Cumulative spring discharge and survey effort influence threatened Suwannee moccasinshell, Medionidus walkeri, occupancy and detection

Freshwater mussels (Unionidae) are among the most imperiled groups of organisms in the world. Unionids are plagued with a lack of basic information regarding species distributions, life history characteristics, and ecological and biological requirements. We assessed the influence of hydrologic factors on the occurrence of the Suwannee Moccasinshell, Medionidus walkeri, a federally threatened freshwater mussel species endemic to the Suwannee River basin in Georgia and Florida. We also assessed the influence of survey effort on detection of M. walkeri during field surveys. All current (2013-2016) mussel survey records in the Suwannee River Basin were compiled, and cumulative discharge contributed by upstream springs was calculated for each of 220 survey locations. The spring discharge predictor variable was combined with M. walkeri detection/non-detection data from each survey location to develop a suite of occupancy models. Modeling results indicated that detection of M. walkeri during surveys was strongly and positively related to survey effort. Modeling results also indicated that sites with cumulative spring discharge inputs exceeding ~28 cms were most likely (i.e., predicted occupancy probabilities >0.5) to support M. walkeri populations; however, occupancy declined in the lowermost reaches of the Suwannee main stem despite high spring discharge inputs, presumably due to greater tidal influences and differences in physicochemical habitat conditions. Historical localities where M. walkeri have presumably been extirpated are all devoid of springs in their upstream watersheds. We hypothesize that springs may buffer extremely tannic, at times polluted surface waters, and maintain adequate flows during periods of drought, thereby promoting the persistence of M. walkeri populations. Our study suggests that springs are a critical resource for M. walkeri and may be more important for conservation planning than previously recognized.