From free-floating to stomach bloating : a study on plastic in (fish in) the river Waal

In this study, trawl nets were used to sample plastic at different positions in the river Waal. The results show that the amount of plastic found was higher at the bottom on the left side of the river. This is in line with previous findings and with expectations if plastic distribution is mostly affected by helical flow and if most of the plastic is higher density than water. Proving or disproving whether these differences in distribution are indeed caused by helical flow will require further studies comparing between the left and right side of the river Waal under circumstances where helical flow would not cause a higher amount of plastic at the bottom on the left side of the river.

The results show differences in plastic distribution between different plastic categories. This shows the importance of learning more about the effect of sampling location on the proportions of plastic found for the different categories. When designing a monitoring strategy, this effect should be considered and corrected for. This could be done either through sampling at varied positions in the river, or through taking these effects into account during interpretation of the results.

Getting a detailed insight into the distribution of plastic in the river requires more information about the plastic found that is currently not gathered with the standard protocols used. Information about the shape and size should be gathered, and methods should be employed to at least estimate the density of the plastic pieces.