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Topic

Assessing the feeding ecology of two myctophid species, Diaphus theta and Stenobrachius leucopsarus, using fatty acids, microscopic analysis, and DNA metabarcoding from two regions around Vancouver Island

School of Earth and Ocean Sciences

Date & location

• Wednesday, April 15, 2026
• 1:00 P.M.
• Clearihue Building, Room A329

Examining Committee

Supervisory Committee

• Dr. John Dower, School of Earth and Ocean Sciences, ̽��ϵ�� (Co-Supervisor)
• Dr. Catherine Stevens, School of Earth and Ocean Sciences, UVic (Co-Supervisor)
• Dr. Stephane Gauthier, Department of Biology, UVic (Outside Member)

External Examiner

• Dr. Francis Juanes, Department of Biology, UVic

Chair of Oral Examination

• Dr. Chris Darimont, Department of Geography, UVic

Abstract

Across the global ocean, mesopelagic fishes are key components of midwater ecosystems. Myctophids dominate mesopelagic fish biomass and play critical ecological roles by transferring energy from zooplankton to higher trophic levels and by transporting carbon from surface waters to the deep ocean via the biological pump. Increasing global demand for seafood has led to growing interest in harvesting mesopelagic fish, despite a limited understanding of trophic connectivity and feeding ecology within this zone. Currently, mesopelagic trophic ecology remains poorly studied in the waters surrounding Vancouver Island, British Columbia. To address this knowledge gap, this thesis provides the first comparative investigation of feeding ecology in the myctophids Diaphus theta and Stenobrachius leucopsarus from the BC/Northern USA shelf and the Strait of Georgia (SoG). Feeding ecology was examined using lipid analysis, microscopic gut content analysis, and DNA metabarcoding of stomach contents sequencing the V4 18S and COI genes. Lipid profile comparisons between D. theta and S. leucopsarus indicate that differences in total lipid were primarily correlated with species-specific storage lipid type and buoyancy requirements. Constant levels of total lipid were noted in both regions in wax ester-storing S. leucopsarus, which utilizes lipids to remain neutrally buoyant as an inactive, semi-migrant myctophid. In contrast, total lipid in triacylglycerol (TAG)-storing D. theta exhibited greater intraspecific variation in both regions, attributed to the “quick use and replacement” properties of TAG and the greater energy requirement for this more active myctophid. Comparison of fatty acid profiles between D. theta and S. leucopsarus showed markedly different levels of saturation, with D. theta containing a greater proportion of saturated fatty acids, whereas S. leucopsarus contained more monounsaturated fatty acids. Comparisons of individual fatty acids representing carnivory, trophic level, and copepod ingestion showed both inter- and intraspecific variation along the latitudinal gradient. Microscopic gut content analysis showed a greater number of prey taxa consumed on the shelf by D. theta compared to S. leucopsarus, with copepods, euphausiids, and amphipods frequently found using this method. DNA metabarcoding of stomach content revealed the occurrence of gelatinous zooplankton in the diet of D. theta both on the shelf and in the SoG, a pattern not exhibited by S. leucopsarus in either region. Regional comparisons between the number of prey taxa found in each myctophid show a greater number of prey taxa eaten on the shelf, despite the SoG having a greater taxonomic diversity of zooplankton prey. These data provide baseline information on the feeding ecology of D. theta and S. leucopsarus from the BC/Northern USA shelf and the SoG, thus providing the foundation to assess future risks of mesopelagic fish extraction in this region.