Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Mary C. Fabrizio
The focus of this dissertation is to investigate relationships among indices of fish condition and examine temporal and spatial dynamics in condition of juvenile summer flounder, striped bass, and Atlantic croaker from Virginia estuaries. In the first two chapters, I compare multiple direct (energy density and proximate components) and indirect (length-based indices, hepatosomatic index, and relative subdermal lipids) approaches for assessing fish health to identify suitable means of estimating the lipid and energy content of juvenile fishes. Because indirect approaches can rapidly estimate the condition of juvenile fishes, these indices were then used to assess spatial and temporal variability in the condition of these species in the third and fourth chapters. Finally, in the fifth chapter I describe the influence of condition on mediating thermal stress of juvenile fishes during winter. Length-based indices, subdermal lipid estimates, and water content of whole fish can accurately assess lipid and energy content for juveniles of these species. Temporal patterns in length-based condition indices, the hepatosomatic index (HSI), and relative estimates of subdermal lipid content revealed energy-storage strategies used by juveniles to prepare for physiologically stressful events: the onset of winter and migration to the continental shelf. Notably, mean subdermal lipid reserves increased in preparation for migration of juvenile Atlantic croaker and summer flounder, whereas mean HSI values increased for all three species during winter. Spatial patterns in condition identified habitats that produced well-conditioned individuals which were energetically prepared for these physiologically stressful events. Prior to migration, Atlantic croaker were in high condition in locations with a low density of conspecifics, whereas summer flounder captured in coastal lagoons of the eastern shore of Virginia exhibited high condition. The mouth of the Rappahannock River and upestuary regions of the York River supported striped bass in high condition during the winter. Regardless of species, individuals that reside in Chesapeake Bay during winter are expected to decline in condition due to low water temperatures and reduced feeding opportunities. Yet, variability in physiological tolerances and energy utilization strategies among species differentially influenced their resistance to thermal stress, such that Atlantic croaker have a high probability of mortality in prolonged winters whereas striped bass do not. Mortality of summer flounder is strongly impacted by a blood flagellate infection common during winter. My findings suggest that the condition of individual fish can be used to understand energy allocation strategies of juvenile fishes and identify areas that may represent essential habitats that support individuals in healthier condition.
© The Author
Schloesser, Ryan W., "Condition of Juvenile Fishes in Estuarine Nursery Areas: Measuring Performance and Assessing Temporal and Spatial Dynamics with Multiple Indices." (2015). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539616842.