Atlantic cod recovery from the Allee effect zone: contrasting ecological and evolutionary rescue

The ability of a population to recover from disturbances is fundamental for its persistence. Impaired population recovery might be associated with a demographic Allee effect. Immigration from adjacent populations could accelerate the recovery not only by promoting population growth beyond the Allee effect threshold but also by bringing in advantageous genotypes. We explore the nature and role of ecological and evolutionary rescue in an Atlantic cod (Gadus morhua) population fished below its Allee effect threshold. We utilize an eco-evolutionary model and simulate scenarios, where the target population evolves in response to selective fishing and sample immigrants from i) a source population similarly adapted to fishing (post-fishing genotypes) or ii) an unexploited source population with natural genetic and phenotypic diversity (pre-fishing genotypes). Immigration of pre-fishing genotypes enhances the recovery due to the larger body sizes and the flow of associated genes. Post-fishing immigrants can promote the population abundance recovery, but they slow down evolutionary recovery from fishing-induced selection and increase uncertainty about recovery times. Our results stress the importance of maintaining genetic and phenotypic diversity and suggest that marine protected areas can serve as an important source of immigrants to promote both the demographic and evolutionary recovery of exploited populations.