Stable coexistence of genetically divergent Atlantic cod ecotypes at multiple spatial scales

Abstract

Coexistence in the same habitat of closely related yet genetically different populations is a phenomenon that challenges our understanding of local population structure and adaptation. Identifying the underlying mechanisms for such coexistence can yield new insight into adaptive evolution, diversification and the potential for organisms to adapt and persist in response to a changing environment. Recent studies have documented cryptic, sympatric populations of Atlantic cod (Gadus morhua) in coastal areas. We analysed genetic origin of 6,483 individual cod sampled annually over 14 years from 125 locations along the Norwegian Skagerrak coast and document stable coexistence of two genetically divergent Atlantic cod ecotypes throughout the study area and study period. A “fjord” ecotype dominated in numbers deep inside fjords while a “North Sea” ecotype was the only type found in offshore North Sea. Both ecotypes coexisted in similar proportions throughout coastal habitats at all spatial scales. The size‐at‐age of the North Sea ecotype on average exceeded that of the fjord ecotype by 20% in length and 80% in weight across all habitats. Different growth and size among individuals of the two types might be one of several ecologically significant variables that allow for stable coexistence of closely related populations within the same habitat. Management plans, biodiversity initiatives and other mitigation strategies that do not account for the mixture of species ecotypes are unlikely to meet objectives related to the sustainability of fish and fisheries.

Fine‐scale population differences in Atlantic cod reproductive success: A potential mechanism for ecological speciation in a marine fish

“…The results of the present study indicate that genetically distinctive populations of Atlantic cod (Knutsen et al, ) can differ considerably in individual reproductive success when competing for mating and reproductive opportunities. These differences appear not to be attributable to phenotypic variability between populations.…”

“…There might also be genetically based differences in reproductive success related to local adaptation, given a high correlation between genetic origin and the presence of three inversion zones in the genome (Sodeland et al, ). Cod in the outer fjord are dominated by a North Sea genomic signature, whereas a coastal‐fjord genotype is predominant in inner Risør (Knutsen et al, ).…”

“…South of 62°, Norwegian cod are considered part of a single North Sea cod management unit ( http://www.ices.dk). Yet cod that inhabit fjords and coastal waters along the southeast Norwegian coast—Skagerrak—can differ genetically from North Sea cod (Knutsen et al, ), a distinction that seems temporally stable, suggesting that cod spawn in, and inhabit, coastal Skagerrak waters throughout the year rather than the North Sea (Cianelli et al, ; Knutsen et al, ; Rogers et al, ).…”

“…There is compelling evidence that cod inhabiting the inner waters of some Skagerrak fjords are phenotypically and genetically distinct from cod inhabiting outer waters of the same fjord (Knutsen et al, ; Øresland & André, ). In this respect, the most extensive data are available for Risør Fjord (58.7°N, 9.2°E).…”

“…Although they are not physically restricted from moving between the two areas, cod inhabiting inner and outer Risør waters can differ genetically from one another (Knutsen et al, ). Recent analyses of single nucleotide polymorphism (SNP) data have revealed that two different genotype clusters of cod coexist in various proportions along coastal Skagerrak (Knutsen et al, ); a “fjord” ecotype dominates the waters of the inner fjords, for which Risør is an excellent example, whereas a “North Sea” ecotype is often predominant in outer‐fjord waters. Phenotypically, cod from inner Risør grow at a slower rate than those from outer Risør (Kuparinen, Roney, Oomen, Hutchings, & Olsen, ).…”