A trawl survey of seafloor macrolitter on the South African continental shelf

https://doi.org/10.1016/j.marpolbul.2019.110741Get rights and content

Highlights

  • We report the first estimates of seabed litter from trawl surveys off sub-Saharan Africa.

  • Only 17% of trawls contained litter, and densities were low (3.4 items·km2).

  • Most litter was packaging, but marine wastes accounted for most of the mass of litter.

  • Proximity to Cape Town, a major urban litter source, best explained litter distribution

  • Most plastic items floated, and biofouling probably caused these plastic items to sink.

Abstract

Demersal trawls provide an index of seafloor macrolitter abundance, but there are no published data from sub-Saharan Africa. We collected litter items from 235 trawls conducted to assess fish abundance off South Africa. Only 17% of trawls contained litter (3.4 items·km2, 2.1 kg·km2 but only 0.2 kg·km2 excluding four megalitter items). Plastic items predominated (88%), of which 77% floated once cleaned of epibionts. One LDPE bag manufactured three months before being caught carried pelagic goose barnacles Lepas anserifera, confirming that biofouling leads to rapid sinking of floating plastics. Fishery/shipping wastes comprised 22% of litter items (98% by mass; 73% excluding megalitter items); the remainder was general waste – mostly packaging or other single-use items – that could come from land- or ship-based sources. Litter was more abundant in deep water close to Cape Town. The annual demersal trawl survey is a useful way to monitor seafloor litter off South Africa.

Introduction

The seafloor is one of the main long-term sinks for plastic litter and is thus an important environmental compartment for which to determine baseline levels of this increasingly ubiquitous type of pollution (Galgani et al., 1996; Pham et al., 2014; Woodall et al., 2014; Bergmann et al., 2017; Ioakeimidis et al., 2017; Peng et al., 2019; GESAMP, 2019). The abundance of macrolitter on the seafloor in shallow water (<30 m) can be assessed by SCUBA divers, but in deeper waters, bottom trawls and underwater cameras (mainly using remotely operated vehicles [ROVs]) offer the best way to assess seafloor litter (GESAMP, 2019). However, both approaches require substantial resources in terms of ships' time, and there have been few dedicated studies to monitor seafloor litter (e.g. Schlining et al., 2013). Most trawl surveys tend to be opportunistic, linked to demersal fish stock surveys. Depending on gear design, these may not be ideal for sampling seafloor litter, but they can provide useful data on long-term trends in seafloor litter provided there is consistency in fishing gear (Maes et al., 2018; GESAMP, 2019).

Information of the abundance and composition of seafloor litter is particularly sparse in the global south. For example, the reviews by Galgani et al. (2015) and Ioakeimidis et al. (2017) each only cite one Southern Hemisphere study of seafloor litter, and only 11% of references to ‘bottom litter’ in Litterbase (https://litterbase.awi.de; n = 116) report data from the Southern Hemisphere. Of these, most studies refer to SCUBA surveys in shallow coastal waters; only two papers report litter in bottom trawls: Acha et al. (2003) used trawls to show that litter is concentrated upriver from the salinity front in the Rio de la Plate estuary off the east coast of South America, and Barnes et al. (2018) collected litter on several South Atlantic seamounts using a mini-Agassiz trawl. Litterbase contains no references to seafloor litter from sub-Saharan Africa, but there have been some SCUBA surveys of litter in coastal waters (Rundgren, 1992; Pfaff et al., 2019). However, as far as we can ascertain, there have been no published assessments of plastic and other macrolitter on the deeper waters of the continental shelf around sub-Saharan Africa. We report the abundance and composition of macrolitter caught in research trawls made to estimate the abundance of hake Merluccius spp. stocks off the west and south coasts of South Africa.

Section snippets

Methods

We collected all man-made items found in trawls conducted during the 2019 South African demersal fish abundance surveys from 17 January to 3 May 2019. The surveys were conducted to estimate the abundance of hakes Merluccius paradoxus and M. capensis and other ground fish on the South African continental shelf from the Namibian border to almost 27°E, where the shelf narrows east of Port Elizabeth (Fig. 1). Trawl sites were selected using a random stratified approach to sample across the

Results

A total swept area of 18.9 km2 was sampled during 235 trawls distributed throughout the study area (Fig. 1). A further eight trawls were discarded due to technical difficulties or damage to gear. Water depth sampled ranged from 33 to 910 m, with 90% of trawls in water 70–660 m deep. Litter items were collected in 16.6% of trawls, with most litter recorded in the region from Cape Town north to St Helena Bay (Fig. 1). Distance to Cape Town was the most important variable influencing the

Discussion

Our study provides the first systematic survey of seafloor litter on the South African continental shelf. Macro and microplastics floating at sea and stranded on beaches have been sampled around South Africa since the 1980s (Ryan, 1988; Ryan and Moloney, 1990), but there is little information on the distribution, abundance and composition of seafloor litter. Recreational divers have conducted clean-ups for some time, mainly in heavily impacted areas such as harbours, but the first attempt to

CRediT authorship contribution statement

Peter G. Ryan:Conceptualization, Investigation, Writing - original draft.Eleanor A. Weideman:Investigation, Visualization, Writing - review & editing.Vonica Perold:Investigation, Visualization, Writing - review & editing.Deon Durholtz:Investigation, Writing - review & editing.Tracey P. Fairweather:Investigation, Writing - review & editing, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the Department of Environmental Affairs demersal survey fish team for collecting the samples; Natasha Karenyi, Luther Adams and Otto Whitehead helped to identify epibionts.

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