*Excludes Observations that spanned multiple years
| Pre 1970 | 56 |
|---|---|
| 1970 | 2 |
| 1971 | 10 |
| 1972 | 3 |
| 1973 | 2 |
| 1974 | 0 |
| 1975 | 2 |
| 1976 | 5 |
| 1977 | 17 |
| 1978 | 6 |
| 1979 | 7 |
| 1980 | 11 |
| 1981 | 6 |
| 1982 | 8 |
| 1983 | 13 |
| 1984 | 9 |
| 1985 | 86 |
| 1986 | 205 |
| 1987 | 322 |
| 1988 | 1134 |
| 1989 | 782 |
| 1990 | 106 |
| 1991 | 559 |
| 1992 | 411 |
| 1993 | 917 |
| 1994 | 2070 |
| 1995 | 1116 |
| 1996 | 960 |
| 1997 | 632 |
| 1998 | 161 |
| 1999 | 137 |
| 2000 | 125 |
| 2001 | 170 |
| 2002 | 138 |
| 2003 | 314 |
| 2004 | 439 |
| 2005 | 429 |
| 2006 | 318 |
| 2007 | 402 |
| 2008 | 444 |
| 2009 | 348 |
| 2010 | 453 |
| 2011 | 361 |
| 2012 | 361 |
| 2013 | 334 |
| 2014 | 350 |
| 2015 | 328 |
| 2016 | 327 |
| 2017 | 307 |
| 2018 | 158 |
| 2019 | 131 |
| 2020 | 67 |
| 2021 | 220 |
| 2022 | 176 |
| 2023 | 108 |
| 2024 | 40 |
| 2025 | 24 |
| 2026 | 6 |
| Total | 16633 |