Oyster feasts and famines - food for thought
Science Centres: Fisheries
Factors affecting productivity of northern harbours.
Oyster growers want to know what makes good and bad fattening years for oysters. So what’s the source of the nutrients that feed the algae which feed the oysters?
Studies of three harbours in the North Island are providing the answers. For example, in Coromandel Harbour, depletion of nutrients in the outer Hauraki Gulf is linked to nutrient declines inside the harbour. Climatic events like the El Niño-La Niña oscillations can determine whether nutrients feed onshore. During a La Niña event in 1999, persistent southeasterly winds caused a ten-fold reduction in nitrates due to onshore winds stopping offshore upwelling of nutrient rich water.
Mahurangi Harbour, however, appears to behave quite differently. Productivity patterns there appear to fluctuate more, especially after heavy rainfall. We think nutrient levels build up on the surrounding farmland, and are discharged to the catchment only after the soil moisture levels reach saturation point after persistent rain. Alternatively, rainfall discharge may flow over the denser saltwater and, through a process of "advection", suck in nutrient-rich water from outside the harbour.
Kaipara Harbour, on the west coast, is different again. The dominant driving force there is probably a combination of high offshore productivity, abundant nutrient-rich, land-based runoff, and the sheer volume of water flowing in and out of this harbour over shallow, wind-swept mud and sand flats. The Kaipara has, at times, almost double the nitrate levels of Mahurangi Harbour.
Understanding these processes will help farmers to stock their farms appropriately, as well as addressing any sustainability issues concerning overstocking of growing areas. However, long-term data sets are invaluable as it’s no good developing sustainability models based only on data from the good years, leaving farmers unable to sell oysters in the lean years. In the future, we hope to be able to predict just how many oysters to put on the farms for predicted climate scenarios.
We are indebted to Biomarine Ltd and Pacific Marine Farms Ltd for collecting data for these studies.