Grey gurnard ( Eutrigla gurnadus ) in the North Sea: an emerging key predator?

Grey gurnard (Eutrigla gurnadus) is a widely distributed demersal species in the North Sea that has been ranked frequently among the 10 dominant species. Since the late 1980s, grey gurnard catch rates in the international bottom trawl surveys showed a pronounced increase and it was included as an "other predator" in the North Sea multispecies virtual population analysis (MSVPA) in 1997. The MSVPA results estimated grey gurnard to be responsible for approximately 60% of the total predation mortality on age-0 Atlantic cod (Gadus morhua). Long-term MSVPA predictions led to the extinction of North Sea cod. As a possible technical reason, the Holling type II functional response implemented in the model was discussed. In the current analysis, it was demonstrated that the Holling type II functional response was not responsible for the extinction of cod in the model, which was rather a true effect of high grey gurnard predation. Further, it was shown that grey gurnard predation had a significant top-down effect on whiting (Merlangius merlangus) and potentially also on cod recruitment, which was linked to the spatial distribution of the three species. Eventually, the implications of the results for North Sea cod stock recovery plans were discussed