Age, growth, and mortality of giant floater (Pyganodon grandis) in eastern South Dakota, USA natural lakes and reservoirs

The freshwater mussel Giant Floater Pyganodon grandis is native to North America. This study documented Giant Floater distribution, age, growth, and mortality rates in eastern South Dakota, USA lakes and reservoirs. The trophic status of water bodies where Giant Floater were collected was also assessed. Live and dead Giant Floater shells were observed in 13 natural lakes and eight reservoirs in the Big Sioux, James, Minnesota, and Missouri River basins. No Giant Floater were collected from waters in the Red River basin. Trophic State Index levels in the water bodies containing Giant Floater ranged from 55.8 to 98.3. Mean values were nearly identical between lakes and reservoirs at 68.9 and 68.7, respectively. Giant Floater ages ranged from 4-to-11 years. Mean age was not significantly different between natural lakes and reservoirs, at 6.1 and 7.8 years, respectively (p = 0.054). Mean estimated length was not significantly different between lakes and reservoirs at 9.71 cm and 12.90 cm, respectively (p = 0.115). Similarly, mean growth coefficient (K) was not significantly different between lakes at 0.27 cm/year and reservoirs at 0.23 cm/year (p = 0.406). Mean annual mortality was 36.7% and was not significantly different between lakes and reservoirs (p = 0.054). A significant negative relationship was found between Giant Floater maximum age and natural lake trophic state (R2 = 0.394, p = 0.022), but no such relationship was observed in reservoirs. There were no significant linear relationships between growth, estimated length, instantaneous mortality, annual mortality, and the trophic state of natural lakes or reservoirs.