Mowing of watercourses (ditches and canals).
Why is mowing done?
To avoid flooding and water shortage (during droughts), we mow water plants and reeds from our 6,000 kilometers of ditches. The water then flows better and eventually ends up in the sea via pumping stations. This is how we keep North Holland water-safe.
How is mowing done?
We consider ecology when mowing. Aquatic plants are important for water quality. They retain soil and suspended particles, they absorb light and nutrients and thus fight (blue) algae. They also have a positive effect on biodiversity. In general, the more species, the better the water quality.
But aquatic plants that take root in the soil and grow to the surface of the water can also cause a nuisance. When it rains a lot, they hold the water, preventing it from reaching the pumping stations. Water recreationists suffer because boating and swimming become more difficult. We are not only responsible for water quantity, but also for water quality. That is why we mainly mow the deeper middle part of the ditch; we leave the vegetation on the sides of the ditch and in the verge. Here animals find a shelter and because the sides are shallower, water plants and reeds have a chance to grow. By giving nature more space on the banks, the ecology and thus the quality of the surface water improves.
When will mowing take place?
The mowing boat comes by, where necessary, twice a year. To take into account the bird nesting season, we start mowing as late as possible and adhere to ecological work protocols. From mid-June to August 1, we mow only where flow is compromised. This round of mowing focuses on keeping the middle part of the ditch free of vegetation. On the sides we leave water plants as much as possible. These are the places where small fish and beetles hide, and we want to disturb them as little as possible.
In September/October is the second round of mowing. At this time the ecology is less sensitive to disturbance and we mow more intensively. We then prepare the water for winter, it rains more then, so we need to be able to drain extra much water.
Fish
Maintenance activities, mostly with mowing boats, cause the bottom to be churned up and bottom material begins to float. The water is then brown/gray in color. Our fish stock management and fish migration advisor Rik Beentjes explains: "The fluffing causes the bacteria to massively absorb the oxygen contained in the floating bottom material. This causes a temporary lack of oxygen in the ditch. Fish in particular sometimes visibly suffer from this. They come to the surface and gasp for oxygen. Sometimes, unfortunately, this means that animals become temporarily paralyzed due to lack of oxygen or die when hit by the mowing bar. Unfortunately, stirring the bottom during mowing operations cannot always be prevented and we try to keep this to a minimum." Fish deaths, of course, we try to prevent.
Upon completion, the suspended solids settle again, the activity of the bacteria decreases and more oxygen becomes available to the fish again. Good news. Rik Beentjes: "Over time, we see that many fish then recover and can swim away. It is of course an unpleasant sight, those floating fish, but a healthy water system usually recovers quickly from mowing. Extraordinarily, a turned over bottom often actually attracts fish again, and in most cases fish are observed again soon after the intervention."
Report
If you see dead fish after mowing, or think the fish are dead, leave them and call us on 072 582 8282. We will go to the ditch to examine the fish situation and remove any dead fish.
We explain how to responsibly mow aquatic plants in this video about the fall survey: