*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 | 167 |
| 1981 | 146 |
| 1982 | 153 |
| 1983 | 267 |
| 1984 | 418 |
| 1985 | 310 |
| 1986 | 193 |
| 1987 | 156 |
| 1988 | 161 |
| 1989 | 300 |
| 1990 | 342 |
| 1991 | 210 |
| 1992 | 314 |
| 1993 | 382 |
| 1994 | 408 |
| 1995 | 440 |
| 1996 | 343 |
| 1997 | 334 |
| 1998 | 372 |
| 1999 | 459 |
| 2000 | 405 |
| 2001 | 686 |
| 2002 | 897 |
| 2003 | 533 |
| 2004 | 741 |
| 2005 | 614 |
| 2006 | 767 |
| 2007 | 775 |
| 2008 | 770 |
| 2009 | 879 |
| 2010 | 757 |
| 2011 | 618 |
| 2012 | 461 |
| 2013 | 334 |
| 2014 | 27 |
| 2015 | 0 |
| Total | 16435 |