UC San Diego

Throughout the global ocean, there is an abundant and diverse assemblage of fishes aggregated at mesopelagic (200-1000 m) depths. These fishes are critical to pelagic food webs and carbon transport. In the southern California Current Ecosystem, with naturally hypoxic mesopelagic waters, mesopelagic fishes may be vulnerable to predicted ocean deoxygenation. Additionally, water property discontinuities at oceanic fronts can disproportionately affect abundance, compositions, and reproduction of marine animals. In this dissertation, I investigate responses of mesopelagic fishes to ocean deoxygenation and fronts.

First, I correlated acoustically-detected Deep-Scattering Layers (DSL) of mesopelagic fishes with midwater oxygen, irradiance, and temperature, and found that the lower DSL boundary correlates most with oxygen. The upper boundary correlates with both oxygen and irradiance. Assuming current deoxygenation rates, I predicted both lower and upper boundaries will shoal. Next, I measured activities of the aerobic enzymes Citrate Synthase and Malate Dehydrogenase, and the anaerobic enzyme Lactate Dehydrogenase to test for changes in metabolic activities of mesopelagic fish in response to dissolved oxygen. There was an apparent suppression of activity at low oxygen concentrations for all species combined. There was no increased reliance on anaerobic activity at depressed oxygen concentrations. Although there is evidence that some mesopelagic fishes may have a rare alternate anaerobic pathway catalyzed in part by Alcohol Dehydrogenase, I did not detect any Alcohol Dehydrogenase activity for 16 species studied.

Finally, I compared responses of mesopelagic fish assemblages at three frontal systems. I found no abundance changes across fronts, except for larvae which were elevated at the most stable system. Non-vertically migratory assemblages were uniform across frontal gradients, while migratory and larval fish assemblages were typically altered across fronts. Changes in population growth potential were detected across the two more stable frontal systems for migrators and larvae, though not for non-migrators.

These results suggest that deoxygenation may cause habitat compression and

metabolic suppression, while changes to frontal frequency could impact the structure and

population growth of mesopelagic fish assemblages. Ongoing monitoring of these

populations using existing and novel technologies will allow further understanding of

mesopelagic fish responses to these and other environmental changes.