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Marine Ecology Progress Series

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MEPS 480:263-275 (2013)  -  DOI:

Spatial analysis of North Sea cod recruitment: concurrent effects of changes in spawning stock biomass, temperature and herring abundance

Dag Ø. Hjermann1,*, Jonathan A. D. Fisher2, Tristan Rouyer1, Kenneth T. Frank3, Nils C. Stenseth1,4

1Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, PO Box 1066 Blindern,
0316 Oslo, Norway
2Centre for Fisheries Ecosystems Research, Fisheries and Marine Institute of Memorial University of Newfoundland, PO Box 4920 St. John’s, Newfoundland A1C 5R3, Canada
3Fisheries & Oceans Canada, Bedford Institute of Oceanography, Ocean Sciences Division, Dartmouth, Nova Scotia B2Y 4A2, Canada
4Institute of Marine Research, Department of Coastal Zone Studies, Flødevigen Research Station, 4817 His, Norway

ABSTRACT: The decline of the North Sea cod Gadus morhua has been attributed to both overfishing and ocean warming. However, another hypothesis is that overfishing of piscivorous cod has caused an increased abundance of formerly predatory-controlled pelagic fish including herring Clupea harengus, which in turn has suppressed the recruitment of cod through predation on the early life stages of cod. Here we analyze 40 yr of trawl survey data in order to explore how the abundance of young herring affects cod recruitment, and how cod biomass affects the abundance of herring. In both cases we also take into account the effects of spawner biomass (of cod and herring, respectively) and sea temperature. We take a novel, explicitly spatial, approach by analyzing these effects on a local (185 × 167 km) scale. Our results indicate large spatial variability in ecological mechanisms. In the German Bight, high cod recruitment is associated with low herring abundance, low temperatures and high overall cod spawner biomass. This area used to contain a large portion of the cod recruits, and there is a strong correlation between the fraction of cod recruits found in this area and overall recruitment. In this area, herring recruitment is also negatively associated with the abundance of large cod. Thus, for this part of the North Sea, our findings are consistent with a reversal of dominance between cod and herring; however, herring may affect the cod by competition rather than by predation.

KEY WORDS: Interspecific interaction · Ecosystem dynamics · Cultivation effect · Predator-prey reversal · Spatial heterogeneity · Warming

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Cite this article as: Hjermann DØ, Fisher JAD, Rouyer T, Frank KT, Stenseth NC (2013) Spatial analysis of North Sea cod recruitment: concurrent effects of changes in spawning stock biomass, temperature and herring abundance. Mar Ecol Prog Ser 480:263-275.

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