genetic impacts and fisheries

fisheries induced evolution

• fisheries-induced evolution (FIE) = genetic changes in fish populations caused by selective harvesting

• fishing targets big fish

• natural predators often target small or weak fish

• this:

  • fishing = opposite selection pressure compared to nature

  • large, fast growing, late maturing fish are removed

  • small, slow growing, early-maturing fish survive and reproduce

• different life history traits are affected by due to FIE

  • fishing selects for earlier maturation, smaller size at age, slower growth, lower reproductive investment

  • this is because large, old, fecud fish are removed, and only small early maturing fish contribute to the gene pool. so overtime there is an evolutionary shift

• FIE is a problem because:

  • smaller adults = low fecundity 

  • early maturation = less energy for growth

  • reduced body size = lower resilience

  • long term reduction in stock productivity

  • slow to reverse, because evolution is heritable

  • FIE can reduce maximum sustainable yield (MSY) and recovery potential

genetic structure and local adaptation

• fish populations are not single homogenous genetic pools

• they are made f local, genetically distinct subpopulations, adapted to temperature, salinity, migration routes, spawning grounds, timing, and depth

• NE atlantic cod has multpile genetically distinct populations    

  • coastal vs migratory cod    

  • spawning groups separate

  • different adaptive genes

• Herring

  • shows AIC-like clines, different spawning times, genetically strucutred populations

• salmon

  • locally adapted to rivers → extremenly strong genetic structure

• overfishing removes some local groups entirely leading to loss of genetic diversity, loss of locally adapted traits, and reduced resilience to climate change

• this is called genetic erosion

how fisheries impact genetic diversity

• there are three pathways that impact genetic diversity

  • direct selection

  • demographic effects

  • habitat changes

• direct selection:

  • this is basically teh selective removal of large/old fish = evolutionary change

• demographic effect:

  • overfishing reduces population size, number of breeders and effective population size (Ne)

  • a low Ne → random genetic drift, loss of rare alleles, inbreeding, reduced adaptability

• habitat changes

  • fishing modifies the seabed, migration pathways, spawning grounds

• this shifts which subpopulation survives, altering the genetic structure

genomic tools reveal hidden impacts

• there are new genomic technologies

  • whole genome sequencing

  • SNP arrays

  • RADseq

  • Environmental DNA (eDNA)

• these tools reveal cryptic population structure, fine sclae adaptation, selective sweeps, signatures of overfishing, and changes happening faster than we though

• case study: atlantic cod

  • cod have supergenens (inversions) associated with migration behaviour

  • overfishing + climate → geographical shifts in supergene frequencies

  • strong spatial genetic structure despite high dispersal potential

gene flow is not enough to rescue overfishes stocks

• even if some fish move long distances, strong local adaptation means that mixing does not guarantee recovery

• this is why some stocks fail to rebuild

fisheries induced evolution is slow to reverse

• once traits (like small size) become genetically embedded

  • reversal takes many generations

  • natural selection must work in opposite direction

  • but fishing pressure often continues

• so we get evolutionary ratchets

management implications

• protect old/large fish → maintain genetic diversity

• protect local spawning units

• use genomic monitoring

• avoid high exploitation rates

• incorporate evolutionary considerations into stock assessments

the papers - Bernatchez et al. 2017 and Helmerson et al. 2025

fisheries have profound genetic impacts on marine fish populations through both selective harvesting and long term demographic reductinos. Bernatchez et al. (2017) outlines the mechanisms of fosheries induced evolution (FIE), where size selective harvesting removes large, late maturing individuals and favours genotypes, local adaptation, and earlier maturation. Genomic toold reveal fine scale population strucutre, local adaptation, and evolutionary shofts that would otherwise remain hidden. Helmerson et al. (2025) provide empirical evidence of these processes by analysing a century of archival and modern atlantic cod genomes. They show major allele frequency shifts, reduced genetic diversity, and changes in key supergenes controlling migration, behaviour, and temperature tolerance.Many cod subpopulations have been genetically homogenised or lost entirely, indicating that human pressures have reshaped cod genomes far more rapidly than natural processes could. Both papers highlight that genetic erosion reduces adaptive potential and makes population recovery under climate change more difficult. Effective management therefore requires protecting large spawners, conserving local populations, maintaining effective population size, and using genomic to monitor evolutionary change