Fish removal and management of fish stocks

The balance of the fish stocks in Lake Tuusula is being restored

As is common in eutrophicated lakes, the fish stock of whitefish in Lake Tuusula was clearly male dominated prior to the restoration. The fish grew slowly and spawning was scarce. The stock was not producing new fish at the rate it should have. Pikeperch stock was large but the population of other predatory fish was little.

The whitefish stock was heavily reduced by fish removal and thus the fish stocks of the lake have clearly become younger and more attractive to recreational fishing.

The perch stock that is favoured by ice fishers has increased and even the pikeperch stock of the lake has been stable. Spawning of pikeperch has been good in recent years and it has grown well. The pikeperch stock has remained large despite the fish removal. There has been some annual variation but this is quite normal in lakes.

The research has shown no evidence that the pikeperch stock would suffer from lack of food. This indicates that as the fish stocks of the lake have grown younger there has been plenty of small fish for pikeperch to prey on.

 

Fish removal a solution to the spread of blue-green algae

Netfishing has shown that the share of predatory fish in the lake has increased from earlier. It is now one fifth of the catch. Ideally it should be one third. The objective of the fish removal is both to make the lake more attractive to recreational fishing and to restrict the growth of blue-green algae.

The fish removal also affects the internal loading of the lake by reducing the whitefish stock that stir up nutrients from the bottom and thus increase the spread of the blue-green algae. Another advantage of the fish removal is that the zooplankton in the water is able to eat the algae more efficiently and the murkiness of the water caused by the algae is reduced.

The effect of the fish removal in Lake Tuusula is monitored by fish research that e.g. The Finnish Game and Fisheries Research Institute and the Helsinki University Department of Limnology have conducted for several years.

The achieved improvements in the fish stocks are maintained e.g. by management of fish stocks, predatory fish stocking, shore seining, trapnet-fishing and autumn seining. Seines are also used in the winter with the Joined Water Associations’ permission. The primary objective is to create a sustainable situation where the predatory fish stock will reproduce and strengthen naturally.

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