Table 1 Monthly mean abundance of Penilia avirostris at Helgoland Roads (54°11′18′′N 7°54 E)
From: Increasing prevalence of the marine cladoceran Penilia avirostris (Dana, 1852) in the North Sea
 | 1990 | 1993 | 1994 | 1998 | 1999 | 2001 | 2002 | 2003 | 2004 |
|---|---|---|---|---|---|---|---|---|---|
January | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
February | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
March | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
April | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
May | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
June | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
July | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1.15 |
August | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0.33 | 3176.4 |
September | 0 | 0 | 0 | 0 | 9.15 | 1.25 | 0.36 | 1.00 | 83.67 |
October | 0.60 | 0.31 | 2.55 | 0.80 | 272.00 | 55.60 | 6.50 | 22.75 | 16.17 |
November | 0 | 0 | 0.26 | 0 | 20.58 | 9.60 | 0.75 | 0.40 | 2.76 |
December | 0 | 0 | 0 | 0 | 0 | 2.67 | 0.18 | 0 | 0 |