Köster, Friedrich W., Huwer, Bastian, Kraus, Gerd, Diekmann, Rabea, Eero, Margit, Makarchouk, Andrei, Örey, Serra, Dierking, Jan , Margonski, Piotr, Herrmann, Jens Peter, Tomkiewicz, Jonna, Oesterwind, Daniel, Kotterba, Paul, Haslob, Holger, Voss, Rüdiger and Reusch, Thorsten B. H. (2020) Egg production methods applied to Eastern Baltic cod provide indices of spawning stock dynamics. Fisheries Research, 227 . Art.Nr.105553. DOI 10.1016/j.fishres.2020.105553.

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Abstract

Highlights:
• Egg production methods have been used for assessing the Eastern Baltic cod stock.
• The analysis contributed to the re-establishment of an analytical assessment for the stock.
• Annual and daily egg production methods gave similar results.
• Results confirmed trends from trawl surveys, questioned earlier because of potential change in catchability.
• Several biological processes impact on absolute spawning stock estimates and research needs are identified to improve these.

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.

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