Most sources give Fly's launch year as 1811, but not all. On 29 December 1811 Scyllla captured Fly. There being an HMS Fly in service, and the navy having lost its last HMS Sealark in 1809, Fly became HMS Sealark.
Lieutenant Thomas Warrand commissioned Sealark in May 1812 for the Lisbon station.
Sealark's most tumultuous moment came on 21 July 1812. That morning, alerted by a shore signal of the presence of an enemy vessel, Warrand set out and within an hour discovered a large lugger flying English colours but chasing and firing at two West Indiamen sailing up the Channel. Sealark caught up with the lugger and eventually an intense engagement ensued that lasted for an hour and a half before a boarding party from Sealark captured the enemy vessel. She was the Ville de Caen, of sixteen guns and 75 men. She belonged to Saint Malo but was just a day out of the Isle de Bas and had taken nothing; she was the same vessel that had fended off the lugger Sandwich at some earlier date.
The engagement was sanguinary. Sealark had seven men killed, and 21 wounded, including Warrand. Ville de Caen had 15 men killed, including her captain, M. Cocket, and 16 wounded. Lloyd's Patriotic Fund awarded Warrand an honour sword worth 50 guineas. In 1847 the Admiralty authorized the award of the Naval General Service Medal with clasp "Sealark 21 July 1812" to the four still surviving claimants from the action.