How does overfishing contribute to biodiversity loss in marine ecosystems?

Overfishing is considered the greatest threat to marine biodiversity, contributing to ecosystem degradation through several primary mechanisms:

• Trophic Cascades: The removal of top predators—such as sharks, tunas, and large predatory fish—disrupts the natural balance of food webs. This can trigger a trophic cascade, where the loss of predators leads to an overabundance of smaller species, which in turn can decimate primary producers or other essential organisms, fundamentally altering ecosystem structure.12
• Habitat Destruction: Commercial methods like bottom trawling physically plow the seafloor, crushing and burying corals, sponges, and other bottom-dwelling species. This causes widespread, long-term damage to critical marine habitats that are often slow to recover.32
• Bycatch: Large-scale fishing gear is often indiscriminate, capturing nontarget species—including sea turtles, seabirds, and marine mammals—as bycatch. This unintentional mortality is a substantial factor in threatening many species with extinction.42
• Population Imbalance: Overfishing disproportionately removes the oldest and largest individuals, which are vital for reproduction. This leaves behind juvenile populations that are more susceptible to predation, reducing the overall resilience and recovery capacity of the species.2