Depleted by injury and suspension, they were pulled apart by a lively Rochdale side who won their second successive game by a 4-0 scoreline.
And the defeat could have been much worse but for some careless finishing from the visitors.
Remarkably, Grimsby had three good chances in the opening eight minutes with Nick Heggarty, Martin Paterson and Gary Harkins all going close.
But it all went wrong for Grimsby from the 13th minute when slack marking from a free-kick enabled former Mariner Simon Ramsden to hook the ball into the top corner.
Grimsby were hoping to reach half-time only a goal behind but a free header from a short corner by Glenn Murray enabled Dale to go in two goals to the good.
Grimsby made two substitutions for the second half by bringing on Danny North and Paul Bolland but it made little difference to the match.
Paterson was unlucky to be denied by keeper Matthew Gilks on 52 minutes but a minute later Chris Dagnall headed wide of an open goal.
But Rochdale made absolutely certain of the points when more atrocious defending gifted them further goals on 66 and 72 minutes.
Adam Rundle made it 3-0 when a cross was knocked down for him to turn in and then even worse happened when the Grimsby defence left a ball from the corner believing it was going behind for a goal-kick and Ramsden tapped in his second of the game from close range.
Remarkably Ramsden didn't score a goal in his spell at Grimsby but has now scored three times against his former side in the last three games.
After that Rochdale attacked at will but Grimsby somehow kept the score down to four and the match showed new boss Alan Buckley just what a task he has on his hands if he is to steady the ship.