Individual-Based Year-Class Strength Estimation

Estimating past year-class strength (YCS) – the number of larvae hatched in a given year – from survey data is key to investigating population dynamics. Traditionally YCS is estimated using catch-curves which, under a series of assumptions, infer an exponential decrease of catch-at-age. In practice, most, if not all, of the assumptions are violated, resulting in spurious estimates. Furthermore, two approximations arise: (i) pooling individuals into age-classes equates their survival and capture probabilities and (ii) age is expressed as an integer. The first issue disregards the uniqueness of individual histories from hatch to capture and results in computations that do not take advantage of the resolution of the available data since sampled fish are aged individually. The second issue will introduce a bias in results if fish are not sampled exactly at yearly intervals from their hatch date, otherwise these individuals are either younger or older. We present a new method to estimate past YCS with an individual-based set of equations. Through the reconstruction of individual histories from hatch to capture, this method takes full advantage of the resolution of survey data, enables the incorporation of the processes that violate the catch-curve assumptions, and, provides more accurate YCS estimates.