*Excludes Observations that spanned multiple years
| Pre 1970 | 106 |
|---|---|
| 1970 | 4 |
| 1971 | 1 |
| 1972 | 3 |
| 1973 | 1 |
| 1974 | 2 |
| 1975 | 3 |
| 1976 | 2 |
| 1977 | 7 |
| 1978 | 3 |
| 1979 | 5 |
| 1980 | 5 |
| 1981 | 3 |
| 1982 | 1 |
| 1983 | 1 |
| 1984 | 4 |
| 1985 | 22 |
| 1986 | 31 |
| 1987 | 81 |
| 1988 | 217 |
| 1989 | 39 |
| 1990 | 45 |
| 1991 | 202 |
| 1992 | 123 |
| 1993 | 186 |
| 1994 | 140 |
| 1995 | 251 |
| 1996 | 64 |
| 1997 | 66 |
| 1998 | 29 |
| 1999 | 28 |
| 2000 | 41 |
| 2001 | 48 |
| 2002 | 52 |
| 2003 | 54 |
| 2004 | 137 |
| 2005 | 62 |
| 2006 | 64 |
| 2007 | 117 |
| 2008 | 116 |
| 2009 | 101 |
| 2010 | 146 |
| 2011 | 114 |
| 2012 | 115 |
| 2013 | 117 |
| 2014 | 100 |
| 2015 | 93 |
| 2016 | 107 |
| 2017 | 103 |
| 2018 | 44 |
| 2019 | 79 |
| 2020 | 52 |
| 2021 | 146 |
| 2022 | 147 |
| 2023 | 146 |
| 2024 | 55 |
| 2025 | 18 |
| Total | 4049 |