Defense Date


Document Type


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


Gulf of Mexico, Cyclothone, abundance, vertical distribution, species composition


The fish genus Cyclothone is considered the most abundant vertebrate taxon on Earth. Despite this assertion, very few detailed ecological studies of this genus exist for any site in the World Ocean, largely due to the lack of expertise (and willingness) to identify specimens from existing sample sets. This study will provide a species-level description of the abundance, vertical distribution, and size structuring of the genus Cyclothone in the Gulf of Mexico, a hyper-diverse, deep-pelagic ecosystem that is increasingly impacted by anthropogenic disturbances. As the putative most-abundant fishes in the ecosystem, this characterization is critically needed for a holistic understanding of the deep Gulf of Mexico as an integrated ecosystem. Data were collected during ONSAP and DEEPEND cruises from 2011 – 2021 using MOCNESS trawls from the surface to 1500 m depth.

In this study, the most abundant Cyclothone species was C. pallida (55.3%), with seven other species each contributing 14 to Cyclothone occurred between 600 and 1000 m depth. Light-colored species were primarily found above the 600-m benchmark depth: C. alba (200 – 600 m), C. braueri (200 – 600 m) and C. pseudopallida (200 – 1000 m). Below the 600-m mark, C. pallida and C. acclinidens were most abundant until another benchmark at 1000 m. Below 1000 m, the dominant species was C. obscura. All Cyclothone species were non-vertically migrating, with no vertical distribution differences between the day and night trawls. This study is the first documentation of C. microdon in the Gulf of Mexico.