Benjamin D. Walther

Interactive effects of ontogeny, food ration and temperature on elemental incorporation in otoliths of a coral reef fish

The potential for environmental and physiological modification of elemental incorporation in otoliths is significant and must be validated before otoliths can be used reliably to estimate water parameters over the life history of a fish. We experimentally manipulated temperature and diet quantity for juvenile, sub-adult, and adult Acanthochromis polyacanthus, a tropical damselfish of the SW Pacific. Significant interactive effects between life history stage, temperature and food quantity were observed for otolith Ba/Ca, while significant interactions between stage and food were observed for Sr/Ca. Specific growth rates were negatively correlated with DBa and DSr for juveniles and sub-adults. These interactions indicated elemental incorporation dynamics varied depending on the life history stage, suggesting variation in effects of stage-specific metabolism or reproductive status. Our results highlight complex responses of elemental incorporation to both endogenous and exogenous factors. Interpretations of life history transects across otoliths must account for these effects to avoid confounding environmental variability with ontogenetic changes in physiology.

Movements of Diadromous Fish in Large Unregulated Tropical Rivers Inferred from Geochemical Tracers

Patterns of migration and habitat use in diadromous fishes can be highly variable among individuals. Most investigations into diadromous movement patterns have been restricted to populations in regulated rivers, and little information exists for those in unregulated catchments. We quantified movements of migratory barramundi Lates calcarifer (Bloch) in two large unregulated rivers in northern Australia using both elemental (Sr/Ba) and isotope (87Sr/86Sr) ratios in aragonitic ear stones, or otoliths. Chemical life history profiles indicated significant individual variation in habitat use, particularly among chemically distinct freshwater habitats within a catchment. A global zoning algorithm was used to quantify distinct changes in chemical signatures across profiles. This algorithm identified between 2 and 6 distinct chemical habitats in individual profiles, indicating variable movement among habitats. Profiles of 87Sr/86Sr ratios were notably distinct among individuals, with highly radiogenic values recorded in some otoliths. This variation suggested that fish made full use of habitats across the entire catchment basin. Our results show that unrestricted movement among freshwater habitats is an important component of diadromous life histories for populations in unregulated systems.

High resolution profiles of elements in Atlantic tarpon (Megalops atlanticus) scales obtained via cross-sectioning and laser ablation ICP-MS: a literature survey and novel approach for scale analyses

Fish scales may be used as nonlethal alternative structures to otoliths for establishing chemical time-series that trace migration and habitat use for euryhaline species such as Atlantic tarpon (Megalops atlanticus). We survey four decades of fish scale studies employing elemental analyses to show the wide diversity of aims, approaches and instrumentation. Analyses are mainly solution-based measurements of whole or subsampled scales or in situ solid-sampling techniques of specific growth regions. Although spot-based transects across scales have been employed for a handful of species, continuous elemental transects across scale growth increments are rare. Laser ablation based sampling methods that assay scale exteriors can be difficult to implement for species with natural topographic variations exceeding the focal range of the laser. These limitations may be overcome by performing laser ablation of scales in cross-section when scale thickness in the well-calcified external layer provides enough surface area for a laser transect. For this study, scales of Atlantic tarpon were embedded in epoxy and longitudinally cross-sectioned to expose the scale growth record of the external layer. Laser ablation ICP-MS was used to quantify continuous profiles of element:Ca ratios across scale growth increments. The analytes surveyed (Mg, P, Ca, Mn, Sr, and Ba) were consistently above the limits of detection (LODs) across each scale with the exception of Pb for one scale. Obtaining high resolution elemental life history profiles from scales will be essential for future efforts to use this structure as a non-lethal substitute for otoliths in Atlantic tarpon and other migratory fishes that possess scales with sufficiently thick calcified external layers to permit a cross-sectional approach.

Environmental Records from Great Barrier Reef Corals: Inshore versus Offshore Drivers

The biogenic structures of stationary organisms can be effective recorders of environmental fluctuations. These proxy records of environmental change are preserved as geochemical signals in the carbonate skeletons of scleractinian corals and are useful for reconstructions of temporal and spatial fluctuations in the physical and chemical environments of coral reef ecosystems, including The Great Barrier Reef (GBR). We compared multi-year monitoring of water temperature and dissolved elements with analyses of chemical proxies recorded in Porites coral skeletons to identify the divergent mechanisms driving environmental variation at inshore versus offshore reefs. At inshore reefs, water Ba/Ca increased with the onset of monsoonal rains each year, indicating a dominant control of flooding on inshore ambient chemistry. Inshore multi-decadal records of coral Ba/Ca were also highly periodic in response to flood-driven pulses of terrigenous material. In contrast, an offshore reef at the edge of the continental shelf was subject to annual upwelling of waters that were presumed to be richer in Ba during summer months. Regular pulses of deep cold water were delivered to the reef as indicated by in situ temperature loggers and coral Ba/Ca. Our results indicate that although much of the GBR is subject to periodic environmental fluctuations, the mechanisms driving variation depend on proximity to the coast. Inshore reefs are primarily influenced by variable freshwater delivery and terrigenous erosion of catchments, while offshore reefs are dominated by seasonal and inter-annual variations in oceanographic conditions that influence the propensity for upwelling. The careful choice of sites can help distinguish between the various factors that promote Ba uptake in corals and therefore increase the utility of corals as monitors of spatial and temporal variation in environmental conditions.

Use of a Natural Isotopic Signature in Otoliths to Evaluate Scale-Based Age Determination for American Shad

Abstract We used &dgr;18O signatures in otoliths as a natural tag for hatch year to evaluate the scale-based age determination method used for adult American shad Alosa sapidissima in the York River, Virginia. Juveniles of the 2002 year-class exhibited high &dgr;18O values in otolith cores that identified adult members of the cohort as they returned to spawn. Recruitment of the 2002 cohort was monitored for three consecutive years, identifying age-4, age-5, and age-6 individuals of the York River stock. The scale-based age determination method was not suitable for aging age-4, age-5, or age-6 American shad in the York River. On average, 50% of the individuals from the 2002 year-class were aged incorrectly using the scale-based method. These results suggest that the standard age determination method used for American shad is not applicable to the York River stock. Scientists and managers should use caution when applying scale-based age estimates to stock assessments for American shad in the York River and throughout their range, as the applicability of the scale-based method likely varies for each stock. This study highlights a promising new direction for otolith geochemistry to provide cohort-specific markers, and it identifies several factors that should be considered when applying the technique in the future.

Frontiers in marine movement ecology: mechanisms and consequences of migration and dispersal in marine habitats

Dispersal and migration are important ecological processes that influence rates of propagule exchange, colonization, extinction and speciation for a wide array of taxa in terrestrial and aquatic systems. The role of movement patterns in the marine environment is particularly critical, because a