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. Most unionids lack basic information regarding species distributions, life history characteristics, and ecological and biological requirements. We examined 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 evaluated the influence of survey effort on detection of Suwannee Moccasinshell during field surveys. All recent (2013-2016) mussel survey records in the Suwannee River Basin were compiled. For each of 220 survey locations cumulative discharge contributed by upstream springs was calculated. The spring discharge predictor variable was combined with Suwannee Moccasinshell detection and non-detection data from each survey location to develop a suite of occupancy models. Modeling results indicated that detection of Suwannee Moccasinshell 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 Suwannee Moccasinshell 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 Suwannee Moccasinshell have presumably been extirpated are all devoid of springs in their upstream watersheds. We hypothesize that springs may buffer extremely tannic, and at times polluted, surface waters, in addition to maintaining adequate flows during periods of drought, thereby promoting the persistence of Suwannee Moccasinshell populations. Our study suggests that springs are a critical resource for Suwannee Moccasinshell and may be more important for conservation planning than previously recognized.