*Excludes Observations that spanned multiple years
| Pre 1970 | 24 |
|---|---|
| 1970 | 1 |
| 1971 | 0 |
| 1972 | 21 |
| 1973 | 12 |
| 1974 | 30 |
| 1975 | 16 |
| 1976 | 23 |
| 1977 | 19 |
| 1978 | 21 |
| 1979 | 28 |
| 1980 | 46 |
| 1981 | 42 |
| 1982 | 73 |
| 1983 | 62 |
| 1984 | 100 |
| 1985 | 116 |
| 1986 | 92 |
| 1987 | 30 |
| 1988 | 34 |
| 1989 | 141 |
| 1990 | 130 |
| 1991 | 114 |
| 1992 | 124 |
| 1993 | 168 |
| 1994 | 123 |
| 1995 | 161 |
| 1996 | 113 |
| 1997 | 79 |
| 1998 | 152 |
| 1999 | 170 |
| 2000 | 142 |
| 2001 | 181 |
| 2002 | 266 |
| 2003 | 207 |
| 2004 | 213 |
| 2005 | 242 |
| 2006 | 273 |
| 2007 | 300 |
| 2008 | 380 |
| 2009 | 375 |
| 2010 | 363 |
| 2011 | 194 |
| 2012 | 306 |
| 2013 | 193 |
| 2014 | 143 |
| 2015 | 300 |
| 2016 | 461 |
| 2017 | 11 |
| 2018 | 1 |
| 2019 | 1 |
| 2020 | 0 |
| 2021 | 0 |
| Total | 6817 |