Abstract:
Plastics make up to 90% of marine litter. Microplastic contamination has emerged as one of
the most serious environmental issues over recent years. Fish are an important source of protein
for many New Zealanders, yet there is little information on contamination of marine fish
species by microplastics. The principal aim of this thesis was to assess the occurrence of
microplastics in coastal inshore fish species and surface waters at the Hauraki Gulf Marine
Park, New Zealand. This study has been structured in two parts. First, it provides baseline
information on plastic ingestion by fish species commonly consumed by New Zealanders, in
order to gain a better understanding of the number of fish species affected by plastic ingestion
and to estimate the average ingestion rate. Secondly, it assesses the abundance and spatial
distribution of microplastic debris in the surface waters. Lastly, it quantifies and characterizes
the composition of recovered plastics by size, form and color.
Samples of six inshore fish species (N = 305) were collected from eleven different sites within
the Hauraki Gulf in 2019. All tissue and other organic material were dissolved using a solution
of 10% potassium hydroxide (KOH), incubating for 48 hours at 40°C. The remaining
homogenate was filtered, and visual analysis was performed to identify suspected plastic
particles. In addition, this study also quantified the amount of floating plastic debris in surface
waters. Sampling was carried out through surface trawls using a zooplankton net with a 330μm
pore size.
Microplastics were found in 70 out of a total of 305 specimens examined. Plastic ingestion rate
(PIR) was expressed as the percentage of fish of the same species that ingested plastic. Average
microplastics (AM) per fish is the arithmetic mean of the total number of microplastic pieces
found in each fish. Plastic load (PL) per individual fish is the arithmetic mean of the total
number of microplastics found in each individual fish that contained microplastics. The
average plastic ingestion rate across all six species was 22.95% with an average of 0.397 pieces
per fish and a plastic load of 1.73 pieces per individual fish. Yellowbelly founder recorded the
highest rate of ingestion at 52.6% while the red gurnard recorded the lowest rate of ingestion
at 6.81%. The gills from pilchards were also separately dissected to investigate the occurrence
of microplastics in the gills as they are filter-feeding fish. Microplastic uptake by the gills of
fish were observed in 4.9% of pilchard samples confirming that filter-feeding fish are at a risk
of consuming plastics. If whole prey were found in the gut, the entire prey item was removed
and analysed for secondary ingestion. Plastic was found in prey, which suggests the trophic transfer of microplastics from prey to predator. Incidence of microplastics in fish caught at
locations closer to Auckland (inshore) and at locations further from Auckland (offshore) were
compared to evaluate if proximity to urban areas influenced the concentrations of microplastic
in fish. Sixty snapper were collected from offshore locations and 43 were collected from
locations closer to Auckland. No significant statistical difference in plastic ingestion rates were
observed between the inshore and offshore locations. Floating microplastic debris were found
in 36 out of 38 sampling stations in the Gulf. In total, this study identified 500 plastic particles
with varying abundances and characteristics. The average number of plastic particles per site
was found to be 13.15 pieces/sample. The total average concentration of plastics across all sites
was estimated at 931.32 plastic pieces/km2.
This study constitutes the first report of floating microplastic debris in New Zealand thus
helping advance the knowledge on plastic pollution in this sparsely sampled region.
Considering that knowledge on microplastic contamination in New Zealand fish and waters is
largely limited, this research has revealed important information on the occurrence of
microplastics in fish helping provide more insight about the safety of seafood products and
their potential impact on human health.