Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
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Item The Role of Connectivity and Spatial Structure on the Population Dynamics of Marine Fishes(2023) Arai, Kohma Herbert; Secor, David H; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Migrations regulate population structure, which can play an important role in conferring stability at aggregate scales via asynchronous responses of population sub-components to perturbation; however, little is known about the importance of spatial structure in population persistence in fishes. My dissertation aims to explore the role of spatial structure on the population dynamics of marine fishes. Two species that exhibit different types of population structure were considered: (i) Atlantic mackerel (Scomber scombrus) in the Northwest Atlantic, comprised of two components that have distinct spawning regions off Canada (northern contingent) and the US (southern contingent); and (ii) striped bass (Morone saxatilis) in the Hudson River (HR), which exhibit early-life partial migration whereby a portion of juveniles remain in their natal freshwater habitats, while others migrate into higher salinity habitats. In Chapter 2, I used otolith stable isotopes (d18O/d13C) to understand contingent mixing of the Northwest Atlantic mackerel over two decades (2000–2019). Prevalent contingent mixing occurred within the US waters, indicating that the northern contingent may provide subsidies to the US mackerel fishery. In Chapter 3, I combined machine learning with otolith d18O isoscapes to predict the geographic origin of the Northwest Atlantic mackerel spanning four decades (1975–2019). Contingent mixing occurred over four decades, including the 1970s when intensive foreign fisheries took place in US waters. Nursery hotspots were detected within spawning regions, but shifted over time. In Chapter 4, for HR juvenile striped bass, I explored the influence of early-life conditions and environmental drivers on partial migration. Otolith chemistry uncovered four dominant early migration modes. Partial migration was associated with larval growth, albeit facultatively controlled by environmental conditions. In Chapter 5, I evaluated how HR striped bass early-stage partial migration influenced recruitment patterns to the adult population over a 3-decade span. As an outcome of partial migration, adults recruited from a variety of nurseries, which exhibited asynchronous dynamics in response to climate variables. Through a comparative analysis of two species that exhibit different types of population structure, I demonstrated how spatial structure can play key roles in the population dynamics of marine fishes, with implications for management and conservation.Item Northwest Atlantic Mackerel (Scomber scombrus) Population Structure Evaluated using Otolith Stable Isotopes(2017) Redding, Stephen Gray; Secor, David H; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Stock assessments for Northwest Atlantic mackerel (Scomber scombrus) assume a single stock, comprised of northern and southern contingents, each with distinct natal regions in the US and Canada. I hypothesized that otolith δ18O and δ13C values would provide discrimination between these regions based upon hydrographic differences, and such information could illuminate seasonal migrations and contingent structure. Otoliths from regions throughout the North Atlantic Ocean were carefully milled to extract carbonates corresponding to the first year of life. Significant differences occurred in otolith isotope composition across the Atlantic basin, and within the Northwest Atlantic stock, despite annual variability. In the Northwest Atlantic, two separate natal habitats and associated contingents were supported for juvenile mackerel, but incursions by the northern contingent into US waters occurred in older fish (age>2). These findings indicate that stock structure assumptions should be revisited in the assessment and management of Northwest Atlantic mackerel.