Doctor of Philosophy
Mahmood Shivji, Ph.D.
Bradley Wetherbee, Ph.D.
Christopher Lowe, Ph.D.
Jeremy Vaudo, Ph.D.
Pelagic apex predators exert strong influences on ecological communities, and often support valuable commercial or recreational fisheries worldwide. Yet, due to their rarity and pelagic lifestyle, many species, such as billfishes, have proven particularly difficult to study at resolutions necessary to define dynamics of recovery from fishery interaction, physical interaction with environmental features and prey exploitation, and competitive interactions among other billfish predators. This leads to a paucity of knowledge on billfish ecology and habitat use, and hinders management efforts. With the ever-improving and miniaturization of technology and oceanographic datasets, the ability to define and quantify these interactions of fish to each other and their environment has never been greater. In this dissertation, I developed a novel tag and deployment method to characterize the recovery dynamics of blue marlin (Makaira nigricans) and sailfish (Istiophorus platypterus) after catch and release in a typical recreational fishery. I also utilized these tags to document fine-scale movement and foraging behavior of these two species in the vertically compressed eastern tropical Pacific, where a large naturally occurring oxygen minimum zone exists. This oceanographic feature compresses the prey of these predators to a narrow range during the day, and I hypothesize that the billfishes are using this to their advantage to increase foraging opportunities. Indeed, animal-borne video and the associated high-resolution data documented a sailfish exploiting the hypoxic boundary, initiating a pursuit at depth, chasing it to the surface, and capturing the prey after a pursuit at the surface. Using physiological and bioenergetic modeling, I estimated the net energetic gain over the 24 hour period the event took place. In the final chapter, I used a multidisciplinary approach to investigate the competitive interactions and mechanisms of niche partitioning employed by blue marlin, sailfish, and black marlin (Istiompax indica) in the vertically compressed environment. This work demonstrated a seasonal response in vertical habitat use, alleviating competition and allowing these sympatric predators to coexist. Together, these works provide unparalleled insights into the fish-fishery interaction, biology, physiology, and ecology of these top pelagic predators, and provide necessary information for ecosystem function and management.
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Ryan Keith Logan. 2023. Movement, Behavior, and Trophic Ecology of a Pelagic Predator Guild in the Eastern Tropical Pacific Ocean. Doctoral dissertation. Nova Southeastern University. Retrieved from NSUWorks, . (124)
Available for download on Wednesday, April 03, 2024