Fisheries Monitoring

Slapton Ley is the largest freshwater lake in south-west England and is separated from the sea at Start Bay by a shingle beach, known as Slapton Sand.

Our client first engaged us to undertake a basic fish population assessment of the ley using conventional methods (seine netting, fyke netting and electrofishing) to establish fish diversity and abundance. It was found that the ley was dominated by small perch at the expense of the roach and rudd population. This phenomenon has been documented for similar waterbodies elsewhere and can often be attributed to poor water quality, including highly eutrophic conditions and subsequent dips in dissolved oxygen concentration.

The initial results led to repeat surveys over 3 years to gain a better understanding of population dynamics and the impacts of water quality in the ley on the fish population. An analysis of long-term water quality data was also undertaken to identify the cause of any shifts in the fish community.

It was found that the relative abundance of perch increased over successive years (with a reduction in roach and rudd), whereas densities of pike and eel remained relatively constant. Certain water quality parameters had become unfavourable in recent years, including temporal spikes in Biological Oxygen Demand, high levels of nitrates and high pH, benchmarks of a eutrophic waterbody.  The worst events were found to typically occur during the summer and autumn, which likely coincides with algal decay, as is typical in eutrophic systems.

The potential sources of elevated phosphate concentration were identified and included a recently expanded sewage treatment plant, agricultural run-off and other point sources in the wider catchment area. Mitigation methods were suggested that included replanting of trees, macrophyte harvesting and fish re-stocking, along with on-going population and water quality monitoring.