Abstract
We propose a new method for estimating the density and biomass of fish larvae and early juveniles when spawning is extended over time and the egg production in spawning groups varies widely with time. As a proof of concept, the method is applied to catch data for age–0 polar cod (Boreogadus saida), collected by the ichthyoplankton survey in the Kara Sea in 2019. The biomass among different size classes of age–0 fish was calculated based on the selectivity of a Bongo net with 500-μm mesh size. The age of larvae and juveniles in each group and average length at hatch (5.5 mm) were determined used for assessing average growth rate (0.18 mm/day) and hatching date for each age–0 fish assemblage. By fitting the population curve to the samples of age–0 polar cod collected in a small area sequentially over ten days, the instantaneous rate of natural mortality was estimated as ∼ 0.05 day-1. Changes in abundance and biomass over time were modeled by the Baranov's equation and power growth function. The average biomass for the time step of interest was calculated by projecting each cohort abundance to a selected point in time and taking a weighted average of all cohort biomasses.
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