The problem of discarded plastic fishing line begins with Abandoned, Lost, or Discarded Fishing Gear (ALDFG), often referred to as “ghost gear.” This thin, strong material is one of the most persistent forms of marine debris found globally. Unlike organic materials, plastic fishing line does not truly disappear or biodegrade on human timescales. Instead, the polymer material fragments into smaller pieces over vast stretches of time, creating a long-term environmental hazard.
Composition and Estimated Lifespans
The material composition of the line determines its longevity. The most common type is monofilament line, primarily made from Nylon, a polyamide polymer. Widely accepted estimates place the breakdown timeline for Nylon monofilament in the range of 500 to 600 years or more.
Another material, fluorocarbon line, is made from Polyvinylidene Fluoride (PVDF) and is chemically more stable than Nylon. This material is inherently more resistant to ultraviolet (UV) light damage, which significantly slows its breakdown process. Braided lines, often composed of Ultra-High-Molecular-Weight Polyethylene (UHMWPE) fibers, are also extremely durable. Due to their chemical structure, both fluorocarbon and polyethylene lines can persist for periods potentially even longer than Nylon.
The Mechanics of Plastic Degradation
The term “break down” for plastic fishing line describes a physical fragmentation process, not a microbial one. These synthetic polymers cannot be broken down chemically by marine microorganisms, meaning true biodegradation does not occur at any meaningful rate. The primary mechanism initiating degradation is photodegradation, which is the breakdown caused by sunlight.
Ultraviolet light provides the energy needed to break the polymer chains. This process is essentially photo-oxidation, where oxygen and UV light weaken the material’s chemical bonds. Water also contributes through hydrolysis, which slowly cleaves the polymer chains, although this is a minor factor in most marine conditions. The initial chemical changes are often confined to the surface layers of the monofilament.
The chemical weakening is then accelerated by constant mechanical action from the ocean itself. The physical forces of waves, currents, and abrasion against rocks or coral reefs help break the material into smaller physical pieces. This combination of sunlight and mechanical stress slowly reduces the line into countless tiny fragments over hundreds of years.
Environmental Factors Influencing Decay Rate
The estimated centuries-long lifespans are highly variable because the rate of degradation depends on local environmental conditions. UV exposure is the most significant accelerator, meaning lines floating on the surface or washed up on sunny beaches deteriorate faster than those submerged. Even with direct sunlight, the process is still measured in multiple human lifetimes.
Temperature also plays a role, as warmer waters increase the rate of chemical reactions like hydrolysis and photo-oxidation. Conversely, lines lost in deep, cold, and dark ocean trenches experience a significantly slower rate of decay. These conditions drastically reduce both UV exposure and temperature-driven chemical changes. Mechanical abrasion against the seafloor or other debris also speeds up the physical fragmentation process.
The Ultimate Fate: Microplastic Formation
The final stage of this long degradation timeline is the formation of microplastics. As the fishing line fragments due to chemical and physical weathering, it breaks down into pieces smaller than five millimeters in length. This process contributes to the immense volume of secondary microplastic pollution in the ocean environment.
These tiny fragments persist in the water column and sediments, posing an ecological risk. Microplastics are readily ingested by marine organisms, including plankton, fish, and filter feeders, allowing the plastic to move up the food chain. Furthermore, these fragments can absorb and leach chemical pollutants from the surrounding seawater, introducing toxic substances into the marine food web. The complete removal of these particles is virtually impossible once they are dispersed throughout the global ocean.