EFFECTS OF FISH HOST AVAILABILITY, WATER TEMPERATURE, AND DISCHARGE ON TWO POPULATIONS OF MUSSELS IN THE ST. CROIX RIVER, MN

Brandon Sansom, Washington and Jefferson College
Daniel J. Hornbach, Macalester College
Mark Hove, Macalester College

Population stability and growth of freshwater mussels are dependent upon a number of biotic and abiotic factors. Population stability is reliant upon juvenile recruitment, which is mainly controlled by fish host availability. Some mussels are considered host generalists, having many host fish species, while other mussels are host specialists, having only one or a few host fish. Annual growth is largely controlled by water temperature and discharge. Population densities of a host specialist mussel, Truncilla truncata, are declining at a location below a hydroelectric power dam in the St. Croix River, MN. However, densities of a host generalist, Actinonaias ligamentina, remain constant both above and below the dam. The objectives of this research were to detect whether differences in host availability is contributing to the differences in mussel density, and to determine how water temperature and discharge are impacting annual growth both above and below the dam. A. ligamentina and T. truncata were collected at sites above and below the dam; mussel length and growth (annual) rings were measured for each mussel. Since shells were eroded, we used x-intercepts (i.e. prediction of eroded area = 0) and their 95% confidence intervals from regressions modeling eroded area with each respective ring to estimate the most likely length of each annual ring. The ring measurements were adjusted accordingly, and mussel age was determined by summing the total number of rings. We used the relationships between shell length and age to convert shell-length frequency histograms to age distributions and used these to examine patterns of recruitment. Host fish CPUE (catch per unit effort) for each species was collected from various state agencies in MN and WI. A. ligamentina has multiple fish hosts and had higher host fish CPUE at both sites, which may be related to the more consistent density noted. Lower abundances of the host fish, Aplodinotus grunniens, for T. truncata were found below the dam, which may imply that the sporadic mussel recruitment noted is closely linked to high year classes of host fish. Annual growth varied between and within species, across sites, and was influenced by the interaction of mussel age and year. Growth variation for A. ligamentina above the dam was related to discharge and water temperature. Below the dam, neither discharge nor water temperature significantly influenced growth for A. ligamentina nor T. truncata, suggesting that other factors, such as water quality or food quantity and/or quality, may be more important in controlling growth rates.