How can we tell how old a fish is and why do we need to know?
Age data is one of the standard components fish managers need to assess the status of a fishery. In order for age-related metrics to be valuable for sustainable management decisions, precise and accurate age data must be generated. Scientists commonly use calcified structures to estimate the age of a fish, and the most accurate and precise structure varies among species. Structures can be extracted from the fish in the field (if a non-lethal structure, such as a pectoral fin ray, is being removed) or in the laboratory (if a lethal structure, such as an otolith, is being removed). The structure is then processed in the laboratory and placed under a microscope to view annuli and estimate age. Different structures are processed differently and the technique used requires careful consideration, as the process selected is a crucial component to viewing annuli consistently.
Few studies have addressed the selection of accurate and precise calcified structures for gars. Therefore, we implemented a study in 2015 to evaluate the relative precision and bias of age estimates obtained from four calcified structures of Longnose Gar, Shortnose Gar, and Spotted Gar. Gars were collected from twelve Illinois waterbodies and branchiostegal rays, pectoral fin rays, cleithra, and sagittal otoliths were extracted for age comparisons.
Accuracy of Calcified Structures Differ by Species
We discovered that age assignments were different among calcified structures for all three gar species. This implies that some structures must be inaccurate and age-related metrics developed from these ages will differ depending on the structure used.
We also compared age assignments between two readers to evaluate relative bias. These data are useful to provide information pertaining to how readable (i.e., how easy is it to count annuli) the structure is. Readers in our study found that annuli in sagittal otolith sections and whole cleithra were more difficult to distinguish relative to annuli in branchiostegal rays and pectoral fin rays. This finding suggests that future studies should modify the processing techniques we used to age otoliths and cleithra to view annuli more clearly.
Our findings reveal the need to improve calcified structure processing methods to increase annuli visibility when using cleithra and otoliths for age estimation. Furthermore, because ages were different among structures, it is crucial future studies examine the accuracy of age estimates, as these data will directly influence management decisions.
This study was completed and published in 2018 in the Transactions of the American Fisheries Society.
King, S. M., David, S. R., and J. A. Stein. 2018. Relative bias and precision of age estimates among calcified structures of Spotted Gar , Shortnose Gar, and Longnose Gar. Transactions of the American Fisheries Society 147(4): 626-638.