Abstract
Microplastics are no longer an emerging pollutant. Instead, they are now considered ubiquitous in the environment. Along with the global distribution of large plastic pieces, microplastics are now in the soil, air, and water of our planet. These plastics are less than five millimetres in size, and as such are easily available to organisms throughout the environment. As more studies are done on the impacts of microplastics, it is becoming apparent that these particles can and do have an impact on the health of our ecosystems. Both primary plastics (plastics directly from manufacturers) and secondary plastics (those worn off from larger pieces of plastic) in the environment are being studied with concerns over their impact on the wellbeing of our planet. Over time, it has become apparent that microplastic exposure is akin to exposure to endocrine disruptors, and that the plastics themselves can interfere with the hormonal balance within individual organisms. As hormones regulate and occasionally catalyse important behaviours within the bodily system, it is important to study and understand the exact risks that microplastic exposure carries to organisms in the environment. In addition to impacts caused by pure microplastics, many plastics in the environment carry additional risks to organisms through phthalates used during the manufacturing process. These phthalates provide plastic items with the flexibility and strength that plastic items are coveted for, but phthalates are readily leached into the environment once a plastic item begins breaking down. Once plastic items have contact with the environment, the phthalates used to create them readily leach out of the polymers. Phthalates act as disrupters of the hormonal balance within organisms when they enter their system, often catalysing changes in physiology and behaviour. For this thesis investigation, I used the mottled triple fin (Fosterygion capito) as a model to investigate behaviour changes under exposure to plastics and the common phthalate di (2-ethylhexyl) phthalate (DEHP).
Chapter 2 investigates the environmental relevance of the laboratory study. I selected three sites (the mouth of the Water of Leith; Mussel Bay, Port Chalmers; and the Purakaunui Harbour jetty) and collected both sediment and fish samples for microplastic quantification. These sites were classified as urban, suburban, and rural, respectively. I predicted that I would find the most microplastic contamination in the sediment and guts closest to the city, a moderate amount at Port Chalmers, and the least amount of plastic at Purakaunui, as Purakaunui is rural and the most pristine of the three. As triplefins are benthic fish, and forage near sediment, I predicted that the amount of microplastics I would find in the intestinal tracts of my collected fish would correlate with the quantities found in the sediment samples. Ten triplefins and ten sediment samples were collected from each of the three sites. Sediment samples were dried and then processed to dissolve majority of the biologicals present, and then filtered through two density gradients. Fish guts were dissolved in 1M KOH and the remaining solids vacuum filtrated onto a glass filter paper. Each sample was then photographed under a microscope using a UV light filter to fluoresce the microplastics within the sample. Using ImageJ, the microplastics in each photograph were counted, and then compared across locations. No significant difference was found in the microplastic count in the sediment across locations, but there was a trend towards less plastics in the Purakaunui sediment. There was, however, a significant difference in the microplastic count in the gut contents, with significantly less plastics found in the fish collected from Purakaunui compared to the fish collected from inside the Otago Harbour. Despite visible trends, no correlation was found between the number of plastics found in the sediments and the fish collected at the same place.
Chapter 3 assessed the behavioural changes in male triplefins caused by microplastic and DEHP ingestion. As territory establishment and guarding is necessary for triplefins during breeding season, the impact of microplastics on territorial behaviour was analysed after exposures. To accomplish this, wild triplefins were captured once a month during the breeding season (June, August, and September), and either exposed to 500-micron pure polystyrene microplastics in their food or 500-micron polystyrene microplastics laced with 15% DEHP by weight for 21 days. A control group was fed food without any microplastics. As DEHP is known as an endocrine disrupter, as most phthalates, I predicted that polystyrene dosed with DEHP would have a compounded impact in feminizing the behaviour of territorial males. After exposures, individuals underwent 20-minute territory competition trials where they had access to a familiar shelter. Videos were analysed to assess the time it took individuals to emerge from acclimation chambers, how quickly individuals laid claim to the familiar territory, the number of times individuals interacted with the territory, the number of bites initiated, the amount of time spent stationary, and time spent in motion throughout the twenty-minute trial. Analysis of behavioural data showed the most significant changes in fish exposed to pure polystyrene, rather than DEHP-dosed polystyrene, as initially hypothesized.
This thesis demonstrates that microplastic exposure is common in the environment, and that ingestion of polystyrene has an impact on triplefin behaviour during the breeding season. I found that fish exposed to polystyrene were slow to emerge from their acclimation chambers, slow to make claims to a familiar territory, and less motile in a competition setting. Surprisingly, only emergence times were impacted in fish exposed to both polystyrene and DEHP. Ultimately, further study is necessary to understand the full impact of microplastic and phthalate exposure on the breeding activities of fish, however, this thesis provides a basis of knowledge for further investigation. Future studies should question the impact of DEHP in combination with multiple types of plastics on behaviour as well as the physiology of aquatic vertebrates.