You have to sell it first in order for the loss (or gain) to be recognized and, as such, a taxable event.
You have to sell it first. You don't have a "realized" gain or loss on a stock until you sell it.
and if you sell at a loss and repurchase like kind stock 30 days before or after the sale the wash sale rules come into play disallowing some or all of the loss. generally the brokers will do this automatically.
