That is not true, near the end of the slave trade, the British outlawed slavery (making it smuggling), also slavers did just go ashore and kidnap African natives once the demand for slaves outpaced supply.
Do you not remember the stories about having all the slaves on a slave ship attached, by the neck, to the same chain so as to more easily throw them all overboard in case of the British Navy? The British said they could smell slave ships from a mile away, so wretched and inhumane was the treatment of the cargo (humans).
Wow. Horrible. We know people are capable of anything - they would casually throw a ship full of captives overboard and make sure they all sank. Very depraved. God is going to be very busy on Judgement Day.