Bycatch describes the unintended capture or mortality of marine life resulting from fishing activity. This includes species not targeted by the fishery, such as invertebrates, juvenile fish, sea turtles, and marine mammals. These animals are often discarded, and many do not survive due to injury, entanglement, or the stress of being brought to the surface. Bycatch presents a widespread ecological problem because this non-selective removal disrupts natural systems and removes biomass from the ocean.
Direct Threat to Protected and Endangered Species
Bycatch poses a severe threat to populations of species that are already vulnerable or possess low reproductive rates. Large, long-lived marine animals are highly susceptible to population decline from even small increases in mortality. For example, sea turtles, such as the loggerhead and leatherback, are frequently caught on longlines or entangled in nets while foraging. Entanglement often leads to drowning or severe injury, creating an unsustainable accidental death rate for these threatened species.
Marine mammals, including dolphins, porpoises, and whales, are severely impacted by entanglement in fishing gear. Approximately 300,000 dolphins and porpoises die annually worldwide from being trapped in nets. Since these are air-breathing animals, being held underwater causes them to drown, drastically slowing population recovery. Similar consequences affect seabirds, such as the Albatross, which become hooked on longlines while trying to catch bait. These populations struggle to rebound because their life histories involve delayed sexual maturity and infrequent breeding cycles.
Sharks are disproportionately affected by bycatch mortality, with nearly half of the global shark catch being unintentional. As slow-growing, late-maturing apex predators, their populations are highly sensitive to overexploitation. Bycatch is a major factor in the decline of species like the blue shark. This direct and unselective removal of threatened megafauna destabilizes marine biodiversity.
Altering the Balance of Marine Food Webs
Bycatch removes immense quantities of non-target fish and invertebrates that are functionally important to the food web. Discards include juvenile fish and forage species, which are the primary food source for many larger predators, seabirds, and marine mammals. Removing this biomass reduces prey availability, negatively affecting the reproductive success and survival of species higher up the food chain.
The removal of apex predators, such as large sharks, can trigger a cascading ecological effect known as a trophic cascade. When these top-level predators are diminished, their prey species, such as smaller fish and rays, experience a population explosion. This increase in intermediate predator populations leads to the overconsumption of organisms at lower trophic levels, altering the entire ecosystem structure. Such disruptions can shift a stable marine environment, for example, transitioning a diverse coral reef to an algae-dominated system.
Bycatch also removes organisms that play specialized roles, such as grazers that maintain algae levels or detritivores that clean the seafloor. The removal of these functional groups destabilizes the natural balance of predator-prey dynamics and competition. This disruption weakens the overall resilience of the ecosystem, making it more susceptible to stressors like pollution and climate change.
Physical Damage to Benthic Habitats
Certain fishing methods cause extensive physical damage to the seafloor, or benthic habitat, resulting in an indirect impact related to bycatch. Gear like bottom trawls and dredges are dragged across the seabed, physically scraping and crushing the organisms and structures that form complex habitats. This destructive contact can penetrate the substrate, resulting in subsurface abrasion that effectively plows the ocean floor.
These benthic environments, including deep-sea corals, sponge grounds, and seagrass meadows, serve as nurseries and refuges for countless marine species. Fragile, slow-growing organisms like deep-sea corals can take decades or centuries to recover from physical disturbance. The destruction of these complex structures removes the shelter and feeding grounds necessary for the survival of many fish and invertebrate populations.
The physical disturbance from bottom-contacting gear is widespread, affecting large percentages of assessed areas. This constant pressure prevents the recovery of long-lived benthic organisms and alters the composition of the seabed community. This habitat degradation undermines the foundational support for the entire local ecosystem.
The Cycle of Ghost Fishing and Persistent Debris
A persistent problem arises from fishing gear that is abandoned, lost, or discarded (ALDFG), widely known as “ghost gear.” Once lost, this gear—including nets, lines, and traps—continues to function indiscriminately, a process called ghost fishing. Ghost gear is engineered to catch marine life and remains active for years or even decades, making it the most lethal form of plastic debris in the ocean.
The lost gear actively traps and kills fish, crustaceans, marine mammals, and sea turtles, leading to death by drowning, starvation, or exhaustion. The decomposing bodies of trapped animals attract scavengers and predators, creating a deadly, self-perpetuating cycle of mortality. Estimates suggest that between 500,000 and 1 million tons of fishing gear are lost annually, constituting about 10 percent of all marine plastic pollution.
Modern fishing gear is primarily made from durable synthetic plastics like nylon and polyethylene. This resilience means that once lost, the gear persists for decades before breaking down into microplastics. Ghost gear also causes physical damage to sensitive habitats, such as coral reefs, by snagging, smothering, and abrading structures as it drifts. This continuous source of mortality and habitat destruction extends the impact of fishing long after vessels have left the area.