Dispersion of freshwater mussel larvae in a lowland river

We examined the dispersal of larvae (glochidia) of a common unionid mussel species, Actinonaias ligamentina, which need to attach to a host fish in order to develop into juveniles, in a lowland river (Sydenham River, Ontario, Canada). Generally, the decline in the number of glochidia captured with distance from release was best described by an inverse power function. The highest proportion was found in the first net 4 m downstream (range 0.1-3.6%), but a small proportion of glochidia was captured 96 m downstream (0-0.03%). This indicates that infestation of host fish may occur several tens to hundreds of meters downstream of the adults' location, even at relatively low flow conditions (mean velocity, 15 cm [s.sup.-1]). Dispersal distances increased with velocity, but the number of glochidia sampled at a given location can vary considerably due to stochastic effects of turbulence, especially at shorter distances. Individual trials could, therefore, deviate considerably from the predictions of an existing turbulent transport model (local exchange model), but overall there was a good correlation between measured data and model prediction. However, model predictions were quantitatively much higher than measured values (i.e., > 50 fold in some cases), which could be in part due to several simplifying assumptions of the model.