*Excludes Observations that spanned multiple years
| Pre 1970 | 60 |
|---|---|
| 1970 | 9 |
| 1971 | 2 |
| 1972 | 5 |
| 1973 | 4 |
| 1974 | 8 |
| 1975 | 6 |
| 1976 | 5 |
| 1977 | 5 |
| 1978 | 7 |
| 1979 | 14 |
| 1980 | 11 |
| 1981 | 11 |
| 1982 | 13 |
| 1983 | 25 |
| 1984 | 40 |
| 1985 | 32 |
| 1986 | 46 |
| 1987 | 67 |
| 1988 | 69 |
| 1989 | 62 |
| 1990 | 71 |
| 1991 | 82 |
| 1992 | 82 |
| 1993 | 73 |
| 1994 | 91 |
| 1995 | 89 |
| 1996 | 97 |
| 1997 | 115 |
| 1998 | 129 |
| 1999 | 137 |
| 2000 | 149 |
| 2001 | 153 |
| 2002 | 152 |
| 2003 | 179 |
| 2004 | 156 |
| 2005 | 164 |
| 2006 | 174 |
| 2007 | 169 |
| 2008 | 134 |
| 2009 | 152 |
| 2010 | 175 |
| 2011 | 141 |
| 2012 | 159 |
| 2013 | 147 |
| 2014 | 147 |
| 2015 | 132 |
| 2016 | 40 |
| 2017 | 0 |
| 2018 | 1 |
| 2019 | 1 |
| 2020 | 1 |
| 2021 | 0 |
| 2022 | 0 |
| Total | 3993 |