In his book “A History of the Oyster and the Oyster Fisheries” published in 1858, author Thomas Cambell Eyton lists Poolbeg on the south side of Dublin’s River Liffey as one of Ireland’s main oyster fisheries. There were also oyster fisheries at Malahide, Sutton and Poolbeg.
Dublin’s relationship with the oyster goes back to its original settlers. Viking excavations at Wood Quay in the 1980s revealed mounds of oyster shells known as ‘middens’ left behind by the Norse invaders. The ‘Native’ or European oyster, ‘ostrea edulis’, was an easily harvestable protein source that thrived at the mouth of the Liffey.
People have fed on oysters so long that the man whom Jonathan Swift called brave for eating the first one is quite out of range of history’s eye. His name was Sergia Orata, a Roman engineer who lived in the first century BC and cultivated oysters in southern Italian lakes by bringing them to spawn on rock piles that he surrounded with twigs, which the cultivator could monitor easily. When the oysters grew to marketable size, they were plucked off and sold.
The reason for the tradition of not eating an oyster in a month without an ‘r’ is to do with the spawning season, which can span from May to August. During these months, the fish are somewhat weaker and the meat tends to be thin and flat-tasting.
They spawn by releasing gametes into the water. A female releases clouds of eggs in a series of wet piffs, while males send forth sperm in a stream. Fertilisation occurs when opposite gametes meet in the water; hence the advantage offered by proximity.
The spawning process takes about 45 minutes during which time a female will emit anything from 10,000 to 60m eggs, only a small fraction of which will be lucky enough to meet their mates. Once the pair of gametes meet, they connect and become a larva that drifts in the tidal current, propelling itself by means of a little organ ringed with cillia called a velium. This is an oyster’s only taste of free movement.
When the larva grows to a size of 300 microns, it extends its foot and seeks a suitable surface on which to set. Having found one, it grows into a spat, which already resembles an oyster, the shape of its shell already visible. It prefers settling on hard, chalky surfaces and given a choice will settle on an old oyster shell.
Overfishing combined with pollution has done huge damage to Dublin’s oyster population. As far as the health of marine ecosystems go, perhaps no single pollutant does more damage than nitrogen. It occurs naturally in human and animal waste.
Fossil fuel combustion produces nitrogen oxides which rise into the atmosphere and come down as nitric acid. In the water, nitrogen serves as a major nutrient for microscopic organisms called phytoplankton.
Individually, they are invisible to the human eye but when present in quantities they cause massive blooms, preventing the sunlight getting down to the plants that need the sun’s energy. When these phytoplankton die, they like all organic matter are eaten by bacteria which deplete the vital oxygen from the water, thus causing more damage to the sealife below.
Fortunately, few species filter nitrogen from the water as effectively as oysters. The oyster is pretty particular about what it eats– not so about what it filters! A single oyster can filter 50 gallons of water per day. As the oyster eats plankton, it draws in everything around it, including nitrogen.
What it does not eat, it expels as solid pellets of waste, which eventually decompose and bubble up into the atmosphere as nitrogen. Maybe it is time to restock the traditional Poolbeg oysterbeds and now with the treatment plant working, this remarkable creature can keep doing what its always done–keep the waters clean and healthy, and provide a sustainable protein source.