Freshwater Mussel Bed Habitat in an Alluvial Sand-Bed-Material-Dominated Large River: A Core Flow Sediment Refugium?

Habitat degradation, organismal needs, and other effects influencing freshwater mussel declines have been subject to intense focus by conservationists for the last thirty plus years. While researchers have studied the physical habitat requirements and needs of mussels in small- to medium-sized rivers with variable levels of success, less research has been conducted on mussel habitat in larger non-wadeable rivers, especially at the reach scale, where core flow environmental conditions provide and maintain habitat for freshwater mussels. We designed a quasi-experimental observational field study to examine seven hydrologic energy and material variables laterally and longitudinally at Current and Extirpated mussel bed habitat reaches in lower White River, Arkansas, a large non-wadeable, sand-bed-material-dominated river. As expected, lateral and longitudinal hydrologic variable differences were identified within a reach. Mean velocity, bed velocity, the Froude number, and stream power were all significantly lower at Current mussel bed habitat stations within a sampling reach. Energy regime differences in shear stress and, marginally, stream power were higher at Extirpated mussel bed habitat reaches. Several factors emerged as important to mussel habitat in the White River. First, bed velocity warrants further exploration in terms of both flow strength and flow direction. Second, bedload appears to be the primary contributor to mussel habitat but requires additional exploration within the context of core and secondary flow pathway interactions. The combined empirical evidence from our study supports the flow refugium concept identified for mussel habitats in smaller systems but expands the concept to large non-wadeable streams and includes reach-scale refuge from sediment transport conditions.

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.

Can larval snapper, Pagrus auratus, smell their new home?

The ability to find a suitable settlement habitat after a pelagic larval period represents a significant challenge to marine settlement-stage larvae, and the mechanisms by which they achieve this are poorly understood. There is good evidence that olfactory cues are used by some coral reef fish larvae to locate suitable settlement habitats; however, the same understanding is lacking for marine temperate fish. Here we show for the first time that the larvae of an important commercial and recreational marine temperate fish, Pagrus auratus, can use olfactory cues to orient to appropriate settlement habitat. Using pairwise choice experiments, naive hatchery reared fish were offered water collected from a range of habitats in the Kaipara Harbour, an important nursery area for P. auratus. Larvae selected to swim towards water taken from over seagrass beds, their preferred settlement habitat, than water taken from the harbour entrance, Asian date mussel habitat, artificial seawater or artificial seawater in which seagrass had been soaked. The preference by the fish for water from the seagrass habitat over artificial seawater in which seagrass had been soaked strongly suggests that chemical cues from sources other than seagrass, such as from prey or conspecifics present in the seagrass habitat, may also be involved.