A team from the School of Freshwater Sciences, University of Wisconsin, Milwaukee, in the US, ran a nine-week experiment to see if they could find some answers. The research, which was published in Animal Nutrition, was led by Professor Dong-Fang Deng.
The team wanted to determine whether after consuming microplastics, do the particles accumulate inside the bodies of the fish.
As far as the researchers can determine, a real assessment of the risks associated with microplastics in farmed fish production, the potential impacts of consumption of such particles on nutritional quality and nutrient metabolism is still lacking.
“The findings of the current study will help to address these gaps and provide baseline information to direct research that investigates how microplastics (MP) impact nutrient utilization in aquaculture fish.”
MP are defined as plastic particles with sizes smaller than 5 mm in 1 dimension and these plastics are increasingly seen as a global environmental problem, noted the research team.
Based on polymer structure, MP are mainly classified as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyamides (PA) (Wagner et al., 2014).
It is estimated that around 8 million tons of plastics enter the oceans every year (Gallo et al., 2018; Jambeck et al., 2015).
Exposure to HDPE
High-density polyethylene (HDPE), a common plastic, is one of the major MP detected in fishmeal, claimed the team, with the researchers citing work by Thiele et al., 2021, in that respect.
So they thus decided to try and identify potential impacts of HDPE exposure on yellow perch.
“We chose yellow perch because they are a favorite fish of people living in the Great Lakes region, and the reduced supply of wild perch has driven commercial aquaculture,” said Deng.
The authors said they wanted to assess the dose–response of yellow perch exposed to HDPE based on growth performance, nutritional quality of the whole fish, liver histopathology and metabolites, and intestinal microbial community analysis.
Juvenile yellow perch were fed four meals daily with feed contaminated with medium and high concentrations of microparticles of HDPE. A control group ate feed that was plastic-free.
At the end of the exposure period, they saw that none of the fish that had consumed the contaminated feed had died, the microplastics had passed out of their bodies and showed up in their feces.
However, the team saw that, after the microplastics were excreted, exposed fish contained lower amounts of protein and minerals than fish fed the diet with no HDPE added.
This could be evidence that the fish’s bodies were not able to break down the microplastics and needed more nutrients than normal to expel them, or the microplastics might have reduced the absorption of these nutrients from their diets, commented the team.
The authors also found evidence of altered liver function in exposed fish and a change in their gut microorganism community. Like it does in humans, the fish gut microbial community performs essential tasks like food digestion and immune system protection, they said.
“So, even though they didn’t get ‘stuck’ in the body,” Deng said, “that doesn’t mean the exposure to microplastics wasn’t harmful.”
Impact on gut microbiota
Ryan Newton, assistant professor of freshwater sciences and a co-author on the paper, investigated the impact of microplastics on the gut microbiota of the perch and said the study raised more questions than it answered.
What the fish eat can change the microbiota found in their digestive systems. But it was difficult to determine, in the study’s time frame, if the changes were linked to the microplastics.
“Our results indicate that it is likely a long period of exposure to the microplastics that may lead to impaired nutrient utilization and fish health. This hypothesis requires further investigation.”
Furthermore, how microplastics interfere with feed intake and its transition time through the GIT, the impact on nutrient digestion and absorption “remains to be elucidated in this species and warrants future study.”