Testing the assumption of annual shell ring deposition in freshwater mussels

We tested the assumption of annual shell ring deposition by freshwater mussels in three rivers using 17 species. In 2000, we notched shell margins, returned animals to the water, and retrieved them in 2001. In 2003, we measured shells, affixed numbered tags, returned animals, and retrieved them in 2004 and 2005. We validated deposition of a single internal annulus per year in all species and in 94% of specimens. Most unvalidated shells were old individuals with tightly crowded rings. Handling produced a conspicuous disturbance ring in all specimens and often resulted in shell damage. Observed growth was similar to but slightly lower than growth predicted by von Bertalanffy length-at-age models developed independently from shell annuli; further, handling specimens in 2 consecutive years reduced growth more than handling only once. These results show that mussels are extremely sensitive to handling. Brief handling does not likely increase short-term mortality, but repeated handling could decrease long-term fitness. Handling effects should be considered in sampling programs or when interpreting results of mark-recapture studies designed to estimate mussel growth. Production of annual shell rings is a pervasive phenomenon across species, space, and time, and validated shell rings can provide accurate estimates of age and growth.