Comparing methods for estimating growth curves in a critically endangered mussel

Background/Question/Methods:

A common method for determining age structures of freshwater mussel populations is the use of external rings. However, some evidence suggests these rings may be produced at different rates by various species of mussels. More reliable methods for examining the age structure of freshwater mussel populations include the use of internal shell rings. Such data are not readily available because the mussel must be killed to observe them. Finally, observed growth increments have been used to estimate models of growth and length-at-age. To understand length-at-age relationships in freshwater mussels, we examined the ring structure of a critically endangered freshwater mussel species, Popenaias popeii. We collected individuals from three sites in the Black River, Eddy County, NM. We conducted surveys between 1997 and 2007, marking individuals and measuring their lengths, annually. We placed mussels in a shallow aquarium and photographed them using tagged image file format (TIFF) with a size standard. Using Image J, we measured the total length and the width of each growth ring. We compared length-at-age curves calculated from photographed growth increments of external rings with those calculated from Ford-Walford plots and internal rings of dead individuals to determine the efficacy of using external rings to estimate age.

Results/Conclusions:

The estimated maximum length for mussels was 96.78, 98.34 and 98.62 mm using Ford-Walford, external, and internal rings, respectively. All models had a strong inflection point at approximately 9 years, indicating a marked difference in growth rate before and after this age. Because growth rate is strongly non-linear, it may be inappropriate to use Ford-Walford relationships to parameterize length-at-age models in this species, because this method estimates growth parameters by comparing length at one year against length at a subsequent year. However, the estimates produced using this method are concordant with the other methods used. We also compared the width of the outermost observable rings with the observed growth increments, but found no relationship. The lack of a relationship between these measurements is likely due to a change in growth rates that prevents easy identification of rings in the larger individuals. This study suggests that for sensitive species or in cases where large numbers of dead individuals are not available, external annuli may produce estimates that are similar to those from more labor-intensive methods.