COMPARISON OF SAMPLING METHODS FOR SEMIPELAGIC ANIMALS IN 2 DEEP BASINS OF LAKE-SAIMAA

The efficiency of different methods for sampling semipelagic animals w as compared in the deep basins of Lake Paasivesi and Lake Puruvesi of the complex Lake Saimaa, Finland, during 1990-92. Certain semipelagic animals, such as Mysis relicta, amphipods and larvae of Chaoborus flav icans, are important bioindicators of water and benthic quality. Howev er, owing to their high mobility and vertical migrations they are diff icult to sample quantitatively with traditional benthic methods. The e quipment used for sampling was comprised of: 1. Vertical nets (0.56 an d 0.13 m(2)) placed on the bottom for 5 minutes or 12 hours and hauled up vertically. 2. Plexiglass traps placed on the bottom for 12-14 hou rs. 3. Mysis-trawl hauled horizontally c. 250-300 metres on the bottom . 4. Ekman-grab. In 1992, parallel samples were taken with vertically hauled nets (12 h and plexiglass traps baited with fish or liver or pr ovided with a chemical light. Large vertically hauled nets were the mo st efficient equipment for catching Gammaracanthus lacustris, M. relic ta and larvae of Ch. flavicans whilst baited or lighted nets were less effective. Lighted plexiglass traps were most effective in catching P allasea quadrispinosa and water mites, but baited and control traps sa mpled inefficiently. The trawl sampled M. relicta and P. quadrispinosa relatively well, but was less efficient in sampling other groups. The Ekman-grab was effective only for sampling Monoporeia affinis. The se mipelagic community differed between the studied basins. The dominant semipelagic animals in the humic, deep L. Paasivesi were M. relicta, a mphipods and larvae of Ch. flavicans whilst the number of water mites was low in the catches. In the shallower, clear-water L. Puruvesi the densities of crustaceans and Chaoborus-larvae were low whilst some wat er mites, such as Piona pusilla, occurred in high numbers. Nocturnal s ampling was more effective, especially for M. relicta.