Practicalities

In practice, biomanipulation can be performed either as an addition of piscivorous fish or as a removal of plank-tivorous fish, or by applying both measures simultaneously. Adding piscivores will, if successful, increase the predation pressure on planktivores, thereby allowing zooplankton to escape predation from planktivores and thus flourish and feed on phytoplankton, thereby increasing the clarity of the water. This method alone has, however, not proved to be very successful. The more common way of performing a biomanipulation is, instead, to remove planktivorous fish. Although labor-intensive, this is a simple method, most efficiently performed by using two boats with a trawl in between (Figure 4). Trawling may result in catches of several tons of fish per day but one problem is that the catch has to be sorted in order to return any piscivorous fish caught. Moreover, the disposal of the fish removed is generally a major problem.

Figure 4 Biomanipulation in practice showing the trawl catch and the table for separating planktivorous fish from piscivorous fish, which are thrown back to the lake. When the catch is sorted, the sorting table is opened and the planktivorous fish put in a container under the table and later frozen (Photo: Richard Nilsson; www.richardnilsson.com).

Figure 4 Biomanipulation in practice showing the trawl catch and the table for separating planktivorous fish from piscivorous fish, which are thrown back to the lake. When the catch is sorted, the sorting table is opened and the planktivorous fish put in a container under the table and later frozen (Photo: Richard Nilsson; www.richardnilsson.com).

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