Wochenbericht AL493

Wochenbericht der Forschungsreise mit R.V. „Alkor“ AL 493 13.05.2017 – 20.05.201

FS ALKOR Cruise Report AL457

R.V. ALKOR Cruise No.: AL 457 Dates of Cruise: 16.05. – 30.05.2014 Areas of Research: Physical, chemical, biological and fishery oceanography Port Calls: Gdynia, Poland, 22.05. – 24.05.201

Cruise Report RV ALKOR Cruise No. AL 493

Dates of Cruise: 13.05. – 29.05.2017 Areas of Research: Physical, chemical, biological and fishery oceanography Port Calls: Visby, Sweden, 19.05. – 21.05.201

Cruise Report RV ALKOR AL 437 [AL437]

12.05. – 27.05.2014 Port Calls: Karlskrona, Sweden, 18.05. – 20.05.2014; Rönne, Danmark, 24.05. – 25.05.201

Cruise Report R.V. Alkor Cruise No.: AL478

Dates of Cruise: 16.05. – 28.05.2016 Areas of Research: Physical, chemical, biological and fishery oceanography Port Calls: Klaipeda, Lithuania, 21.05. – 23.05.201

Drivers, dynamics and management of Eastern Baltic cod (Gadus morhua) reproduction habitat

The overall aim of my thesis was to improve our understanding of environmental drivers causing the dynamics of the Eastern Baltic cod reproduction habitat and to assess their relevance for a possible application in the sustainable stock management. A novel approach to map the reproductive habitat of Eastern Baltic cod, the Buoyancy Depending Reproductive Layers (BDRL), was developed and used to propose an alternative stock indicator, the effective Spawning Stock Biomass (eSSB). The eSSB was found to improve the fit of a recruitment model compared to the model using the conventional Spawning Stock Biomass. Oxygen depletion was also negatively impacting the available size of nursery areas for juveniles. The mechanism was able to partly explain the observed decline of the condition of juveniles in the nursery areas, because a high population density in the remaining habitat could increase the impact of density depending effects. Furthermore, by the application of the novel approach of BDRLs the spawning habitat was shown to be sensitive to eutrophication and that this sensitivity is strongly depending on the size of the female spawner using the habitat. As predicted, the BDRL approach was superior to the “classic” approach, the Reproductive Volume (RV) because it was more sensitive to environmental change, able to incorporate stock structure, was not overestimating the spawning habitat in the eastern spawning areas and could provide estimates of other stressors depending on the female spawner size. Due to a permanently installed measurement platform in the Arkona Basin, the new methodology could be used to establish a new environmental indicator on the spawning habitat conditions. It was recommended to be used in future stock assessments

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 Baltic Sea prone to much stronger interannual variations in the abiotic environment, than other spawning habitats of Atlantic cod (Gadus morhua). 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 hydrographic process knowledge to predict the annual spawning conditions for Eastern Baltic cod 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 assessment and management. Here we used a hydrodynamic model to hindcast hydrographic property 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 Baltic cod 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 Baltic cod 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 Baltic to timely provide scientific knowledge on the spawning conditions of Eastern Baltic cod. Furthermore it could be a tool to assist ecosystem-based fisheries management with a cost-effective implementation by including the short term predictions as a simple indicator in the annual assessments

Oxygen depletion in coastal seas and the effective spawning stock biomass of an exploited fish species

Environmental conditions may have previously underappreciated effects on the reproductive processes of commercially exploited fish populations, for example eastern Baltic cod, that are living at the physiological limits of their distribution. In the Baltic Sea, salinity affects neutral egg buoyancy, which is positively correlated with egg survival, as only water layers away from the oxygen consumption-dominated sea bottom contain sufficient oxygen. Egg buoyancy is positively correlated to female spawner age/size. From observations in the Baltic Sea, a field-based relationship between egg diameter and buoyancy (floating depth) could be established. Hence, based on the age structure of the spawning stock, we quantify the number of effective spawners, which are able to reproduce under ambient hydrographic conditions. For the time period 1993–2010, our results revealed large variations in the horizontal extent of spawning habitat (1000–20 000 km2) and oxygen-dependent egg survival (10–80%). The novel concept of an effective spawning stock biomass takes into account offspring that survive depending on the spawning stock age/size structure, if reproductive success is related to egg buoyancy and the extent of hypoxic areas. Effective spawning stock biomass reflected the role of environmental conditions for Baltic cod recruitment better than the spawning stock biomass alone, highlighting the importance of including environmental information in ecosystem-based management approaches