*Excludes Observations that spanned multiple years
| Pre 1970 | 36 |
|---|---|
| 1970 | 1 |
| 1971 | 4 |
| 1972 | 14 |
| 1973 | 23 |
| 1974 | 26 |
| 1975 | 29 |
| 1976 | 24 |
| 1977 | 27 |
| 1978 | 18 |
| 1979 | 20 |
| 1980 | 56 |
| 1981 | 49 |
| 1982 | 95 |
| 1983 | 101 |
| 1984 | 197 |
| 1985 | 187 |
| 1986 | 141 |
| 1987 | 78 |
| 1988 | 118 |
| 1989 | 286 |
| 1990 | 334 |
| 1991 | 256 |
| 1992 | 350 |
| 1993 | 643 |
| 1994 | 695 |
| 1995 | 587 |
| 1996 | 359 |
| 1997 | 385 |
| 1998 | 458 |
| 1999 | 672 |
| 2000 | 604 |
| 2001 | 969 |
| 2002 | 1164 |
| 2003 | 702 |
| 2004 | 983 |
| 2005 | 716 |
| 2006 | 876 |
| 2007 | 995 |
| 2008 | 1062 |
| 2009 | 1153 |
| 2010 | 1062 |
| 2011 | 718 |
| 2012 | 865 |
| 2013 | 463 |
| 2014 | 430 |
| 2015 | 520 |
| 2016 | 186 |
| 2017 | 18 |
| 2018 | 1 |
| 2019 | 2 |
| 2020 | 9 |
| 2021 | 0 |
| Total | 19767 |