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

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 inshoreoffshore 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.

Spatial scale of genetic structuring in coastal cod Gadus morhua and geographic extent of local populations

“…between the Georges Bank and Browns Bank, about 80 km apart (Ruzzante et al 1998) and over a saline gradient (Nielsen et al 2003). It may also be of relevance that Bjelland was sampled further outside the fjords than were the other samples (perhaps outside the spawning area, as suggested by recent egg-density 234 Knutsen et al 2007). This sample also contained a lower percentage of mature fish (< 50%: Table 1), and it is unclear to what extent it represented a spawning population.…”

Escapes of fishes from Norwegian sea-cage aquaculture: causes, consequences and prevention

“…Wright et al 2006); and (3) sea-cage cod farms are often located within short distances of known wild cod spawning grounds . Recent research also suggests that cod eggs may be entrained in the vicinity of the spawning grounds long after spawning (Knutsen et al 2007). Therefore, there is considerable potential for larvae from escaped cod eggs to experience favourable conditions for survival and recruitment to coastal cod stocks if spawning in sea-cages occurs during the natural spawning season of wild cod.…”

Infection of wild fishes by the parasitic copepod Caligus elongatus on the south east coast of Norway

“…Tagging of Norwegian coastal Atlantic cod (Moksness & Øiestad 1984, Danielsen & Gjøsaeter 1994 and genetic studies (Knutsen et al 2007) show that these fish have high site fidelity. This suggests that few Atlantic cod from the south east Norwegian coast migrate to open waters and become infected with Genotype 1 Caligus elongatus.…”