Population Density and Growth of the Freshwater Mussel Anodonta californiensis in a Flow-Fragmented Stream

Abstract

ABSTRACT I studied the endemic California floater (Anodonta californiensis) density and growth in pools of a flow-fragmented stream that differed in amount of surface water inflow. Mussel densities, estimated using mark-and-recapture, were lowest in pools having intermittent summer inflows, and in one of those pools a length-frequency histogram of mussel shell lengths was truncated compared to measurements from pools with perennial inflows. In a summer experiment to measure monthly responsiveness of mussel growth to the range of temperature, dissolved oxygen, and pH conditions present in three pools with differing inflows, mean monthly instantaneous growth of caged mussels, based on whole body weight, ranged from 0.22%/day to 0.99% day and was significantly higher in a pool with intermittent inflows than in those with perennial inflows. Mussel shell growth and percentage of dry tissue weight, an indicator of energy stores in mussels, were also highest in the pool with intermittent inflows. Caged mussel growth was compared to un-caged counterparts. Caging did not have a detrimental effect on growth; growth of caged mussels in one pool exceeded by 20% that of un-caged mussels, while growth was nearly identical in another pool. Mussel growth was strongly correlated with water temperature (R2 = 0.67) but was only weakly and non-significantly correlated with other habitat variables. My results suggest that flow-fragmentation is responsible for reduced California floater densities in pools with intermittent inflows in Wenas Creek. However, mussel growth and condition was also highest in a pool with intermittent inflows, suggesting that monitoring growth and condition alone may not sufficiently indicate population responses to habitat change.