On 13 July 1823, a young Royal Navy officer called Cheesman Binstead noticed a large number of sharks in the water as his ship patrolled in the seas off west Africa. His superiors left him in no doubt about the cause. To avoid a fine, an intercepted slave ship had thrown its human cargo into the waves and the jaws of the predators.
Amid the barbarity of a trade that brought 11 million Africans to the New World in chains, what Midshipman Binstead witnessed was not rare. But what was unusual was that he wrote it down as part of an account of the reality of transatlantic slavery and attempts to bring it to a halt.
This week, the diary kept by Binstead for two years while serving on the Royal Navy's West Africa Squadron, charged with intercepting slave ships, goes on display for the first time since it was written. It forms part of a new exhibition at the Royal Naval Museum in Portsmouth to mark the abolition of the slave trade by Britain in 1807.
Binstead was serving on the HMS Owen Glendower between 1823 and 1824. The vessels in the squadron were empowered to impose a fine of £100 for every slave found on a British ship.
Describing the aftermath of one such encounter, Binstead wrote: "Many large whales and sharks about us, the latter is owing to the number of poor fellows that have lately been thrown overboard. The ship is now truly miserable, many of our own crew very sick and the decks crowded with black slaves who are dying in all directions and apprehensive - their cases of fever are contagious."
A month earlier, Binstead had learned of the fear caused by the slave trade among Africans when he gave chase to a convoy of canoes on the Congo river as the British sailors looked for enslaved tribesmen. He wrote: "Observed many large canoes, one of which I went in chase of. On my coming up with her, the whole crew jumped overboard and I fear they met a watery grave. These poor wretches were fearful we were going to make slaves of them."
The diaries were donated to the museum by Binstead's great-great-granddaughter, 78-year-old Rosa Lee, whose mother discovered them after the Second World War. Ms Lee, from Maidenhead, Berkshire, said: "I'm so glad I kept the diaries and they are of interest and that they have found their natural home."
Some historians have argued that the West Africa Squadron, established a year after the 1807 Abolition of the Slave Trade Act, was as important as the campaigners led by William Wilberforce in bringing slavery to an end. Others point out that the squadron's anti-slavery role was twinned with the task of establishing British influence in West Africa and dominating maritime trade routes. The squadron was tasked with patrolling the 3,000 miles of the west African coast, blockading ports and giving chase to slave vessels.
Binstead described the "wretched state" in which slaves were discovered and the privations of his shipmates as they succumbed to diseases such as yellow fever and malaria. More than 1,500 naval ratings lost their lives over the next 60 years while serving in the squadron.
Initially, the unit was limited to intercepting only British slave ships but London eventually signed a host of bilateral arrangements, sub-contracting the Royal Navy to target slavers from all other countries.
The exhibition, Chasing Freedom: The Royal Navy and the Suppression of the Transatlantic Slave Trade, opens at the Royal Naval Museum in Portsmouth Historic Dockyard on Saturday.