Experimental validation of otolith-based age and growth reconstructions across multiple life stages of a critically endangered estuarine fish

Wilson Xieu; Levi S Lewis; Feng Zhao; Rachel A Fichman; Malte Willmes; Tien-Chieh Hung; Luke Ellison; Troy Stevenson; Galen Tigan; Andrew A Schultz; James A Hobbs

doi:10.7717/peerj.12280

Experimental validation of otolith-based age and growth reconstructions across multiple life stages of a critically endangered estuarine fish

PeerJ. 2021 Nov 17:9:e12280. doi: 10.7717/peerj.12280. eCollection 2021.

Authors

Wilson Xieu^#¹, Levi S Lewis^#¹, Feng Zhao¹, Rachel A Fichman¹, Malte Willmes^{2

3}, Tien-Chieh Hung⁴, Luke Ellison⁴, Troy Stevenson⁴, Galen Tigan⁴, Andrew A Schultz⁵, James A Hobbs^{1

6}

Affiliations

¹ Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, CA, United States of America.
² Institute of Marine Sciences, University of California, Santa Cruz, CA, United States of America.
³ Southwest Fisheries Science Center, National Marine Fisheries Service, Santa Cruz, CA, United States of America.
⁴ Department of Biological and Agricultural Engineering, University of California, Davis, CA, United States of America.
⁵ Bay-Delta Office, United States Bureau of Reclamation, Sacramento, CA, United States of America.
⁶ Bay-Delta Region, California Department of Fish and Wildlife, Stockton, CA, United States of America.

^# Contributed equally.

Abstract

Background: The application of otolith-based tools to inform the management and conservation of fishes first requires taxon- and stage-specific validation. The Delta Smelt (Hypomesus transpacificus), a critically endangered estuarine fish that is endemic to the upper San Francisco Estuary (SFE), California, United States, serves as a key indicator species in the SFE; thus, understanding this species' vital rates and population dynamics is valuable for assessing the overall health of the estuary. Otolith-based tools have been developed and applied across multiple life stages of Delta Smelt to reconstruct age structure, growth, phenology, and migration. However, key methodological assumptions have yet to be validated, thus limiting confidence in otolith-derived metrics that are important for informing major water management decisions in the SFE.

Methods: Using known-age cultured Delta Smelt and multiple independent otolith analysts, we examined otolith formation, otolith-somatic proportionality, aging accuracy and precision, left-right symmetry, and the effects of image magnification for larval, juvenile, and adult Delta Smelt.

Results: Overall, otolith size varied linearly with fish size (from 10-60 mm), explaining 99% of the variation in fish length, despite a unique slope for larvae < 10 mm. Otolith-somatic proportionality was similar among wild and cultured specimens. Aging precision among independent analysts was 98% and aging accuracy relative to known ages was 96%, with age estimates exhibiting negligible differences among left and right otoliths. Though error generally increased with age, percent error decreased from 0-30 days-post-hatch, with precision remaining relatively high (≥ 95%) thereafter. Increased magnification (400×) further improved aging accuracy for the oldest, slowest-growing individuals. Together, these results indicate that otolith-based techniques provide reliable age and growth reconstructions for larval, juvenile, and adult Delta Smelt. Such experimental assessments across multiple developmental stages are key steps toward assessing confidence in otolith-derived metrics that are often used to assess the dynamics of wild fish populations.

Keywords: Delta Smelt; Estuarine; Fish; Freshwater; Growth; Otolith; San Francisco; Validation.

Grants and funding

This work was supported by the U.S. Bureau of Reclamation Directed Outflow Project (No. R17AC00129). A. Schultz (USBR) provided logistical support for carrying out the work and valuable feedback in the drafting of the final manuscript.