Understanding the spatiotemporal recruitment dynamics of commercially important fish species in the North Sea

Nearly 75% of marine fish stocks are overexploited and management based on sound science is needed to promote stock recovery and sustainability. Understanding recruitment, the number of fish entering the fishable stock every year, is key to sustainable management strategies. During the early-life period of fish, rates of natural mortality are extremely high and even small fluctuations in these rates can generate orders-of-magnitude differences in recruitment. For more than a century, fisheries scientists have developed various hypotheses explaining recruitment variability. At the present time, it is widely acknowledged that observed variations in fish recruitment most likely result from a combination of bottom-up and top-down processes acting at various temporal and spatial scales during fish early life. However, the variability in the magnitude of these processes remains largely undocumented and poorly understood leading to competing or sometimes even conflicting hypotheses on recruitment drivers. This thesis addressed the need for further investigation of the spatial and temporal variability of the processes driving recruitment variability of commercially important fish species in the North Sea. In Chapter 4, a solid basis for analyses of the spatial and temporal variability of North Sea hydrography was produced. A physical-statistical model (Adjusted Hydrography Optimal Interpolation, AHOI) was developed to produce gridded hydrography based on available oceanographic observations...