Nearly 50% of plastic in the Great Pacific Garbage Patch comes from fishing

Nature.com publishes an article by L. Lebreton et al, 22nd March 2018, titled Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic. The authors state:

Ocean plastic can persist in sea surface waters, eventually accumulating in remote areas of the world’s oceans. Here we characterise and quantify a major ocean plastic accumulation zone formed in subtropical waters between California and Hawaii: The Great Pacific Garbage Patch (GPGP). Our model, calibrated with data from multi-vessel and aircraft surveys, predicted at least 79 (45–129) thousand tonnes of ocean plastic are floating inside an area of 1.6 million km2; a figure four to sixteen times higher than previously reported.

We explain this difference through the use of more robust methods to quantify larger debris. Over three-quarters of the GPGP mass was carried by debris larger than 5 cm and at least 46% was comprised of fishing nets.

Microplastics accounted for 8% of the total mass but 94% of the estimated 1.8 (1.1–3.6) trillion pieces floating in the area. Plastic collected during our study has specific characteristics such as small surface-to-volume ratio, indicating that only certain types of debris have the capacity to persist and accumulate at the surface of the GPGP.

Finally, our results suggest that ocean plastic pollution within the GPGP is increasing exponentially and at a faster rate than in surrounding waters.

Global annual plastic consumption has now reached over 320 million tonnes with more plastic produced in the last decade than ever before1. A significant amount of the produced material serves an ephemeral purpose and is rapidly converted into waste. A small portion may be recycled or incinerated while the majority will either be discarded into landfill or littered into natural environments, including the world’s oceans.

While the introduction of synthetic fibres in fishing and aquaculture gear represented an important technological advance specifically for its persistence in the marine environment, accidental and deliberate gear losses became a major source of ocean plastic pollution. Lost or discarded fishing nets known as ‘ghostnets’ are of particular concern as they yield direct negative impacts on the economy and marine habitats worldwide.

Around 60% of the plastic produced is less dense than seawater. When introduced into the marine environment, buoyant plastic can be transported by surface currents and winds, recaptured by coastlines, degraded into smaller pieces by the action of sun, temperature variations, waves and marine life, or lose buoyancy and sink. A portion of these buoyant plastics however, is transported offshore and enters oceanic gyres.

A considerable accumulation zone for buoyant plastic was identified in the eastern part of the North Pacific Subtropical Gyre. This area has been described as ‘a gyre within a gyre’ and commonly referred to as the ‘Great Pacific Garbage Patch’ (GPGP). The relatively high concentrations of ocean plastic occurring in this region are mostly attributed to a connection to substantial ocean plastic sources in Asia through the Kuroshio Extension (KE) current system as well as intensified fishing activity in the Pacific Ocean.

Most available data on quantities and characteristics of buoyant ocean plastic are derived from samples collected with small sea surface trawls initially developed to collect neustonic plankton. Due to their small aperture (0.5–1 m width, 0.15–1 m depth) and limited surface area covered, they could underestimate loads of rarer and larger plastic objects such as bottles, buoys and fishing nets. In an attempt to overcome this misrepresentation, a research team combined net tow data with information from vessel-based visual sighting surveys.

They found that while small, millimetre-sized pieces (<4.75 mm) count in trillions at global scale, they only represent a small mass portion (13%) of the total available buoyant material. Nevertheless, vessel-based sightings data yielded high uncertainties due to differences in survey protocols across research groups and difficulties in estimating the mass of sighted objects. Historical datasets on buoyant ocean plastic are also sparse in space and time.
   
To circumvent such limitations, recent studies have coupled datasets with dispersal models to predict ocean plastic pollution levels worldwide. Outputs from ocean plastic transport model are generally integrated over several years and calibrated against datasets collected during different seasons, years and decades. However, such method may misrepresent ocean plastic transportation and accumulation as these processes are closely associated with seasonal and inter-annual variability.

In this study, we characterised and quantified buoyant ocean plastics inside the GPGP.

Between July and September 2015, we conducted a multi-vessel expedition to collect surface trawl samples within and around the GPGP region and obtain a representative distribution of buoyant plastic concentrations in this region.

In October 2016, we conducted an aerial survey to obtain geo-referenced imagery that sampled greater sea surface area and improved estimations for debris larger than 0.5 m.

Our final dataset, containing measured concentrations for ocean plastic of various sizes and types, was used to calibrate a multi-source and multi-forcing ocean plastic transport model. We calibrated our numerical model using monthly averages of predicted concentrations that reflected seasonal and inter-annual changes of the GPGP position.

As such, this study is a first attempt at introducing a time-coherent dynamic model of floating debris accumulation in the GPGP. This allowed us to compare our findings with historical observations (1970s to present) and assess the long-term evolution of ocean plastic concentrations within and around the GPGP.

 

Source:  www.nature.com,  22nd March 2018.  To read the full report, see  www.nature.com/articles/s41598-018-22939-w

 


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