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Pelagic larval duration (PLD) likely influences evolutionary processes including dispersal, genetic connectivity, and extinction in aquatic organisms. PLD has been well studied in marine systems, but very few freshwater species have been studied. Darters are a diverse group of freshwater North American fish with available information on the length of this stage from propagation efforts. There is surprising variation in the length of this stage ranging from 0 to 60 days. By compiling information from Conservations Fisheries, Inc. (Knoxville, TN) and the literature, we were able to make comparisons between the PLD of 23 species and other life history characteristics. We hypothesized that 1) PLD will influence developmental characteristics and reflect tradeoffs at these critical stages 2) a higher extinction risk should associate with lower PLD. Size at yolk absorption was found to have a marginally significant, relationship with PLD, but juvenile size and maximum size were found to have positive relationships. These relationships indicate a possible developmental tradeoff between larval survival and potential size and that pelagic larval duration in darters is greatly influenced by life history characteristics. Additionally, through a phylogenetic comparative analysis, we found lower PLD in darter lineages was evolutionarily associated with extinction risk, which is similar to patterns commonly recovered from the marine fossil record. These findings provide some of the first support in extant taxa for the hypothesis that PLD is an important determinant of extinction. This study indicates that PLD is highly influential in species life history characteristics and predictions of species’ persistence.

A frequency domain method of detecting and spatially locating structural nonlinearities using measurements made during a typical modal test is further developed and evaluated in this thesis. The combined coherence function contains drops due to digital signal processing (DSP) errors, however it is unaffected by nonlinearities. By comparing combined coherence to ordinary coherence (or multiple combined coherence to multiple coherence) one can determine whether or not a drop in ordinary coherence is due to a nonlinearity or a DSP error. Previous work was done by Tom Roscher and Dr. Randy Allemang with theoretical data generated from a lumped parameter (M, K, C) model [1, 2]. This paper expands on that work in two ways: 1) By applying the formulation to a “real-world” test structure. 2) By expanding the theoretical background to enable the use of post-processed data for multiple input testing with Multiple Combined Coherence (MCCOH) In applying combined coherence to two physical substructures with linear and/or nonlinear connections, the results of several testing scenarios are examined to see how they affect the ability of combined coherence to spatially locate nonlinearities. These include: 1) Cases with and without leakage errors 2) Varying the number of spectral averages 3) Reducing the number of nonlinear paths 4) Varying the input force locations 5) Changing the spatial density of the responses