Skate are members of the ray, and hence the shark, family. The sharks or elasmobrachs (...!...) use urea in the body to regulate the intake of water through the skin / mouth / gills so they don't get saturated.
As there is a lot of urea therefore naturally in the flesh it needs to be purged - use a weak bicarb of soda if there is a smell you want to remove for fresh skate ( or shark meat).
But if the flesh is suspect and you aren't sure of the age or how fresh it is - chuck it as ammonia smell is also a sign of decomposition.
http://www.elasmo-research.org/education/topic s/p_sharks_in_fresh.htm if you get bored one night and want to bone up on sharks...