*Excludes Observations that spanned multiple years
| Pre 1970 | 152 |
|---|---|
| 1970 | 26 |
| 1971 | 24 |
| 1972 | 37 |
| 1973 | 26 |
| 1974 | 48 |
| 1975 | 16 |
| 1976 | 31 |
| 1977 | 35 |
| 1978 | 29 |
| 1979 | 53 |
| 1980 | 88 |
| 1981 | 110 |
| 1982 | 118 |
| 1983 | 151 |
| 1984 | 153 |
| 1985 | 149 |
| 1986 | 267 |
| 1987 | 229 |
| 1988 | 218 |
| 1989 | 366 |
| 1990 | 405 |
| 1991 | 285 |
| 1992 | 276 |
| 1993 | 234 |
| 1994 | 284 |
| 1995 | 367 |
| 1996 | 415 |
| 1997 | 409 |
| 1998 | 350 |
| 1999 | 394 |
| 2000 | 514 |
| 2001 | 458 |
| 2002 | 628 |
| 2003 | 434 |
| 2004 | 594 |
| 2005 | 569 |
| 2006 | 640 |
| 2007 | 592 |
| 2008 | 766 |
| 2009 | 722 |
| 2010 | 737 |
| 2011 | 549 |
| 2012 | 536 |
| 2013 | 632 |
| 2014 | 152 |
| 2015 | 2 |
| 2016 | 0 |
| 2017 | 0 |
| 2018 | 0 |
| Total | 14270 |