Spawning grounds of Atlantic cod (Gadus morhua) in the North Sea

The protection of spawning fish from anthropogenic impacts requires information on the location, timing, and interannual persistence of spawning aggregations. In this study, generalized additive models were used to predict the spawning habitat of North Sea cod, based on the abundance of spawning fish within three population subareas and nine environmental layers. The length of spawning cod differed among population subareas, consistent with published evidence of maturation differences. In the northeast North Sea (Viking), the peak in the spawning season was estimated to occur after the survey which made the prediction of spawning grounds for this area less certain. Cod were found to prefer areas with temperatures around 5–7°C for spawning and there was a general preference for high salinity waters. Persistence of cod spawning grounds over the study period was related to interannual stability in temperature, with high variability in the use of Southern Bight spawning grounds. As such, cod appear to minimize interannual variability in the initial environmental conditions affecting offspring survival by selecting suitable cold spawning grounds. Seabed conditions also affected spawning distribution with cod selecting coarse sand and avoiding areas of very high tidal flow. The model prediction was compared with the distribution of cod aggregations during the spawning season reported by fishing boats. Seventy per cent of the aggregations was located in areas classified as occasional or recurrent spawning grounds. The predicted distribution confirmed the widespread occurrence of spawning in the North Sea and showed good agreement with recent and past studies of cod egg distribution, suggesting that nearly all major historical areas of spawning still appear in use today. However, the study also found that the recent recovery of spawning-stock biomass was not uniform across the stock, being centred in the northwest subarea