Ranked by ERA+, a 100-based statistic that adjusts a pitcher's ERA for league and ballpark and compares it to the average, where <100 is bad and >100 is good. Minimum: 20 innings.
Jeff Weaver, 29 (2007)
Paul Abbott, 35 (2002)
Travis Blackley, 43 (2004)
Bob Galasso, 46 (1977)
Edwin Nunez, 52 (1988)
Clay Parker, 53 (1992)
Steve Trout, 53 (1988)
Mac Suzuki, 53 (1999)
Gene Nelson, 54 (1983)
Mike Campbell, 56 (1989)
Jim Converse, 56 (1994)
Mike Parrott, 57 (1980)
Brett Hinchliffe, 57 (1999)
Jim Beattie, 58 (1985)
Lee Guetterman, 58 (1986)
Dick Pole, 59 (1978)
Clint Nageotte, 59 (2004)
Dennis Martinez, 59 (1997)
Honorable Mention: Mike Hampton, 46 (1993, 17 IP)
Before 2007, the worst single-season ERA+ for any Mariner who started at least six games was Blackley's 43. The worst for a guy who lasted most or all of the year in the rotation goes to Jeff Fassero's 1999, at 68. Special mention of Dick Pole's 1978, where he put up his 59 ERA+ in 18 starts and three relief appearances.