Defense Date

8-5-2022

Document Type

Thesis

Degree Type

Master of Science

Degree Name

Marine Science

First Advisor

Tracey Sutton, Ph.D

Second Advisor

Rosanna Milligan, Ph.D

Third Advisor

Matthew Johnston, Ph.D

Abstract

Diel‌ ‌vertical‌ ‌migration,‌ or DVM, ‌is‌ ‌defined‌ ‌as‌ ‌the‌ large-scale changes in the depth distribution of a species or an assemblage with respect to the time of day. DVM‌ ‌is‌ ‌the‌ ‌largest‌ ‌active movement‌ ‌of‌ ‌biomass‌ on Earth, driven by the need for food balanced against predator avoidance and metabolic constraints. Asynchronous‌ ‌diel‌ ‌vertical‌ ‌migration, in the context of this study,‌ ‌refers‌ ‌to‌ the phenomenon where only a portion of a species’ population migrates upwards at night while others remain at depth. ‌The‌ extent that factors such as temporal variation, ontogenic variation, and methodological variation explain this migratory pattern is the focus of this study.‌ ‌Data for five numerically dominant mesopelagic fishes species (four lanternfishes, Benthosema suborbitale, Ceratoscopelus warmingii, Lampanyctus alatus, and Lepidophanes guentheri, and one bristlemouth, Sigmops elongatus) were analyzed from two extensive deep-pelagic research programs in the Gulf of Mexico.‌ A size-depth relationship, with larger individuals in a population residing deeper during daytime, was clearly apparent for four of the five species examined, and likely applied to the fifth. Two species, L. guentheri and B. suborbitale, were synchronous, or near-synchronous vertical migrators. The remaining three species were asynchronous migrators whose diel migration fidelity appeared tied primarily to size. In the two asynchronously migrating lanternfishes the largest size class migrated daily while the smallest migrated least, while the pattern was opposite in the bristlemouth, S. elongatus. A possible ecological explanation for these patterns is presented based on fluid mechanics theory. Given the importance of diel vertical migrators in the global sequestration of carbon via the biological pump, and the increasing sophistication of individual-based models of carbon flux, quantifying the variability in DVM and AVM behavior is essential, as these values drive the models. Quantifying this variability will greatly enhance the accuracy (and likely precision) of carbon flux models, which are vitally important in a rapidly changing deep ocean subjected to increasing human disturbance.

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