Photo of man fishing

Stratification and the importance of real-time water quality monitoring in lake fisheries management

The ever increasing risk of stratification makes it more difficult for fisheries, lake and pond owners to manage their fish stocks sustainably. Maintaining healthy lake ecosystems is vital both for fisheries managers who risk losing hundreds of thousands of pond stock, and to the over three million people who take up fishing as a recreational activity in the UK alone. Fish die-offs due to stratification – the layering of water with depth – is becoming an increasing issue with longer warm summer periods due to climate change. A recently published study in Nature Communications predicts that the period of yearly stratification will be lengthened up to two months by the end of this century, increasing pressures on fish.

Fish survival in lakes is very dependent on stable dissolved oxygen (DO) concentrations. In spring, when ambient temperatures become warmer, lake surface waters start to warm up, decreasing oxygen concentrations at the top. As a result, fish move to deeper water layers, where oxygen is more soluble and thus more available than in warmer top waters. Over time, the layering of water with warmer waters at the top and cooler waters at the bottom intensifies, Figure 1. In autumn storms break up the layers, mixing and oxygenating the lake water again.

The risk of intensification and uptake of DO in deeper waters during summer are fish die-offs . Fish overstocking in commercialized ponds can also lead to an increased uptake of DO at depth and increased fecal pollution enhancing the threat to experience fish die-offs. It has also been observed that a longer warm period changes the community structure of phytoplankton. In waters with high nitrate and phosphate concentrations, algae blooms can lead to unforeseen rapid uptakes of DO and can be toxic to fish. Stressed fish experiencing low water DO concentrations are also more prone for disease development.

Graphic showing summer lake stratification
Figure 1. Summer lake stratification (RMB, 2021. Lakes Monitoring Program – Stratification and Mixing. [online] Available at: ).

Monitoring cost effective in real time

One of the most common methods to keep DO concentrations high enough for fish to survive is the usage of aeration equipment: bubble diffusers, turbines and paddles. Aerating the water breaks the ‘physical barrier’ that is created during stratification and helps to transfer oxygen to deeper layers in the water column. With water and air temperatures changing due to climate change and the resulting still unknown DO variability in each water body, weekly lab tests or daily handheld-probe measurements might not be sufficient anymore to secure fish survival. A continuous monitoring of only oxygen and temperature would already give insight of when to aerate the water in time and when to decrease fish stockings to reduce the stress on fish. Linking the output of the monitors to the aeration control system will ensure that DO levels are maintained at an optimal whilst also reducing the economic costs of continuous aeration or fish loss.

The new generation of lower cost monitoring systems allow fisheries managers to obtain the vital data that they need in real-time. WATR is a comparably low-cost modular water quality technology. The monitor is floating in the water taking live data and sending it to an app and dashboard. Next to Temperature, pH and DO, Ammonia, Conductivity and Turbidity can also be measured, giving deeper insight into fisheries water management.

Hilke Schweer
Environmental Scientist - WATR