme de Terre River produced densities ranging from 0 to 113 mussels/person/hour, the mean number of mussels found at each site was 19 mussels/person/hour. In the Chippewa River, timed search densities ranged from 0 to 167 mussels/person/hour, the mean number of mussels found at each site was 41.3 mussels/person/hour.
Live mussel diversity at each site ranged between 1 and 11 in the Pomme de Terre River, the average being 4.0, with 14 live species observed in the drainage. Live mussel diversity in the Chippewa River ranged between 0 and 12 species, the average was 6.4, with 16 live species observed in the drainage. Including records of dead species, the number of species found in the Pomme de Terre River was 17 and in the Chippewa River was 21.
The mussel density in the Pomme de Terre River was low. We suspect habitat degradation and overharvest most responsible for the decline. In general, recruitment was poor for many species in both rivers. We suspect fluctuating flows have most severely affected recruitment. We recommend no harvest of shell material from either river until the populations show evidence of recruitment and have restored themselves, especially in the Pomme de Terre River.