Aspects of the Physiology and Behavior of an Endothermic Fish, the Opah, Lampris guttatus.

The opah, Lampris guttatus, is a large mesopelagic predatory fish inhabiting oceans worldwide and is an important species in many commercial fisheries. Despite its importance, little is known about the biology of this species, and this study addresses aspects of its physiology and behavior. The opah is the only fish known to exhibit a whole-body form of endothermy and must therefore meet increased metabolic demands to maintain an elevated internal temperature relative to the highly variable temperatures encountered in the water column. Accordingly, opah gill structure is specialized to uptake sufficient oxygen to drive the metabolic reactions within its tissues and to optimize heat retention. In this study, gill surface area of the opah was determined across a range of body sizes for comparison with other fishes and to determine the relationship of gill surface area to body mass. It was found that the opah has much larger gills than those of most other fishes and a high gill surface area to body mass scaling exponent (1.16, with most teleosts being between 0.8-0.9). This high scaling exponent suggests a disproportionate increase in oxygen demand as the fish grows, which may consequently allow for an increased capacity for endothermy with size. In addition to gill measurements, a number of opah were outfitted with customized PSATs (pop-up archival satellite tags) with an intermuscular thermocouple to better understand their daily movements and habitat preference in relation to internal body temperature. Together gill and tagging data provide needed insight into the correlation of gill morphology, metabolism, life-history and habitat preference, yielding a better understanding of opah specialization for endothermy in the mesopelagic and providing important information that can aid in the future management of this species.