Complex sentence: Compared to a 1972 survey, 20 new species were recorded, including two alien species (Mozambique tilapia and largemouth bass); however 34 species recorded in 1972 were absent in 2010, and overall fish density was lower.
Split into simpler sentences: Compared to an earlier survey in 1972,20 new species were recorded (including two alien species, Mozambique tilapia and largemouth bass). However, 34 species recorded in 1972 were not recorded in 2010, and the overall fish density was lower.