Even as tough fishing restrictions in New England’s Georges Bank fishing ground are beginning to restore populations of cod and other groundfish, a new report by federal and state biologists suggests that still tighter limits should be imposed. Scientists say that fish populations have been so severely depleted that it will take years to restore them to levels that can sustain fishing over the long term. But many cod fishers say they are having their best summer season in a decade and argue that the current restrictions are too tight, and all types of fishers argue that their livelihoods are being destroyed by tight rules. The New England Fisheries Management Council, which governs fishing in the region, has not yet decided on its next move.