*Excludes Observations that spanned multiple years
| Pre 1970 | 4 |
|---|---|
| 1970 | 1 |
| 1971 | 2 |
| 1972 | 0 |
| 1973 | 0 |
| 1974 | 1 |
| 1975 | 0 |
| 1976 | 1 |
| 1977 | 3 |
| 1978 | 1 |
| 1979 | 1 |
| 1980 | 1 |
| 1981 | 2 |
| 1982 | 2 |
| 1983 | 3 |
| 1984 | 0 |
| 1985 | 2 |
| 1986 | 0 |
| 1987 | 1 |
| 1988 | 2 |
| 1989 | 3 |
| 1990 | 7 |
| 1991 | 2 |
| 1992 | 7 |
| 1993 | 9 |
| 1994 | 6 |
| 1995 | 26 |
| 1996 | 31 |
| 1997 | 14 |
| 1998 | 16 |
| 1999 | 16 |
| 2000 | 20 |
| 2001 | 10 |
| 2002 | 10 |
| 2003 | 4 |
| 2004 | 14 |
| 2005 | 11 |
| 2006 | 13 |
| 2007 | 15 |
| 2008 | 20 |
| 2009 | 17 |
| 2010 | 19 |
| 2011 | 22 |
| 2012 | 26 |
| 2013 | 29 |
| 2014 | 26 |
| 2015 | 32 |
| 2016 | 26 |
| 2017 | 19 |
| 2018 | 21 |
| 2019 | 20 |
| 2020 | 14 |
| 2021 | 25 |
| 2022 | 14 |
| 2023 | 0 |
| 2024 | 0 |
| Total | 591 |