Mapping and Modeling the Distribution, Abundance, and Habitat Associations of the Endangered Fat Threeridge in the Apalachicola River System

Large, Coastal Plain rivers of the southeastern United States contain some of the most diverse freshwater communities in North America; however, surveying the fauna of these large rivers presents numerous logistical and statistical challenges. We assessed the contemporary distribution, abundance, and habitat associations of the endangered fat threeridge mussel Amblema neislerii throughout the Apalachicola River system in northwestern Florida. To achieve this goal, we used side scan sonar to map the distribution of mesohabitats and conducted a system-wide, quantitative survey to define mussel habitat associations. We then used habitat and mussel data to develop predictive models of spatial distribution and to estimate the abundance of fat threeridge across the entire Apalachicola River system. Findings revealed a broadly distributed (i.e., 128 river kilometers occupied), robust population of approximately 9 million individuals (95% CI = 5–12 million), with a center of distribution (i.e., where abundance and occurrence were highest) approximately 45–80 river kilometers upstream of the river mouth. Fat threeridge primarily occupy fine sediment mesohabitats characterized by smooth/plane bedforms that are clearly definable via sonar habitat mapping. We hypothesize that this species may be particularly sensitive to the availability of stable, fine sediments during one or more critical life history phases and that the availability of this habitat may explain its restricted distribution in tributary rivers. Our study provides a quantitative, replicable foundation upon which future population and habitat monitoring can be based.

Carbon Reservoir Effects in Eastern Oyster from Apalachicola Bay, USA

Eastern oyster (Crassostrea virginica) is an ubiquitous estuarine shellfish taxon in eastern North America and one of the most abundant materials available for radiocarbon (14C) dating. We examine spatiotemporal variability in carbon reservoir effects among pre-bomb oysters from Apalachicola Bay, USA, a river-influenced estuary on the northern Gulf of Mexico. Shells were sampled at multiple points along the valve to produce time-series records of 14C variation during the lives of the mollusks. Conventional ages within shells differed by as little as 36 14C yr to as much as 295 14C yr. Reservoir offsets varied sub-regionally within the estuary, increasing from 92±37 yr in the eastern edge of study region to 227±110 yr in the west, reflecting the influence of 14C-depleted dissolved inorganic carbon from the Apalachicola River. Dynamic carbon reservoirs can pose problems for the estimation of ΔR and for building coastal chronologies. Estimating sub-regional ΔR values can be useful for assessing the range of variability in reservoir offsets within an estuary, and for correcting sample ages if the shell origin is known. Greater variability and/or uncertainty in ΔR lead to greater uncertainty in the calibrated age.

Validation of Annulus Formation in Spotted Sucker Otoliths

Spotted suckers Minytrema melanops are found in many river systems from the Midwest to the Southeast United States. Though they are a relatively abundant species, otolith age validation methods have not been reported. The objective of this study was to validate annulus formation on the lapillar otolith for spotted suckers. We collected spotted suckers monthly from December 2005 to November 2006 in the Apalachicola River, Florida. Using marginal incremental analysis on whole browned lapillar otoliths, we validated that spotted suckers form one annulus per year between May and June. We also report population characteristics including a length–weight regression. This study validates otolith aging to provide an avenue for accurately assessing the age composition of spotted sucker populations, which will facilitate a better understanding of growth, recruitment, and mortality of the species.