Can watercress farming directly affect fish communities?

Watercress (Nasturtium officinale) has previously been shown to produce chemical outputs thought to be toxic to macroinvertebrate species. Exposure to these toxins can create an imbalance in invertebrate populations. It has been proposed that the reduction in macroinvertebrate communities is having a detrimental effect on fish populations.

One question that still remains largely unanswered is whether it is just the invertebrate declines that are leading to the decline of fish populations. There remains much contradiction in the literature as to whether fish populations remain stable or decline downstream of watercress beds. Early life stages of fish, especially at the fertilisation phase, are known to be highly vulnerable to contaminants in the water even when they occur at low levels.

Outputs from electro-fishing surveys to ascertain the impacts of watercress beds on fish populations have been sporadic and have yielded conflicting results. Much research to date has focused on salmonid species, but little else. This project therefore sets out to address the impacts of watercress beds on a range of species and deign to proffer advice on management strategies to mitigate the impacts, if any, on fish populations.

The aims of the project are to answer the following questions:-

> Does watercress factory wash water impact early phase development in a range of fish species?
> Can phenethyl isothiocyanate and phosphorus impact the development of fish embryos?
> Can watercress discharge water affect fish assemblages?
> Are any effects a direct or indirect result of effluent exposure?
> How should watercress outfalls be managed to mitigate the impacts of watercress discharge on fish species?

This project enabled a better understanding of the impacts, if any, on fish populations and will help to determine whether the output from the farming of N. officinale has any direct detrimental effect on neighbouring fish communities or whether they may be indirectly affected by impacts on river macroinvertebrate communities.

The project represents a significant advance in our understanding of the impacts of watercress farming on aquatic organisms; currently, there is a spread of literature on the impacts on invertebrate and plant species, but very little data on fish, especially the early life stages. The outcomes of this work will result in three scientific publications and the subsequent management plan for watercress farms to aid fish conservation.