Pesticide Residue from Farms and Towns End Up in Fresh Oysters, Study Reveals

Oysters have been hailed as one of the most sustainable and healthy seafood options. These bivalve mollusks also do a lot for our environment by acting like vacuum cleaners of the sea. However, this shellfish is only as healthy as the environment it is grown in.

Looming Environmental Threat

In a new research conducted by experts, it has been revealed that oysters are at risk of soaking up pesticide residue. In the study "Pesticide occurrence in a subtropical estuary, Australia: Complementary sampling methods", scientists investigated the occurrence of pesticides in the Richmond River estuary in Australia and their effects on fresh oysters.

Led by Endang Jamal, the research team collected water samples from six sites during two sampling periods. From January to March 2020, they used composite water samples and oyster collections, while water sample collection and Chemcatcher deployment were done from February to March 2021.

A total of 21 different pesticides were detected across all sites using all methods. In the 2020 sampling, the number of pesticides detected was higher in oyster samples than in water samples. The same result was also observed by the Chemcatcher samplers in 2021.

On average, detectable amounts of nine different pesticides were found in each oyster. While the full health risks of eating oysters from Richmond River are not yet clear, experts are aware that five of these pesticides are potentially dangerous. In fact, they are not even allowed to be present in meat.

The researchers noted that the risk is higher for oysters taken from the wild. Commercially farmed oysters could be safer, since they are regulated by shellfish quality assurance programs and can only be harvested when the quality of water is good.

Most of the chemicals found in this study are routinely used by farmers, land managers, and council workers. However, the team also discovered the fungicide benomyl which has been illegal in Australia since 2006 due to the high risk in the environment and to human health. Detecting this substance means that it is being used illegally.

How Do Pesticides Get Into Oysters?

As filter-feeding shellfish, oysters eat by straining particles from water. These bivalve mollusks pump water through their bodies and eat the plankton, bacteria, and other particles they filter out. An individual oyster can filter up to five liters of water an hour and over 250,000 liters in their lifetime.

Before the period of colonization, oyster reefs were abundant in Australia. Most of these reefs were pulled out for the shell lime and meat. In the Richmond River, poor water quality and a disease killed off most of these shellfish until a new, disease-resistant strain emerged.

Filter-feeding works well if the oysters are just filtering out what is found naturally. If the water they inhabit is contaminated, oysters can end up storing pathogens and pollutants in their bodies. Oysters prefer to live in brackish water which contains a mixture of fresh and saltwater. This is the reason why they are extensively farmed in many estuaries. However, many of the coastal catchments in Australia currently contain farms, cities, or towns, so the used pesticides and other chemicals get washed into rivers after rain.