*Excludes Observations that spanned multiple years
| Pre 1970 | 212 |
|---|---|
| 1970 | 23 |
| 1971 | 10 |
| 1972 | 14 |
| 1973 | 11 |
| 1974 | 19 |
| 1975 | 13 |
| 1976 | 10 |
| 1977 | 9 |
| 1978 | 28 |
| 1979 | 27 |
| 1980 | 28 |
| 1981 | 28 |
| 1982 | 26 |
| 1983 | 33 |
| 1984 | 42 |
| 1985 | 53 |
| 1986 | 57 |
| 1987 | 69 |
| 1988 | 82 |
| 1989 | 73 |
| 1990 | 71 |
| 1991 | 59 |
| 1992 | 65 |
| 1993 | 64 |
| 1994 | 71 |
| 1995 | 98 |
| 1996 | 109 |
| 1997 | 105 |
| 1998 | 103 |
| 1999 | 118 |
| 2000 | 140 |
| 2001 | 166 |
| 2002 | 123 |
| 2003 | 234 |
| 2004 | 242 |
| 2005 | 281 |
| 2006 | 403 |
| 2007 | 185 |
| 2008 | 114 |
| 2009 | 166 |
| 2010 | 270 |
| 2011 | 268 |
| 2012 | 267 |
| 2013 | 211 |
| 2014 | 195 |
| 2015 | 270 |
| 2016 | 293 |
| 2017 | 182 |
| 2018 | 173 |
| 2019 | 178 |
| 2020 | 232 |
| 2021 | 209 |
| 2022 | 123 |
| 2023 | 2 |
| 2024 | 3 |
| Total | 6660 |