Validation of Scale‐Derived Ages in Wild Juvenile and Adult Steelhead Using Parental‐Based Tagging

Abstract

Accurate age information is a critical component in fisheries monitoring, management, and research. The ability to assign accurate ages to fish by using nonlethal structures is vital in calculating cohort productivity for fish species with low abundances and variable life histories. A common nonlethal method to assign ages is by interpretation of patterns in scales. Validation of ages is not easily obtained for free‐ranging fishes; however, the development of genetic “tags” has expanded age validation opportunities. In this study, we used parental‐based genetic tagging to validate ages determined from scales collected from juvenile and adult steelhead Oncorhynchus mykiss in two streams within the Snake River basin. Juvenile scale ages were in agreement with 93% of the known ages. Adult scale ages were in agreement with 89% of the known ages. Although overall bias in juvenile ages was very low, we saw a slight positive bias in younger fish (young of the year and age 1) and a small negative bias in older juveniles (ages 2–4). A small negative bias in ages of adults was a consequence of errors in freshwater age. The errors observed did not significantly bias age compositions because the 90% confidence intervals about the age proportions based on scales contained the known proportions; therefore, scale analysis is an acceptable method for assigning ages to Snake River steelhead. We discuss the value of validated known‐age scales as a reference collection, and as an illustrative use, we constructed a circulus count guide to aid technicians in identifying missing annuli and distinguishing true from false annuli. To demonstrate the use of the circulus guidelines, we reanalyzed samples with age discrepancies, applying the circulus count data and the known age to identify mistakes.

Applications of Age–Length Keys for Assigning Ages to Juvenile Steelhead

“…Although not examined here, published findings of the methods used to age scales of juvenile steelhead (Reinhardt et al. 2022) reported low bias (CV = 4.7%) and high accuracy (93%), thus validating the age data used in this study. The duration of freshwater rearing of wild juvenile steelhead can range up to 7 years (Peven et al.…”

“…Scales were processed and aged using methods from Reinhardt et al. (2022). Standardized methods for aging salmonid scales have reported low bias (CV <5.0%) and high accuracy (>90%; McNicol and MacLellan 2010; Reinhardt et al.…”

“…For example, if catch was low, the proportion of fish sampled was increased to ensure that sample goals were met. Scales were processed and aged using methods from Reinhardt et al (2022) Age-length keys.-For ALKs, juvenile steelhead were categorized into appropriate 10-mm length intervals from 80 to 250 mm. Fish aged from scales were used to construct ALKs for each seasonal period and for data pooled across summer and fall trapping periods for each year.…”

“…Brood year production can then be estimated if an accurate age composition is available. Juvenile salmonids are often sampled across multiple seasons, and ages are typically determined from a smaller subsample of fish whose scales are collected and aged (Reinhardt et al 2022). Movement and survival downstream of trapping sites can be assessed with the use of PIT tags and detection systems installed in streambeds and at hydrosystem dams.…”

Age, growth, reproduction and status of resource development of Ptychidio jordani, a critically endangered freshwater fish in the Hongshui River, China

“…Scales are one of the most important and accurate materials for the age determination of fish, and have been used in the age determination of many freshwater fishes (Duan et al ., 2015; Reinhardt et al ., 2022). In this study, scales were used as age‐identification materials, and there was a significant positive correlation between scale diameter and body length, indicating that scales can be used as an effective material for age identification.…”