[9] Two areas are nature reserves managed by the Hampshire and Isle of Wight Wildlife Trust, Farlington Marshes[10] and Southmoor Nature Reserve[11] West of Portsmouth is Portsmouth Harbour and the three linked harbours are important recreational and conservation areas as well as supporting commercial fishing and shipping.

[12] The eastern boundary with Chichester Harbour is defined by a historic causeway known as the Wade Way, which was originally the only crossing between Hayling Island and the mainland.

With the end of the last ice age sea levels rose until sometime between 4000 and 3500BC the harbour took on the form it would have until the 18th century.

[16] The Domesday Book records three salthouses around the harbour[17] and by the early 17th century a saltern at Copnor was well established.

[19] Oyster farming began in the harbour around 1820 with winkle and clam cultivation probably starting around much the same time.

[23] During the Second World War the harbour was used as Starfish decoy site to misdirect German bombers.

[1][25] A ferry has operated between Portsmouth and Hayling Island across Langstone Harbour for more than 200 years; in 2022-23 it carried more than 47,000 passengers.

[28] American hard-shelled clams have been found in the harbour spreading from an initial release site on the lower River Test.

A decision was made to give the whale a lethal injection as a blood test revealed that it was suffering from kidney failure.

[31][32][33] The quality of water in the harbour was called into question when samples taken in 2022 were found to contain traces of numerous chemical compounds including prescribed medicines.

[36][37] The tug named the Irishman was sunk by a magnetic mine and now rests partially submerged at low tide.