Author Type

Graduate Student

Date of Award

Summer 7-17-2026

Document Type

Dissertation

Publication Status

Version of Record

Submission Date

July 2026

Department

Biological Sciences

College Granting Degree

Charles E. Schmidt College of Science

Department Granting Degree

Biological Sciences

Degree Name

Doctor of Philosophy (PhD)

Thesis/Dissertation Advisor [Chair]

Matthew Ajemian

Abstract

Estuaries are productive but highly variable ecosystems that support recreationally and commercially valuable fisheries, as well as species of conservation concern. However, coastal development, habitat loss, altered freshwater inflow, and declining water quality can modify the environmental gradients that structure fish distributions. In the St. Lucie River and southern Indian River Lagoon, located in southeast Florida, managed freshwater discharges, watershed runoff, shoreline modification, and degradation of essential fish habitats (e.g., seagrass, oyster, and mangrove forests) have altered estuarine conditions.

This dissertation evaluates how four estuarine fish species with contrasting life histories respond to environmental variability, hydrologic disturbance, and habitat structure within an urbanized coastal system. In Chapter 1, I examined movement and habitat use for sheepshead (Archosargus probatocephalus), spotted seatrout (Cynoscion nebulosus), and juvenile goliath grouper (Epinephelus itajara). In Chapter 2, long-term fisheries independent monitoring data were used to assess the environmental drivers of abundance and occurrence of sheepshead and spotted seatrout within the estuary. Lastly, the nursery function of the estuary for the Critically Endangered smalltooth sawfish (Pristis pectinata) was evaluated via acoustic telemetry and spatial analysis in Chapter 3.

Acoustic telemetry showed that movement and space use of sheepshead, spotted seatrout, and juvenile goliath grouper were influenced by freshwater inflow, salinity, dissolved oxygen, and temperature, with responses varying by species and their location within the estuary. Individuals often used small core areas within broader estuarine ranges, suggesting that suitable habitat may be spatially constrained under changing environmental conditions. Environmental variability can alter not only where fishes occur, but also how they maintain connectivity across the various habitats within an estuary system.

Long-term Fisheries Independent Monitoring data from 1997–2024 provided broader context for species-level habitat associations across the SLR and southern IRL. Sheepshead were common and broadly distributed across both sampling zones within the study site, with occurrence and positive-catch abundance related to DO, temperature, depth, seasonality, and spatial position. In contrast, spotted seatrout were less frequently encountered, particularly in the St. Lucie River, and there was a strong association with specific bottom types, vegetation, and shoreline features.

The reemergence of the St. Lucie River and southern Indian River Lagoon as a nursery area for small juvenile smalltooth sawfish was documented in Chapter 3. Public encounter reports and acoustic telemetry showed increasing juvenile observations since 2015, strong site fidelity, year-round residency, and concentrated use of the South Fork and western Middle Estuary.

Together, this work demonstrates that fish responses to estuarine disturbance are species-specific, nonlinear, and shaped by both dynamic water quality gradients and static habitat structure. These findings inform fisheries management by identifying habitat needs, movement thresholds, and high-use areas that support recreationally important fishes and endangered species. They also provide guidance for habitat restoration and freshwater discharge planning by linking fish habitat use to salinity, DO, temperature, flow, and structurally complex habitats. Conservation strategies should prioritize protection of persistent core-use areas, restoration of functional nursery habitats, and maintenance of estuarine connectivity under changing environmental condition.

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