Colin P. Shea

Misidentification of freshwater mussel species (Bivalvia:Unionidae): contributing factors, management implications, and potential solutions

Abstract Surveys of freshwater mussel populations are used frequently to inform conservation decisions by providing information about the status and distribution of species. It is generally accepted that not all mussels or species are collected during surveys, and incomplete detection of individuals and species can bias data and can affect inferences. However, considerably less attention has been given to the potential effects of species misidentification. To evaluate the prevalence of and potential reasons for species misidentification, we conducted a laboratory-based identification exercise and quantified the relationships between mussel species characteristics, observer experience, and misidentification rate. We estimated that misidentification was fairly common, with rates averaging 27% across all species and ranging from 0 to 56%, and was related to mussel shell characteristics and observer experience. Most notably, species with shell texturing were 6.09× less likely than smooth-shelled species to be misidentified. Misidentification rates declined with observer experience, but for many species the risk of misidentification averaged >10% even for observers with moderate levels of experience (5–6 y). In addition, misidentification rates among observers showed substantial variability after controlling for experience. Our results suggest that species misidentification may be common in field surveys of freshwater mussels and could potentially bias estimates of population status and trends. Misidentification rates possibly could be reduced through use of regional workshops, testing and certification programs, and the availability of archived specimens and tissue samples in museum collections.

Estimation of mussel population response to hydrologic alteration in a southeastern U.S. stream

The southeastern United States has experienced severe, recurrent drought, rapid human population growth, and increasing agricultural irrigation during recent decades, resulting in greater demand for the water resources. During the same time period, freshwater mussels (Unioniformes) in the region have experienced substantial population declines. Consequently, there is growing interest in determining how mussel population declines are related to activities associated with water resource development. Determining the causes of mussel population declines requires, in part, an understanding of the factors influencing mussel population dynamics. We developed Pradel reverse-time, tag-recapture models to estimate survival, recruitment, and population growth rates for three federally endangered mussel species in the Apalachicola–Chattahoochee–Flint River Basin, Georgia. The models were parameterized using mussel tag-recapture data collected over five consecutive years from Sawhatchee Creek, located in southwestern Georgia. Model estimates indicated that mussel survival was strongly and negatively related to high flows during the summer, whereas recruitment was strongly and positively related to flows during the spring and summer. Using these models, we simulated mussel population dynamics under historic (1940–1969) and current (1980–2008) flow regimes and under increasing levels of water use to evaluate the relative effectiveness of alternative minimum flow regulations. The simulations indicated that the probability of simulated mussel population extinction was at least 8 times greater under current hydrologic regimes. In addition, simulations of mussel extinction under varying levels of water use indicated that the relative risk of extinction increased with increased water use across a range of minimum flow regulations. The simulation results also indicated that our estimates of the effects of water use on mussel extinction were influenced by the assumptions about the dynamics of the system, highlighting the need for further study of mussel population dynamics.

An Evaluation of the Relative Influence of Habitat Complexity and Habitat Stability on Fish Assemblage Structure in Unregulated and Regulated Reaches of a Large Southeastern Warmwater Stream

Abstract River regulation and development are the foremost problems threatening lotic fishes and other aquatic biota in the United States. The operation of hydroelectric facilities can influence both habitat availability and environmental stability in downstream reaches. We evaluated the relative influence of habitat complexity and environmental stability on fish assemblage structure at unregulated and hydropower-regulated reaches of the Flint River in southwestern Georgia. The availability of different habitat types was highly variable at the regulated reach owing to large, daily fluctuations in discharge. Habitat-specific fish assemblages also differed between reaches, as a greater number of species occupied identical habitat types at the unregulated reach, most notably in shallow, slow-flowing habitats. Differences in fish assemblage structure between study reaches in comparable habitat types were explained equally well by patterns of habitat structure and variability. Within-reach patterns of fish ass...

