Modeling Catchability Differences Between High-Speed Rope Trawls and Rectangular Midwater Trawls in Deepwater Ecosystems

Author

• Jacob Hack, Nova Southeastern UniversityFollow

Defense Date

8-4-2026

Document Type

Thesis

Degree Type

Master of Science

Degree Name

Marine Science

First Advisor

David Kerstetter

Second Advisor

Rosanna Milligan

Third Advisor

Tracey Sutton

Keywords

Midwater Trawl, Catchability, Bathypelagic, Gulf of Mexico, Pelagic, Teleost fish

Abstract

Midwater trawls differ in geometry, mesh configuration, and towing mechanics, creating gear specific patterns in encounter and retention that shape how pelagic fish communities are sampled. This thesis quantifies catchability differences between a 10-m² MOCNESS and the Irish Herring Trawl (IHT) across shallow and deep strata in the Gulf. The MOC10 consistently retained small bodied mesopelagic taxa, with Cyclothone spp., myctophids, and sternoptychids dominating catches across all depths and diel periods. The IHT sampled larger, more mobile taxa including stomiids, carangids, and large myctophids such as Notoscopelus resplendens and Ceratoscopelus warmingii, and produced substantially larger standard lengths in every gear pairing. Species specific effect sizes showed strong and repeatable separation between gears, and exclusive species occurred only in IHT800 and MOC1000 in the shallow stratum and in both gears at 1500 m, with Notolychnus valdiviae dominating the deep MOC10 assemblage and Engraulis eurystole occurring only in IHT samples. These patterns reflect how ecological traits such as body size, swimming mode, and sensory ability interact with gear mechanics to determine encounter and retention. The IHT emphasized larger, visually oriented taxa, while the MOC10 resolved smaller, and compressible fishes that dominate numerically and often occupy narrow depth strata. Because no single net captures the full mesopelagic community, integrating complementary gears through trait-based interpretation and size structured analyses provides a more complete and ecologically grounded representation of mesopelagic biodiversity. The combined results provide a framework for comparing datasets collected with different midwater gears and improving interpretations of deep pelagic biodiversity and ecosystem function.

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.

NSUWorks Citation

Jacob Hack. 2026. Modeling Catchability Differences Between High-Speed Rope Trawls and Rectangular Midwater Trawls in Deepwater Ecosystems. Master's thesis. Nova Southeastern University. Retrieved from NSUWorks, . (247)
https://nsuworks.nova.edu/hcas_etd_all/247.