*Excludes Observations that spanned multiple years
| Pre 1970 | 12 |
|---|---|
| 1970 | 1 |
| 1971 | 8 |
| 1972 | 0 |
| 1973 | 1 |
| 1974 | 0 |
| 1975 | 4 |
| 1976 | 4 |
| 1977 | 16 |
| 1978 | 9 |
| 1979 | 11 |
| 1980 | 6 |
| 1981 | 13 |
| 1982 | 1 |
| 1983 | 16 |
| 1984 | 7 |
| 1985 | 30 |
| 1986 | 93 |
| 1987 | 132 |
| 1988 | 526 |
| 1989 | 83 |
| 1990 | 81 |
| 1991 | 445 |
| 1992 | 476 |
| 1993 | 530 |
| 1994 | 130 |
| 1995 | 548 |
| 1996 | 240 |
| 1997 | 275 |
| 1998 | 112 |
| 1999 | 175 |
| 2000 | 172 |
| 2001 | 288 |
| 2002 | 100 |
| 2003 | 129 |
| 2004 | 199 |
| 2005 | 114 |
| 2006 | 117 |
| 2007 | 145 |
| 2008 | 135 |
| 2009 | 155 |
| 2010 | 399 |
| 2011 | 239 |
| 2012 | 107 |
| 2013 | 179 |
| 2014 | 172 |
| 2015 | 133 |
| 2016 | 272 |
| 2017 | 329 |
| 2018 | 293 |
| 2019 | 350 |
| 2020 | 283 |
| 2021 | 356 |
| 2022 | 394 |
| 2023 | 51 |
| 2024 | 0 |
| Total | 9096 |