Use of existing hydrographic infrastructure to forecast the environmental spawning conditions for Eastern Baltic cod

The semi-enclosed nature and estuarine characteristics, together with its strongly alternating bathymetry, make the BalticSea prone to much stronger interannual variations in the abiotic environment, than other spawning habitats of Atlantic cod( Gadusmorhua). Processes determining salinity and oxygen conditions in the basins are influenced both by long term gradual climate change, e.g. global warming, but also by short-term meteorological variations and events. Specifically one main factor influencing cod spawning conditions, the advection of highly saline and well-oxygenated water masses from the North Sea, is observed in irregular frequencies and causes strong interannual variations in stock productivity. This study investigates the possibility to use the available hydrographicprocess knowledge to predict the annual spawning conditions for Eastern Balticcod in its most important spawning ground, the Bornholm Basin, only by salinity measurements from a specific location in the western Baltic. Such a prediction could serve as an environmental early warning indicator to inform stock assessmentand management. Here we used a hydrodynamic model to hindcast hydrographicproperty fields for the last 40+ years. High and significant correlations were found for months early in the year between the 33m salinity level in the Arkona Basin and the oxygen-dependent cod spawning environment in the Bornholm Basin. Direct prediction of the Eastern Balticcod egg survival in the Bornholm Basin based on salinity values in the Arkona Basin at the 33 m depth level is shown to be possible for eggs spawned by mid-age and young females, which currently predominate the stock structure. We recommend to routinely perform short-term predictions of the Eastern Balticcod spawning environment, in order to generate environmental information highly relevant for stock dynamics. Our statistical approach offers the opportunity to make best use of permanently existing infrastructure in the western Balticto timely provide scientific knowledge on the spawning conditions of Eastern Balticcod. Furthermore it could be a tool to assist ecosystem-based fisheries managementwith a cost-effective implementation by including the short term predictions as a simple indicator in the annual assessments.