*Excludes Observations that spanned multiple years
| Pre 1970 | 60 |
|---|---|
| 1970 | 10 |
| 1971 | 4 |
| 1972 | 25 |
| 1973 | 30 |
| 1974 | 276 |
| 1975 | 326 |
| 1976 | 231 |
| 1977 | 92 |
| 1978 | 139 |
| 1979 | 103 |
| 1980 | 169 |
| 1981 | 149 |
| 1982 | 159 |
| 1983 | 276 |
| 1984 | 472 |
| 1985 | 328 |
| 1986 | 216 |
| 1987 | 173 |
| 1988 | 201 |
| 1989 | 346 |
| 1990 | 400 |
| 1991 | 252 |
| 1992 | 375 |
| 1993 | 420 |
| 1994 | 499 |
| 1995 | 488 |
| 1996 | 391 |
| 1997 | 377 |
| 1998 | 419 |
| 1999 | 554 |
| 2000 | 478 |
| 2001 | 785 |
| 2002 | 997 |
| 2003 | 611 |
| 2004 | 824 |
| 2005 | 665 |
| 2006 | 823 |
| 2007 | 838 |
| 2008 | 925 |
| 2009 | 948 |
| 2010 | 814 |
| 2011 | 654 |
| 2012 | 527 |
| 2013 | 372 |
| 2014 | 55 |
| 2015 | 12 |
| 2016 | 29 |
| 2017 | 16 |
| 2018 | 0 |
| Total | 18333 |