Grey gurnard (Eutrigla gurnadus) in the North Sea: an emerging key predator? Jens Floeter, Alexander Kempf, Morten Vinther, Corinna Schrum, and Axel Temming Abstract: 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 responsi- ble 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 re- sponse 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. Résumé : Le grondin gris (Eutrigla gunardus) est une espèce démersale à grande répartition dans la mer du Nord, qui a souvent été classée parmi les 10 espèces dominantes. Depuis la fin des années 1980, les taux de captures du grondin gris dans les inventaires internationaux de chaluts de fond ont augmenté de façon marquée et le grondin gris a été in- clus parmi les « autres prédateurs » dans l’analyse des populations virtuelles plurispécifiques (MSVPA) de la mer du Nord en 1997. Les résultats de MSVPA estiment que le grondin gris est responsable d’environ 60 % de la mortalité to- tale due à la prédation chez les morues (Gadus morhua) d’âge 0. Les prédictions à long terme de MSVPA prévoient l’extinction de la morue de la mer du Nord. Nous discutons de l’inclusion de la réponse fonctionnelle de Holling de type II comme raison technique possible de cette extinction. Dans notre analyse, nous démontrons que la réponse fonc- tionnelle de type II de Holling n’est pas responsable de l’extinction dans le modèle, mais que l’extinction est de fait due à la forte prédation par le grondin gris. De plus, nous montrons que la prédation par le grondin gris a un effet des- cendant significatif sur le merlan (Merlangius merlangus) et potentiellement aussi sur le recrutement de la morue, qui est lié à la répartition spatiale des trois espèces. Finalement, les conséquences de ces résultats sur la récupération du stock de morues de la mer du Nord font l’objet d’une discussion. [Traduit par la Rédaction] Floeter et al. 1864 Introduction Grey gurnard (Eutrigla gurnadus) is a widely distributed demersal species in the North Sea that has been ranked among the 10 dominant species (Yang 1982; Daan et al. 1990). Since the late 1980s, North Sea grey gurnard catch rates in the international bottom trawl surveys (IBTS) showed a pronounced increase (International Council for the Exploration of the Sea (ICES) 2002). Possibly, grey gurnard was able to occupy substantial parts of the ecological niche freed by the diminishing demersal gadoids, especially after the regime shift in the late 1980s (Reid et al. 2001a, 2001b; Beaugrand et al. 2003). Grey gurnard shows a clear seasonal shift in distribution; in winter, it concentrates in the western part of the central North Sea, whereas these concentrations disperse in spring and summer (Knijn et al. 1993). The quarterly IBTS in the North Sea revealed that in the second and third quarters, larger sized grey gurnard predominantly occur in high abundances along the 50-m-depth contour as well as at the Doggerbank, the Fisher Banks, off Jutland, and in the German Bight (Heessen and Daan 1994). The most extensive description of grey gurnard diets orig- inate from the multinational sampling efforts under the aus- pices of ICES during the second year-of-the-stomach in 1991, when grey gurnard was included in the stomach sam- pling program as a priority by-catch species (de Gee and Can. J. Fish. Aquat. Sci. 62: 1853–1864 (2005) doi: 10.1139/F05-108 © 2005 NRC Canada 1853 Received 6 August 2004. Accepted 11 April 2005. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 26 August 2005. J18252 J. Floeter, 1 A. Kempf, and A. Temming. Zentrum für Meeres und Klimaforschung, Institut für Hydrobiologie und Fischereiwissenschaft, Olbersweg 24, D22767 Hamburg, Germany. M. Vinther and C. Schrum. Danish Institute for Fisheries Research, Jægersborg Allé 1, Charlottenlund, Denmark, DK 2920. 1 Corresponding author (e-mail: jfloeter@uni-hamburg.de).