Cocherell, D. E. | Search bioRxiv

bioRxiv2024

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.

physiology↗

bioRxiv2021

Survival of a threatened salmon is linked to spatial variability in river conditions

Extirpation of the Central Valley spring-run Chinook Salmon ESU (Oncorhynchus tshawytscha) from the San Joaquin River is emblematic of salmonid declines across the Pacific Northwest. Habitat restoration and fish reintroduction efforts are ongoing, but recent telemetry studies have revealed low outmigration survival of juveniles to the ocean. Previous investigations have focused on modeling survival relative to river discharge and geographic regions, but have largely overlooked the effects of habitat variability. To evaluate the link between environmental conditions and survival of juvenile spring-run Chinook Salmon, we combined high spatial resolution habitat mapping approaches with acoustic telemetry along a 150 km section of the San Joaquin River during the spring of 2019. While overall outmigration survival was low (5%), our habitat-based classification scheme described variation in survival of acoustic-tagged smolts better than other candidate models based on geography or distance. There were two regional mortality sinks evident along the longitudinal profile of the river, revealing poor survival in areas that shared warmer temperatures but that diverged in chlorophyll-, fDOM, turbidity and dissolved oxygen levels. These findings demonstrate the value of integrating river habitat classification frameworks to improve our understanding of survival dynamics of imperiled fish populations. Importantly, our data generation and modeling methods can be applied to a wide variety of fish species that transit heterogeneous and diverse habitat types.

ecology↗