Connectivity in a locally adapted fish species, Menidia menidia, as revealed through otolith geochemistry

Patterns of connectivity are important in understanding the geographic scale of local adaptation. High connectivity, or gene flow, is assumed in most marine species due to life histories that include widely dispersive stages. Studies showing evidence of local adaptation in Menidia menidia raise questions concerning the degree of connectivity in this species. To address the question of connectivity, I first examine the usefulness of otolith signatures to serve as a unique natural tag and then apply the method to otoliths collected from locations along the northeastern coast of the United States. In Chapter 1, I examine the influence of both genotype and genotype x environment interactions on the chemical composition of otoliths from lab-reared stocks. In Chapter 2, I characterize the geochemical otolith signatures from juvenile fish of known origin and identify the spatial and temporal patterns that allow these signatures to act as natal chemical fingerprints. In Chapter 3, I use the signatures developed in the previous chapter to classify the otoliths from captured adults of unknown origin back to natal origin. Using this information, I calculate the mixed stock composition for each location and the probability of migrating certain distances and directions. This reveals the level of connectivity present in this species. Finally, in Chapter 4, I examine the spatial scale at which factors influence the abundance of Menidia menidia given these dispersal patterns. Using over 25 years of data collected by two different beach seine surveys, I investigate factors influencing the intra-and inter-annual variation in abundance of Menidia menidia in the Hudson River. I then compare the patterns present in the Hudson River to those of other estuaries from Virginia to Rhode Island to examine the effect of local versus regional factors. The findings presented in this dissertation suggest that otoliths are useful natural tags for continuously distributed coastal fish species, such as Menidia menidia. Otolith signatures show among population differences due to genetic differences. Furthermore, signatures varied spatially along the coast allowing for them to be used as a tool to examine connectivity in the adults. Dispersal patterns showed that adult fish had the highest probability of migrating back to their site of natal origin, but there was also evidence of extensive mixing throughout the range. These findings suggest marine species with largely open populations are capable of local adaptation despite high gene flow. Additionally, results show that abundance patterns in populations along the east coast fluctuate largely independently of one another, but evidence of a "coastwide" signal could be the result of dispersal and/or a Moran effect.