Ecological effects of offshore wind farms on Atlantic cod (Gadus morhua) in the southern North Sea.

Evaluating the ecological effects of the rapid expansion of offshore renewables at local, regional and ecosystem-wide scales is essential to understand the overall socio-ecological trade-offs also for other sectors such as fisheries. Hence, little is known about the ecological impact on demersal fish. To shed light on this topic, we studied the effects of an offshore wind farm in the southern North Sea on different life stages of Atlantic cod (Gadus morhua) using a combination of sampling methods at varying spatial and temporal scales. Our investigations of diet composition and trophic niches indicate that cod utilizes wind turbine piles with scour protection as feeding grounds. Furthermore, collected information on cod adults and early life stages during winter spawning season suggest that spawning activity occurred in winter across the wider wind farm area. We conclude that wind turbine foundations with a scour protection can function as artificial reefs that have local positive effects on the resilience of local cod populations. With our study we contribute to urgently needed observational evidence regarding the ecological impact of offshore wind farm installations to inform area-based management and future monitoring activities.

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 km(2)) 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.