*Excludes Observations that spanned multiple years
| Pre 1970 | 57 |
|---|---|
| 1970 | 10 |
| 1971 | 0 |
| 1972 | 31 |
| 1973 | 81 |
| 1974 | 248 |
| 1975 | 192 |
| 1976 | 152 |
| 1977 | 74 |
| 1978 | 121 |
| 1979 | 89 |
| 1980 | 144 |
| 1981 | 124 |
| 1982 | 108 |
| 1983 | 184 |
| 1984 | 382 |
| 1985 | 209 |
| 1986 | 173 |
| 1987 | 67 |
| 1988 | 100 |
| 1989 | 167 |
| 1990 | 186 |
| 1991 | 109 |
| 1992 | 179 |
| 1993 | 190 |
| 1994 | 155 |
| 1995 | 202 |
| 1996 | 164 |
| 1997 | 119 |
| 1998 | 192 |
| 1999 | 207 |
| 2000 | 186 |
| 2001 | 210 |
| 2002 | 315 |
| 2003 | 230 |
| 2004 | 240 |
| 2005 | 260 |
| 2006 | 327 |
| 2007 | 301 |
| 2008 | 410 |
| 2009 | 372 |
| 2010 | 406 |
| 2011 | 252 |
| 2012 | 251 |
| 2013 | 213 |
| 2014 | 12 |
| 2015 | 0 |
| 2016 | 1 |
| 2017 | 0 |
| Total | 8402 |