MEPS 333:249-255 (2007)  -  doi:10.3354/meps333249

Egg distribution, bottom topography and small-scale cod population structure in a coastal marine system

Halvor Knutsen1,2,*, Esben Moland Olsen1,2, Lorenzo Ciannelli2,4, Sigurd Heiberg Espeland2, Jan Atle Knutsen1, Jan Henrik Simonsen1, Stig Skreslet3, Nils Chr. Stenseth1,2

1Institute of Marine Research, Flødevigen, 4817 His, Norway
2Centre for Ecological and Evolutionary Synthesis (CEES), University of Oslo, PO Box 1066, Blindern, 0316 Oslo, Norway
3Bodø University College, Department of Fisheries and Natural Sciences, 8049 Bodø, Norway
4Present address: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA

ABSTRACT: Coastal marine species with pelagic egg and larval stages, such as the Atlantic cod Gadus morhua, can be structured into genetically distinct local populations on a surprisingly small geographic scale considering their dispersal potential. Mechanisms responsible for such small-scale genetic structure may involve homing of adults to their natal spawning grounds, but also local retention of pelagic eggs and larvae. For example, spawning within sheltered fjord habitats is expected to favour local retention of early life stages. Here, we studied the distribution of cod eggs along inshore– offshore transects in 20 Norwegian fjords. The general pattern exhibited across all fjords was a higher concentration of cod eggs inside the fjords than further offshore. In particular, fjords with shallow sills (model threshold 37 m) show an abrupt reduction in egg density over the sill. This study provides empirical support for an offspring retention hypothesis, which may help to explain the maintenance of local population structure in pelagic marine systems.

KEY WORDS: Atlantic cod · Gadus morhua · Egg distribution · Population structure

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