Just remove/delete the 'color' aesthetic
this will be as below;
ggplot(data = x) +
geom_bar(
mapping = aes(x = month, y = numb_fish, fill = LifeStage), stat = "identity") +
labs(title = "Monthly Data",y = "Total") +
scale_x_date(date_breaks = "1 month", date_labels = "%b -%y")