Population structure in Atlantic cod in the eastern North Sea-Skagerrak-Kattegat: early life stage dispersal and adult migration

Abstract

BackgroundIn marine fish species, where pelagic egg and larvae drift with ocean currents, population structure has been suggested to be maintained by larval retention due to hydrographic structuring and by homing of adult fish to natal areas. Whilst natal homing of adults has been demonstrated for anadromous and coral reef fishes, there are few documented examples of philopatric migration in temperate marine fish species.ResultsHere, we demonstrate temporally stable genetic differentiation among spawning populations of Atlantic cod (Gadus morhua L.), and present genetic and behavioural evidence for larval drift and philopatric migration in the eastern North Sea-Skagerrak-Kattegat area. We show that juvenile cod collected in the eastern Skagerrak and central Kattegat are genetically similar to cod from offshore spawning areas in the eastern North Sea. Genetic assignment of individual 2–5 year old fish indicates that cod residing at, or migrating towards, spawning areas in Kattegat and the North Sea display philopatric behaviours.ConclusionsTogether these findings suggest a loop between spawning, larval drift and adult return-migrations to spawning areas and underlines that both oceanographic processes and migratory behaviour in the adult phase may be important for stock separation and integrity in marine temperate fishes such as Atlantic cod.Electronic supplementary materialThe online version of this article (doi:10.1186/s13104-016-1878-9) contains supplementary material, which is available to authorized users.

Genome architecture enables local adaptation of Atlantic cod despite high connectivity

“…On a much smaller spatial scale within the Skagerrak and Kattegat, two confined seas connecting the brackish Baltic Sea with the saline North Sea (Figure ), evidence has recently accumulated for the presence of yet another pair of coexisting Atlantic cod ecotypes (André et al., ; Rogers, Olsen, Knutsen, & Stenseth, ; Sodeland et al., ). These coexisting fish are characterized by distinct lifestyles, with mobile oceanic (offshore) individuals foraging along the coast but possibly returning to North Sea or offshore Skagerrak spawning sites, and sedentary coastal individuals that remain close to the coast and local spawning sites at all times (Espeland et al., ; Knutsen et al., ; Neuenfeldt et al., ; Rogers et al., ).…”

“…In these cases, the maintenance of differentiation has been associated with seascapes, coastal topography and hydrographic features such as salinity gradients (Ciannelli et al., ; Howe et al., ; Knutsen et al., ; Rogers et al., ). Limited migration of coastal cod (Espeland et al., , ), spawning site fidelity (Espeland et al., ; Skjæraasen, Meager, Karlsen, Hutchings, & Fernö, ) and pronounced natal homing behaviour (André et al., ; Bonanomi et al., ; Svedäng, Righton, & Jonsson, ) could further aid differentiation of coastal and oceanic ecotypes by reducing the potential for gene flow. Interestingly, allelic frequency shifts of large chromosomal rearrangements have recently been described between western Skagerrak cod residing in coastal vs. oceanic environments (Sodeland et al., ).…”

“…Interestingly, allelic frequency shifts of large chromosomal rearrangements have recently been described between western Skagerrak cod residing in coastal vs. oceanic environments (Sodeland et al., ). In contrast, studies have so far failed to delineate genetic structuring of coastal and locally adapted populations within the fine geographical scale along the eastern Skagerrak‐Kattegat coast and fjords (André et al., ; Svedäng, André, Jonsson, Elfman, & Limburg, ), although spawning site fidelity was supported by otolith chemistry (Svedäng et al., ).…”

Complex post-larval dispersal processes in Atlantic cod revealed by age-based genetics and relatedness analysis

“…However, genetic drift cannot account for local family structures. Another likely possibility is that, at some point after the pelagic larval stage, ad mixed populations of juvenile cod sort into in shore and offshore ecotypes (André et al 2016, Bonanomi et al 2016. Cod larvae from offshore spawning areas are often transported by ocean currents into nearshore areas (Pepin & Helbig 1997), where they may reside as juveniles.…”

“…Nevertheless, post-settlement processes that maintain population structure in the face of extensive mixing during juvenile stages remain largely unexamined. Whether nearshore juvenile cod originating from offshore transition into inshore cod, or eventually return to join offshore spawning populations, remains un known (but see André et al 2016, Bonanomi et al 2016. The disconnect between pre-adult and adult life-history stages represents one of the most significant gaps in our knowledge of cod population connectivity (Robichaud & Rose 2004).…”