Jesstimation: A novel approach for estimating more accurate fish ages from otolith zone counts and measurements

Accurate estimates of age and growth are essential for conducting age-structured stock assessments and for developing effective management and conservation strategies. One of the most common methods for estimating fish age is by counting growth increments that have been deposited either daily or annually in otoliths. However, counts of daily increments become unreliable as fish grow older (typically beyond one year), and annual counts only provide integer age estimates (in years). For many research questions, a more precise fractional age is desirable. In theory, this can be calculated from the annual increment count if the fish’s birthday and the time of year when its annual increments were formed are known. However, in practice, individual birthdays are unknown and vary widely in species with protracted spawning seasons. Moreover, if the timing of the annual increment formation is protracted or variable, it becomes difficult to determine whether the most recent growth increment was deposited before or after the fish’s most recent birthday. To address these challenges, we present a novel method for converting annual increment counts into fractional ages using daily counts of young-of-the-year (YOY) fish and otolith measurements. Our approach does not require knowing the birthday of a fish or the precise timing of increment formation and provides a more accurate conversion of zone counts to age estimates - particularly for species with variable spawning periods. We demonstrate the method for western Pacific bigeye tuna (Thunnus obesus), a species that can spawn throughout the year in tropical waters and forms annuli in its otoliths over a six-month period.