how to tell Male from Female Walleye??

Ok, been catchig some eyes lately and thinkin of keepin a few for the table. however I would like to let the Females go.. The fish I am catching are in the 15-20 inch class.. Some are fat, some are average. Any easy way to tell a female so I can let her go?

Females have 2 small holes including the anus and males have 3 small holes including the anus. The other holes are the urogential system.

The holes are very tiny so careful inspection is manditory.