*Excludes Observations that spanned multiple years
| Pre 1970 | 69 |
|---|---|
| 1970 | 38 |
| 1971 | 26 |
| 1972 | 46 |
| 1973 | 25 |
| 1974 | 44 |
| 1975 | 27 |
| 1976 | 39 |
| 1977 | 46 |
| 1978 | 41 |
| 1979 | 71 |
| 1980 | 87 |
| 1981 | 122 |
| 1982 | 129 |
| 1983 | 156 |
| 1984 | 180 |
| 1985 | 192 |
| 1986 | 215 |
| 1987 | 134 |
| 1988 | 136 |
| 1989 | 302 |
| 1990 | 314 |
| 1991 | 271 |
| 1992 | 308 |
| 1993 | 249 |
| 1994 | 240 |
| 1995 | 247 |
| 1996 | 285 |
| 1997 | 298 |
| 1998 | 282 |
| 1999 | 336 |
| 2000 | 298 |
| 2001 | 349 |
| 2002 | 377 |
| 2003 | 339 |
| 2004 | 404 |
| 2005 | 419 |
| 2006 | 432 |
| 2007 | 475 |
| 2008 | 524 |
| 2009 | 547 |
| 2010 | 568 |
| 2011 | 423 |
| 2012 | 472 |
| 2013 | 342 |
| 2014 | 357 |
| 2015 | 388 |
| 2016 | 123 |
| 2017 | 0 |
| 2018 | 0 |
| Total | 11792 |