Development and Evaluation of a Stream Channel Classification for Estimating Fish Responses to Changing Streamflow

Abstract Current approaches to assessing the potential effects of river regulation and water use on stream fish communities are based on physical habitat simulation and are not feasible for estimating these effects over large spatial scales. We developed a channel classification for streams in the lower Flint River basin, Georgia, based on gross channel morphology and geology and evaluated its usefulness at 23 study sites representing the four channel types in the basin. Our channel classification separated stream channel types based on dominant geology (upland residuum versus Ocala limestone) and channel form (confined versus unconfined). Fish were sampled and habitat measured at the study sites in spring, summer, and winter from 2001 to 2004, a period that included some of the lowest and highest seasonal flows ever recorded. The channel types differed with respect to diurnal variability in temperature and dissolved oxygen concentration as well as habitat characteristics (substrate, large wood). Statisti...

Imperfect Recapture: A Potential Source of Bias in Freshwater Mussel Studies

Abstract Assessing the population status and dynamics of species is an important component of monitoring efforts aimed at improving understanding of relationships between freshwater mussels (Bivalvia:Unionidae) and their environment. Most freshwater mussel population assessments are conducted using raw count (density, abundance) or presence/absence data but relatively few studies account for potential biases associated with incomplete detection of individuals or species during sampling. We conducted a capture-mark-recapture study over 7 y to assess survival and recapture probabilities of three federally endangered freshwater mussels in a small southeastern U.S. stream. Although similar numbers of mussels were collected among sampling occasions, only a small proportion of the individuals present within the sampling site were collected on any given occasion. Mean apparent survival was 0.81 and did not vary among species or among years. Modeling results indicated that recapture probabilities varied among species, through time, and among individuals as a function of shell length. Recapture probabilities for Pleurobema pyriforme and Medionidus penicillatus increased with increased shell length, whereas recapture probabilities for Hamiota subangulata decreased with increasing shell length. Under the generally accepted assumption of constant recapture among sampling occasions, apparent survival estimates ranged from 0.89 to 0.99 among the three species and decreased for H. subangulata with increased shell length. Our study demonstrates that failure to account for incomplete recapture of freshwater mussels could bias inferences regarding the status and trajectory of mussel populations, which may in turn result in the implementation of ineffective management and conservation strategies.

Multispecies Occupancy Modeling as a Tool for Evaluating the Status and Distribution of Darters in the Elk River, Tennessee

AbstractSixteen darter species, including the federally endangered Boulder Darter Etheostoma wapiti, are known to occur in the Elk River, a large, flow-regulated tributary of the Tennessee River, Tennessee–Alabama. Since the construction of Tims Ford Dam (TFD) in 1970, habitat modification caused by cold, hypolimnetic water releases and peak-demand hydropower generation has contributed to population declines and range reductions for numerous aquatic species in the main-stem Elk River. We developed Bayesian hierarchical multispecies occupancy models to determine the influence of site- and species-level characteristics on darter occurrence by using presence–absence data for 15 species collected from 39 study sites. Modeling results indicated that large-river obligate species, such as the Boulder Darter, were 6.92 times more likely to occur for every 37-km increase in the distance downstream from TFD. In contrast, small-stream species were 2.35 times less likely and cosmopolitan species were 1.88 times less ...

Cumulative Spring Discharge and Survey Effort Influence Occupancy and Detection of a Threatened Freshwater Mussel, the Suwannee Moccasinshell

Abstract Freshwater mussels (Unionidae) are among the most imperiled groups of organisms in the world, and the lack of information regarding species distributions, life-history characteristics, and ecological and biological requirements may limit the protection of remaining mussel populations. 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. We compiled all recent (2013–2016) mussel survey records in the Suwannee River Basin. We calculated cumulative discharge contributed by upstream springs for each of 220 survey locations. We combined the spring discharge predictor variable with Suwannee Moccasinshell detection and nondetection data from each survey location to develop a suite of occupancy models. Modeling results indicat...

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.