I suppose that the treatment depends upon the severity of the problem. I've seen shakes where the only solution was to glue splines into them, but yours might be smaller than that. I hope that they are. A shake won't threaten the soundness of the mast, so a fine one can be treated with something spreadable worked into it. There are fillers in various shades to match timbers available from paint shops. Very fine wood dust blended into epoxy would be suitable for a wider one. I'm sure that others will have their favourite fillers, too.
Peter.