Defense Date


Document Type


Degree Type

Master of Science

Degree Name

Marine Science

First Advisor

Rosanna Milligan, Ph.D

Second Advisor

Tamara Frank, Ph.D

Third Advisor

Tracey Sutton


Body size is one of the main determinants of marine ecosystem structure and is correlated with many behavioral processes such as diel vertical migration (DVM). Myctophidae, a highly abundant, speciose, and globally distributed fish family, perform diel vertical migrations between the epipelagic zone at night and the mesopelagic zone during the day with vertical distributions varying with ontogeny, and therefore body length. Understanding how DVM contributes to an ecosystem’s structure is important to understanding ecosystem functioning, especially in response to anthropogenic impacts such as the Deepwater Horizon oil spill. The aim of this study was to investigate changes in myctophid body size in relation to their diel vertical migration distributions and species identity, using an existing and extensive myctophid dataset collected from the Gulf of Mexico during the ONSAP (2011) and DEEPEND (2015 – 2018) research programs. Using Generalized Least Squares models, patterns of fish body size were examined in relation to diel vertical migration and mesoscale environmental variables for the 12 most abundant myctophid species. All myctophid species exhibited diel vertical migration behaviors, ranging from 200 – 1000 m depth during the day and ascending to 0 – 200 m at night, and species-specific patterns were observed. Each species was grouped according to vertical distribution pattern and overall, it appeared that size does not dictate vertical distribution nor has size significantly differed between the ONSAP and DEEPEND programs. These findings help us understand the structure of deep-sea fauna and how they may change naturally or in the event of anthropogenic impacts.