Fish length back-calculation from scales: advancing methodology and correction of bias

Abstract

Fish length back-calculation from hard structures (e.g., scales, otoliths, spines, etc.) is a commonly used method to reconstruct individual growth rates and sizes at age/stage in the absence of an intensive sampling–resampling timeseries. However, reliable estimates via these methods require the empirical validation of multiple assumptions about the growth of these hard structures. Here, we focus on reducing bias in scale-based back-calculations and validating proposed improvements using archived scales from a wild population of anadromous steelhead trout ( Oncorhynchus mykiss). We first describe a series of back-calculation problems and propose solutions that can be integrated into common back-calculation methods. We then compare back-calculation accuracy, precision, and bias between proposed solutions and traditional forms of two back-calculation methods: Fraser–Lee and Body-Proportional. We discovered that the assumption that rapid growth begins immediately after juvenile steelhead pass downstream of the fish fence (300 m upstream of the ocean) was invalid and required a correction factor to prevent overestimating fish length by an average of 14%. The proposed modified methods we described performed better than traditional back-calculation methods. Overall, these findings can improve estimates of fish length from scale-based back-calculations and illustrate the importance of validating key assumptions.

Evaluating the influence of environmental and biological factors on migration behavior and residence duration of wild subyearling Chinook Salmon in a fjord estuary using miniature acoustic transmitters

“…Measurements were repeated if the sum of distances between points differed by more than 2.5% from the length of the transect. The Fraser–Lee method (Bailey et al 2022) was then used to estimate fork length at the time the antepenultimate circuli was formed and recent growth represents the difference between these two measurements. Previous studies have indicated that subyearling Chinook Salmon deposit scale circuli in proportion to somatic growth and that four circuli may be formed per month during the spring and summer (Walker and Sutton 2016).…”