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Research to understand the effects of glyphosate on trout

Based on experiments conducted with several generations of fish, an ANSES team explored the effects of glyphosate and glyphosate-based herbicides on the health of rainbow trout. This research showed that whether used pure or in herbicides, glyphosate could have effects on multiple generations of this species.

The Fish Virology, Immunology and Ecotoxicology (VIMEP) Unit of ANSES's Ploufragan-Plouzané-Niort Laboratory carried out several experiments with rainbow trout to assess the health effects on this species of pure glyphosate and two commercial glyphosate-based herbicides.

The study was undertaken as part of the GlyphoTAC project, which was the subject of a university thesis defended by Jessy Le Du-Carré. This work was financed by the Brittany Region, the Côtes d’Armor Departmental Council, and the Saint-Brieuc Urban Community. Few studies of this type, focusing on multiple generations of aquatic animals, have been conducted to date due to the complexity of implementing them.

“We studied the exposure of several generations of rainbow trout. We considered the effects on directly exposed fish and on the next two generations, whether or not they themselves had been exposed” explains Morgane Danion, a scientist from the unit who jointly supervised the thesis. “Trout are a good model for ecotoxicological studies, because their biological functioning is well known and they are sensitive to the quality of their environment”.

A wide range of effects on the health and survival of fish

The fish were exposed over a long period, to concentrations comparable to those measured in natural environments. The researchers did not observe any increase in mortality directly due to exposure to the active substance (glyphosate) or to the two tested products containing it; they also did not note any negative effects on reproduction. However, the behaviour of the exposed fish was modified, as were several of their biological parameters.

In particular, a decrease in the ability to respond to changes in light levels was observed in larvae from F1 and/or F0 generations that had been exposed to pure glyphosate or the herbicides. In nature, this could result in a reduced escape response to predators.

Another experiment showed a reduction in the diversity of the micro-organisms present on the gills of fish that had been exposed to pure glyphosate or the studied herbicides. This was significant considering that gills are essential organs responsible for exchanges with the external environment; they also play a role in keeping fish in good health.

Consequences for the descendants of exposed fish and different effects observed for pure glyphosate and for herbicides

This research also showed that the most severe effects did not occur in the first generation (F0) of exposed fish but for the most part in the F1 and/or F2 generation. Moreover, the active substance alone and the commercial products, which were more complex, were shown to have different effects. Depending on the experimental conditions, the product’s formula increased or reduced the effects of glyphosate and even caused no effects to be observed in trout exposed to the pure active substance.

For example, while no changes in survival were observed in the first generation of fish following experimental infection with a virus, mortality due to viral infection in trout whose parents had been exposed to the herbicides was either higher or lower than that observed in the control group, depending on the tested product. However, this mortality was not affected in fish exposed to pure glyphosate or their descendants.

Exposure also had an effect on the expression of enzymes involved in energy metabolism and on the quantity and/or efficiency of certain immune cells; developmental disruptions (malformations) were also observed, especially in larval stages.

Questions concerning the mechanisms responsible for these effects

Further studies will need to be conducted to try to understand the mechanisms behind these findings, in particular how toxic effects are passed down from generation to generation. “The two commercial products tested were withdrawn from the market in 2018 and 2019. This decision was independent of our study results. However, we can imagine that other products still on the market that contain glyphosate or other active substances could have comparable effects. It seems particularly appropriate to continue working on long periods of exposure to environmental concentrations by undertaking multi-generational studies” concludes Thierry Morin, Head of the VIMEP Unit.

Better knowledge of the toxicological mechanisms responsible for these findings would enable these effects to be taken into account in the assessment of active substances and products prior to their authorisation.