*Excludes Observations that spanned multiple years
| Pre 1970 | 127 |
|---|---|
| 1970 | 4 |
| 1971 | 11 |
| 1972 | 3 |
| 1973 | 2 |
| 1974 | 0 |
| 1975 | 3 |
| 1976 | 5 |
| 1977 | 18 |
| 1978 | 6 |
| 1979 | 11 |
| 1980 | 14 |
| 1981 | 8 |
| 1982 | 8 |
| 1983 | 15 |
| 1984 | 9 |
| 1985 | 86 |
| 1986 | 206 |
| 1987 | 324 |
| 1988 | 1134 |
| 1989 | 784 |
| 1990 | 106 |
| 1991 | 560 |
| 1992 | 411 |
| 1993 | 917 |
| 1994 | 2070 |
| 1995 | 1118 |
| 1996 | 960 |
| 1997 | 632 |
| 1998 | 161 |
| 1999 | 137 |
| 2000 | 125 |
| 2001 | 170 |
| 2002 | 139 |
| 2003 | 314 |
| 2004 | 439 |
| 2005 | 430 |
| 2006 | 318 |
| 2007 | 422 |
| 2008 | 464 |
| 2009 | 355 |
| 2010 | 454 |
| 2011 | 361 |
| 2012 | 393 |
| 2013 | 355 |
| 2014 | 350 |
| 2015 | 328 |
| 2016 | 327 |
| 2017 | 307 |
| 2018 | 159 |
| 2019 | 131 |
| 2020 | 67 |
| 2021 | 220 |
| 2022 | 176 |
| 2023 | 108 |
| 2024 | 40 |
| 2025 | 24 |
| 2026 | 6 |
| Total | 16832 |