Air Pocket for a Non-Air-Breathing Fish: Developing Respirometry Methodology for the critically endangered Delta Smelt

Respirometry quantifies O2 consumption to estimate metabolic activity across stressor(s), and the resulting data is central for devising conservation strategies for fishes. Due to its rapid adoption and widespread utility, protocols have been established to foster quality control and increase consistency. However, the application of established respirometry protocols on the Delta Smelt (Hypomesus transpacificus) have been largely unsuccessful, with reports over the past 12 years documenting high mortality rates that can only be circumvented with very short measurement duration ([~]4 h). This prevents the measurement of standard metabolic rate (SMR) and critical O2 tension (Pcrit), and ultimately obstructed physiological research, management actions and conservation tools development. In this study, we detailed the anecdotal, behavioral and empirical evidence that culminated in successful measurements of Delta Smelt SMR and Pcrit at 10, 12, 15, 17 and 19{degrees}C. We discovered the Delta Smelt is physotomous and requires frequent refilling of its air bladder to maintain buoyancy. Inspired by air-breathing fish respirometry, the inclusion of an air pocket within their chamber led to considerably more robust respirometry measurements compared to past attempts. The influence of fasting and time of day on O2 consumption were also examined. Altogether, this methodology paves the way for acquiring critical knowledge needed to inform Delta Smelt research and conservation (e.g. the development of metabolic index). The lessons learned may also have implications for other aspects of Delta Smelt conservation such as decreasing conservation aquaculture operating costs and improving survival during fish transport and supplementation.