*Excludes Observations that spanned multiple years
| Pre 1970 | 9 |
|---|---|
| 1970 | 2 |
| 1971 | 11 |
| 1972 | 1 |
| 1973 | 0 |
| 1974 | 0 |
| 1975 | 1 |
| 1976 | 0 |
| 1977 | 5 |
| 1978 | 6 |
| 1979 | 2 |
| 1980 | 5 |
| 1981 | 0 |
| 1982 | 4 |
| 1983 | 0 |
| 1984 | 3 |
| 1985 | 90 |
| 1986 | 120 |
| 1987 | 223 |
| 1988 | 720 |
| 1989 | 106 |
| 1990 | 58 |
| 1991 | 327 |
| 1992 | 224 |
| 1993 | 663 |
| 1994 | 340 |
| 1995 | 482 |
| 1996 | 336 |
| 1997 | 244 |
| 1998 | 53 |
| 1999 | 139 |
| 2000 | 142 |
| 2001 | 137 |
| 2002 | 105 |
| 2003 | 58 |
| 2004 | 105 |
| 2005 | 65 |
| 2006 | 38 |
| 2007 | 48 |
| 2008 | 82 |
| 2009 | 78 |
| 2010 | 81 |
| 2011 | 96 |
| 2012 | 94 |
| 2013 | 108 |
| 2014 | 106 |
| 2015 | 93 |
| 2016 | 88 |
| 2017 | 101 |
| 2018 | 59 |
| 2019 | 39 |
| 2020 | 15 |
| 2021 | 70 |
| 2022 | 76 |
| 2023 | 8 |
| Total | 6066 |