Christopher Costello, John M. Drake, and David M. Lodge. 2007. Evaluating an invasive species policy: ballast water exchange in the Great Lakes. Ecological Applications 17:655–662.

Appendix A. Shipping traffic (number of vessels) in the Great Lakes from 1959 to 2000.

Year	No. vessels
1959	1061
1960	1061
1961	987
1962	1104
1963	1019
1964	1236
1965	1410
1966	1426
1967	1436
1968	1357
1969	1325
1970	1125
1971	1362
1972	1341
1973	1162
1974	713
1975	829
1976	924
1977	1009
1978	1098
1979	961
1980	887
1981	718
1982	797
1983	671
1984	718
1985	576
1986	634
1987	565
1988	570
1989	523
1990	417
1991	364
1992	380
1993	417
1994	576
1995	396
1996	486
1997	470
1998	591
1999	644
2000	676