By Summer Lindelien
In Florida, Largemouth Bass (LMB) are mainly aged by interpreting annular rings (i.e., annuli) deposited in sagitta otoliths, but extraction can only be accomplished via lethal dissection. Development of non-lethal aging techniques would be a stepping-stone for new research. For example, FWC partners with bass anglers through TrophyCatch, tournaments, and tagging studies which largely, if not explicitly, involve catch and live-release of LMB. Incorporating a non-lethal aging method in these activities would increase the breadth and application of data collected by anglers and scientists; thus, it would be a vital tool for LMB management and conservation in Florida.
Our current non-lethal aging investigations are focused on dorsal spines III–V. Dorsal spines are accessible, easy to remove and process, and when cross-sectioned they can provide relatively precise and accurate ages. The initial step towards implementation was to evaluate the assumption of non-lethality of dorsal spine removal. The objectives of our study were to 1) determine the effects of dorsal spine excision on LMB survivability, and 2) determine if LMB size affects survival following dorsal spine excision.
To evaluate survival after dorsal spine removal, we collected 36 wild LMB across a range of sizes (30–57 cm total length; TL). For positive fish identification, we implanted each LMB with a passive integrated transponder tag and associated them with a weight (g) and TL (mm). After transporting the LMB, we randomly established them in six identical 1200-gallon outdoor tanks where they acclimated for a week. Next, we removed dorsal spines III–V (n = 18) with a pair of cutting pliers and surgical scissors. During the experiment, each tank was occupied by three LMB with excised dorsal spines and three with no excised dorsal spines (i.e., controls). We fed LMB a mixture of wild-caught crayfish, tadpoles, and Bluegill, which were evenly apportioned among tanks at each feeding.
Over the 35-day study, no mortalities were observed for LMB with excised dorsal spines, and experiment-wide survival was 0.94. Survival was not affected by LMB size, so we proceeded to look for differences in survival between groups without the effect of TL. Ultimately, survival was not different between excised and non-excised LMB (p = 0.15). Despite LMB being fed throughout the trial, all fish exhibited a significant decrease in weight after the study (p < 0.001). On average, LMB lost 105 g, but there was not a significant difference in weight loss between treatment groups. The areas of excision healed with no visible wounds or sublethal effects; however, we noted some LMB with potential handling sores on other parts of their bodies. Consequently, proper care and handling of fish should be kept in mind moving forward with this non-lethal aging technique.
Our current research is focused on continued validation of dorsal spine aging accuracy and precision in LMB across a diverse suite of Florida waterbodies: Lake Griffin, Stick Marsh/Farm 13 Reservoir, Fellsmere Reservoir, L-67A Canal, Escambia River Marsh, and Apalachicola River. As our accuracy is better documented, removal of dorsal spines likely will be taught to fisheries biologists and citizens who handle trophy-bass (≥ 3.63 kg) frequently, allowing an avenue for collection of age data without sacrificing bigger LMB.