*Excludes Observations that spanned multiple years
| Pre 1970 | 36 |
|---|---|
| 1970 | 1 |
| 1971 | 4 |
| 1972 | 14 |
| 1973 | 23 |
| 1974 | 26 |
| 1975 | 29 |
| 1976 | 24 |
| 1977 | 27 |
| 1978 | 18 |
| 1979 | 20 |
| 1980 | 56 |
| 1981 | 47 |
| 1982 | 91 |
| 1983 | 101 |
| 1984 | 197 |
| 1985 | 187 |
| 1986 | 141 |
| 1987 | 78 |
| 1988 | 118 |
| 1989 | 286 |
| 1990 | 334 |
| 1991 | 256 |
| 1992 | 350 |
| 1993 | 643 |
| 1994 | 694 |
| 1995 | 586 |
| 1996 | 359 |
| 1997 | 385 |
| 1998 | 458 |
| 1999 | 672 |
| 2000 | 604 |
| 2001 | 969 |
| 2002 | 1164 |
| 2003 | 702 |
| 2004 | 983 |
| 2005 | 716 |
| 2006 | 876 |
| 2007 | 994 |
| 2008 | 1059 |
| 2009 | 1147 |
| 2010 | 1060 |
| 2011 | 717 |
| 2012 | 865 |
| 2013 | 455 |
| 2014 | 415 |
| 2015 | 516 |
| 2016 | 108 |
| 2017 | 16 |
| 2018 | 0 |
| Total | 19627 |