Contents lists available at ScienceDirect Fisheries Research journal homepage: www.elsevier.com/locate/ fishres Egg production methods applied to Eastern Baltic cod provide indices of spawning stock dynamics Friedrich W. Köster a, *, Bastian Huwer a , Gerd Kraus b , Rabea Diekmann c , Margit Eero a , Andrei Makarchouk d , Serra Örey b , Jan Dierking e , Piotr Margonski f , Jens Peter Herrmann g , Jonna Tomkiewicz a , Daniel Oesterwind h , Paul Kotterba h , Holger Haslob b , Rüdiger Voss i , Thorsten B.H. Reusch e a Technical University of Denmark, National Institute of Aquatic Resources, Kemitorvet, Bygning 202, 2800 Kgs., Lyngby, Denmark b Thünen-Institute of Sea Fisheries, Herwigstraße 31, 27572, Bremerhaven, Germany c Hochschule Bremerhaven, An der Karlstadt 8, 27568, Bremerhaven, Germany d Institute of Food Safety, Animal Health and Environment, Fishery Resources Research Department, Daugavgrivas 8, Riga, 1048, Latvia e GEOMAR Helmholtz Centre for Ocean Research Kiel, Marine Evolutionary Ecology, Düsternbrooker Weg 20, Kiel, 24105, Germany f National Marine Fisheries Research Institute, Kollataja 1, Gdynia, 81-332, Poland g Institute of Marine Ecosystems and Fishery Science, Olbersweg 24, Hamburg, 22767, Germany h Thünen Institute of Baltic Sea Fisheries, Alter Hafen Süd 2, Rostock, 18069, Germany i Christian-Albrechts-Universität zu Kiel, Wilhelm-Seelig-Platz 1, Kiel, 24118, Germany ARTICLE INFO Handled by George A. Rose Keywords: Ichthyoplankton surveys Egg production method Fecundity Spawning stock biomass Fish stock assessment Eastern baltic cod ABSTRACT Egg production methods (EPM) provide fishery independent estimates of spawning stock sizes and dynamics of fish populations. Such methods are commonly used for short-lived pelagic species, less so for demersal fish such as cod. In this paper, we apply EPMs on cod in the eastern Baltic Sea, using a long time series of ichthyoplankton data. Stock assessment of Eastern Baltic cod has been challenged due to changing productivity of the stock invalidating some of the standard procedures, e.g. age determination and input variables, e.g. natural mortality. We demonstrate that EPMs, based on other data and assumptions than standard stock assessments, provide useful information on stock status and dynamics. We apply both the annual and daily egg production methods, which yielded similar results and were in line with stock trends derived from bottom trawl surveys. However, the EPM based spawning stock estimates were consistently lower compared to results from the latest analytical stock assessment. We identified processes introducing uncertainties in EPM estimates and their effects on the resulting estimates, and conclude that they mainly affect the absolute estimates but less the relative trends in stock dynamics. Therefore, we consider that EPMs are useful for providing relative indices for stock assessment purposes, with the Eastern Baltic cod being the first case where such indices are included in an official stock assessment of a demersal gadoid species. We also identify knowledge gaps in order to be able to derive absolute stock size estimates from EPMs in the future. 1. Introduction Egg production methods (EPM) allow for estimation of fish stock sizes based on egg abundance data from ichthyoplankton surveys (Bernal et al., 2012; Dickey-Collas et al., 2012). Additionally, they provide estimates of the stock reproductive potential (Marshall et al., 2003) and give an account of spatial dynamics of spawning activity (Maxwella et al., 2012). A variety of egg production methods have been developed, accounting for different reproductive strategies of fish spe- cies (Lasker, 1985; Lo et al., 1992; Armstrong and Witthames, 2012). The daily egg production method (DEPM) has frequently been applied to estimate the spawning stock biomass of short lived, pelagic fish species, such as anchovy and sprat (Lasker, 1985; Alheit, 1993; Stratoudakis et al., 2006). In principle, this method can be applied to any fish species, both indeterminate and determinate spawners that release pelagic eggs in a series of batches and for which the daily spawning fraction and fecundity can be estimated with sufficient ac- curacy (Bernal et al., 2012; Kraus et al., 2012). The annual egg pro- duction method (AEPM), developed by Lockwood et al. (1981), based on a concept of Saville (1964), is designed to estimate stock sizes of https://doi.org/10.1016/j.fishres.2020.105553 Received 21 September 2019; Received in revised form 28 February 2020; Accepted 4 March 2020 ⁎ Corresponding author. E-mail address: fwk@aqua.dtu.dk (F.W. Köster). Fisheries Research 227 (2020) 105553 Available online 12 March 2020 0165-7836/ © 2020 Elsevier B.V. All rights reserved. T