The Otago study aimed to understand whether species in particular locations were more affected than others.
While previous studies have found microplastic levels in sediments increased near urban areas, few studies examined whether this resulted in fish ingesting more plastics in those areas.
The research sampled three sites around Dunedin, to demonstrate that fish collected 2.5km and 10km from the city centre ingested significantly more microplastics than fish collected 25km away from the city.
Adult mottled triplefins — a nest-guarding intertidal fish — were collected at each of the three sites: Dunedin city centre, Port Chalmers, and a rural site, Pūrākanui.
Lead author and University of Otago zoology master’s graduate Fletcher Munsterman said researchers initially set out to discover whether there was a correlation between the level of microplastics in the sediment at each location, and how much plastic fish had ingested.
However, despite finding no significant relationship between the two, the fish collected at Pūrākanui had ingested 8.5 times less plastic overall than those in Dunedin city, and 6.3 times less plastic than Port Chalmers.
He said fish dwelling in the more urbanised areas of Otago Harbour, were exposed to far more plastic in the sediment, in correlation with the human population density.
"Even if they aren’t ingesting it directly, they’re still being exposed to the effects from the presence of these plastics — whether that’s the physical impact or chemical.
"If a fish is eating large amounts of microplastics, and the plastics are too large to pass directly through the system, the fish themselves will hold on to those plastics in their systems, and pass these on to the next level up the food chain when they are eaten."
Of the total samples collected from all three sites, 83% contained microplastics.
While the Pūrākanui microplastic counts were similar to counts found in the guts of various New Zealand commercial fish species, the Port Chalmers and Dunedin city counts were much higher, with fish averaging 20 microplastics per gut.
Research supervisor Assoc Prof Sheri Johnson said she was surprised by the high levels of microplastic concentrations in Otago compared to other areas in New Zealand.
"The concentration of microplastics in sediments and ingested by the triplefins here in Otago are similar to the microplastic levels in the highly-polluted Venetian Lagoon.
"We need to understand more about why Otago sediments and marine life have so much higher concentrations of microplastics."
She hoped the study would make people think more about their use and disposal of plastics.
"It would be great if people would at least collect plastic rubbish when they see it on beaches or shorelines."
