PhD Dissertation Defense: Martin Benavides

The PhD Dissertation Defense of Martín Benavides is presented by the University of North Carolina at Chapel Hill’s, Department of Marine Sciences and the Institute of Marine Sciences. This event will be held on Wednesday, April 1st at 2:00 pm. This event will be held ONLINE ONLY and it will be available online via Zoom (Use Zoom Meeting ID: 252 726 6841).

Title: Variability in coastal shark populations across multiple spatiotemporal scales

Abstract: The importance of understanding variability across spatiotemporal scales has been recognized by oceanographers as a fundamental issue in understanding ecosystem dynamics. Sharks have emerged as a conservation concern as population declines and their controls on ecosystem dynamics have become more apparent. Efforts to better manage and understand shark populations and their response to anthropogenic pressures have been hindered by a lack of understanding of cross-scale dynamics. This dissertation aimed to describe patterns of variability in coastal shark populations across multiple spatiotemporal scales.

Chapter 1 exploited a 45-year time series database of shark monitoring to describe patterns of seasonality in the coastal shark community in Onslow Bay, North Carolina, and how seasonality has shifted over longer timescales (interannual-decadal). Seasonal turnover in coastal shark community composition was correlated with temperature changes, with spring/autumn species appearing first and subsequently being replaced by summer species before appearing again in autumn; both transitions occurred at approximately 25 °C. On interannual scales, this seasonal pattern was overshadowed by increases in abundance of the Atlantic sharpnose shark (Rhizoprionodon terraenovae), a non-seasonal species caught during all months of sampling. Chapter 2 utilized the long-term data set to investigate within-species size structure over four decades. My analyses suggest declining trends in size for all species analyzed, with the strongest evidence for size declines in two small coastal shark species, Atlantic sharpnose shark and blacknose shark (Carcharhinus acronotus). These results provide insight on assemblage-level response to fishing pressure via environmental or genetic mechanisms.

Chapter 3 employed acoustic telemetry to decipher bonnethead shark (Sphyrna tiburo) movement and behaviors (e.g., residency and habitat use) over multiple spatiotemporal scales. My results suggest individual bonnethead sharks show fidelity across years to specific areas within estuaries in North Carolina and Georgia during seasonal residency and have affinity for areas nearest inlets. Finally, Chapter 4 evaluated shark detection probabilities from aerial drone surveys and how these were affected by environmental factors in a temperate estuary. Shark detection from drone surveys was most influenced by depth, wind speed, and time of day, with highest detection probabilities occurring at shallow depth, low wind speed, and mid-day flight times.