*Excludes Observations that spanned multiple years
| Pre 1970 | 121 |
|---|---|
| 1970 | 10 |
| 1971 | 4 |
| 1972 | 9 |
| 1973 | 13 |
| 1974 | 47 |
| 1975 | 49 |
| 1976 | 29 |
| 1977 | 12 |
| 1978 | 21 |
| 1979 | 34 |
| 1980 | 15 |
| 1981 | 22 |
| 1982 | 18 |
| 1983 | 30 |
| 1984 | 114 |
| 1985 | 40 |
| 1986 | 62 |
| 1987 | 44 |
| 1988 | 57 |
| 1989 | 57 |
| 1990 | 55 |
| 1991 | 67 |
| 1992 | 63 |
| 1993 | 74 |
| 1994 | 75 |
| 1995 | 87 |
| 1996 | 66 |
| 1997 | 116 |
| 1998 | 88 |
| 1999 | 117 |
| 2000 | 122 |
| 2001 | 127 |
| 2002 | 150 |
| 2003 | 154 |
| 2004 | 157 |
| 2005 | 157 |
| 2006 | 139 |
| 2007 | 118 |
| 2008 | 171 |
| 2009 | 160 |
| 2010 | 193 |
| 2011 | 198 |
| 2012 | 175 |
| 2013 | 76 |
| 2014 | 115 |
| 2015 | 102 |
| 2016 | 20 |
| 2017 | 1 |
| 2018 | 0 |
| 2019 | 6 |
| 2020 | 0 |
| 2021 | 2 |
| 2022 | 9 |
| 2023 | 0 |
| Total | 3968 |