Microplastic concentrations on the urban coastline of KwaZulu–Natal, South Africa, and its impact on juvenile fish.

Abstract

The global production of plastics per annum has increased from 1.5 million tonnes in the 1950‘s to 300 million tonnes today. Following this increasing production trend, plastic concentrations have increased over time in marine environments. Improper sewage treatment, industrial spillages, garbage and fishing activities among many others, have made the marine environment a sink for plastic debris. The main aims of this study were to determine (1) microplastic levels within five estuaries along the Durban coastline and on intervening beaches, (2) the incidence of plastic ingestion by estuarine mullet, (3) the effects of plastic ingestion on long–term fish health and (4) the plastic concentrations along the KwaZulu–Natal coastal shelf. To achieve these aims (1) plastic was isolated from estuarine sediment, beach sediment and the surface water of each estuary, (2) fish from the most polluted estuary were dissected to investigate the incidence of plastic ingestion, (3) small juvenile fish were kept in tanks and fed plastics for three months to monitor their growth and survival and (4) coastal water samples were collected using a manta trawl net to quantify floating debris in the ocean. Overall, an attenuating plastic concentration trend away from the city centre was found, with the Durban Harbour, Isipingo and uMgeni Estuaries having the highest contamination levels. The highest recorded plastic levels were found in the Bayhead area of the harbour, with 745.4 ± 129.7 particles per 500 mL, which mostly consisted of plastic fragments. Fibres dominated other estuaries with proportions ranging from 38% of total plastics in the uMgeni Estuary to 66% in the Mdloti. Plastic particle concentration in estuarine sediment generally increased from larger to smaller size classes. High plastic concentrations were also found on the coastal shelf of KwaZulu–Natal, with sites south of the harbour having the highest plastic concentrations, however no seasonal differences were found. There is also evidence pointing toward long range movement of particles and thus pollution at the source must be dealt with before it reaches the open ocean. Seventy three percent of the mullet sampled at the harbour ingested plastic particles with an average of 3.751 ± 4.667 (S.D.) particles per fish. Particles that were ingested were mainly fibres that are thought to come from sewage inputs to the harbour. Juvenile fish in microplastic feeding experiments had lower growth and survival than control fish. This has possible economic and ecological consequences for future fish stocks that use urban estuaries as nursing areas.