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.