Exploitation of Marine Species (Cambridge (CIE) IGCSE Environmental Management): Revision Note

Overfishing & overharvesting

• The rising global demand for seafood has led to the use of unsustainable harvesting practices

  • These methods often damage marine ecosystems and lead to overexploitation of fish stocks

• Overfishing occurs when fish are harvested at a rate faster than they can reproduce

  • This leads to population declines

• Marine overharvesting usually refers to the excessive removal of other marine organisms, such as shellfish, and other ocean resources at a rate faster than they can naturally replace themselves

• These activities threaten marine and freshwater ecosystems, affecting both biodiversity and human livelihoods

• Overfishing and overharvesting can also affect both target species and non-target species, disrupting entire food chains

Impacts of overfishing & overharvesting

Effect on target species

• Target species (the species being fished) may fall below sustainable levels

  • Smaller populations make it harder for species to recover

• Removal of large, breeding adults reduces reproductive success

  • This slows population recovery, even if fishing stops

• Genetic diversity decreases as fewer individuals remain

  • Lower diversity makes species more vulnerable to disease and environmental change

Effect on bycatch species

• Bycatch refers to species caught unintentionally during fishing

  • Examples include dolphins, turtles, seabirds or unwanted fish species

• Bycatch species often die after being caught in nets or hooks

  • This threatens their populations even though they are not being targeted

Effect on food chains

• Overfishing disrupts marine food chains because removing one species affects the species above and below it

  • Marine ecosystems rely on balanced predator–prey relationships

When the overfished species is a predator

• Many target fish species are predators that keep smaller populations under control

  • If these predators are removed, prey species can increase rapidly

• This rapid growth can lead to overgrazing of sea plants or overcrowding

  • For example, if a predator fish that eats small crustaceans is overfished, crustacean numbers may rise and damage seagrass beds

When the overfished species is prey

• Some target species are important food sources for larger predators

  • Removing too many of these fish means predators have less to eat

• Predator populations may decline due to starvation

Wider ecosystem impacts

• As predator–prey relationships change, the entire ecosystem becomes less stable

  • Loss of one key species can cause cascading effects on many others

  • This reduces overall marine biodiversity