Spawning in the Pacific Oyster

Pacific oysters occur globally and spawn during the summer throughout their range, which in New Zealand means between November and March, the Southern Hemisphere summer.

Spawning is temperature dependent and begins when the water temperature reaches approximately 18-20ºC. Pacific oysters are a broadcast spawner, which means the eggs and sperm of mature oysters is released into the water column where they mix and the eggs are fertilized.

Fertilization produces free-swimming larvae that can spend up to three weeks looking for suitable substratum to settle on. Once settled the free-swimming larvae become spat which cement themselves to the substratum.

Pacific oysters have a very high growth rate (size), approximately 25mm per year, but can grow to over 75mm in their first 18 months. This growth occurs mainly during the summer once spawning has occurred. Growth is also temperature dependent (a function of nutrient abundance) and slows during the winter, May to September in the Southern Hemisphere, this is also when Pacific oyster are ‘in season’. During winter when water temperature is between 8-12ºC oysters ‘condition up’, this is where the oysters put their growth effort into gamete production – the production of eggs and sperm.

Conditioning in the Pacific Oyster

When the Pacific oyster is ‘in season’ they are often referred to as having ‘conditioned up’ or being in ‘good condition’. An oyster in good condition will have meat/mantle that fills the shell and is porcelain white in colour, the gills will be a dark shade of cream and the mantle edge will be highly defined and be dark brown or black in colour.

Pacific oysters are in good condition in New Zealand from July through to November, the beginning of Southern Hemisphere summer when they spawn. As spawning is temperature dependent oysters on one side of an oyster farm may spawn days to weeks before oysters on the other side of the same farm. Factors influencing this, i.e influencing water temperature experienced by an oyster, can include; height of the oysters from the sea bed; or exposure to different sea conditions.

During the early summer and the early stages of spawning oysters that do not spawn immediately are said to be ‘holding condition’. Oysters can hold their condition through to early January under some environmental conditions but this is uncommon and it is usually during November and December that oysters will ‘lose condition’.

An oyster that has lost condition is an oyster that has spawned. The mantle of the oyster will be opaque and appear to be ‘stringy’, these are the remains of eggs/sperm in the mantle, the overall size of the meat/mantle will decrease and the gills will appear opaque and watery.

Once an oyster has spawned it will begin the process of conditioning usually from March through to July. During this time water temperature is between 8-12ºC, the ideal temperature range for production of eggs and sperm. With the egg/sperm production the mantle will fatten and the gills will gradually lose the opaque/ watery appearance.

Clevedon Coast Oysters Summer Long-line Harvesting

During the warm summer months of January through March Clevedon Coast Oysters primarily harvest oysters that have been grown on our sub-tidal ‘long-line’ lease. Being sub-tidal the oysters on the long-line remain underwater for the duration of their time there, approximately ten weeks. The long-line differs from the inter-tidal leases where the oysters are exposed at low tide for up to eight hours a day.

The long-line offers a stable temperature environment for oysters during the summer as although the water temperature does increase it is gradual and the oysters do not experience the rapid fluctuations that occur on inter-tidal leases. Temperature fluctuations in the oysters are caused by the rising and falling tides and the effects of sea bed temperature and air temperature as well as wind and rainfall.