Temporal patterns of arrival of beachcast green-lipped mussel ( Perna canaliculus) spat harvested for aquaculture in New Zealand and its relationship with hydrodynamic and meteorological conditions

Abstract

The substantial Greenshell mussel aquaculture industry in New Zealand is heavily reliant on one major source of wild spat at Ninety Mile Beach, at the northern end of the country. The spat of Perna canaliculus arrives intermittently at the beach attached to seaweed and other debris whereupon it is harvested for seeding mussel farms around the country. Periods of low or non-existent spat arrival at the beach have caused major disruption to the aquaculture production of this species. In an attempt to better understand the daily, monthly, and inter-annual patterns in the arrival of spat at Ninety Mile Beach, the harvesting records of spat harvesters for 1990 to 1999 were analyzed in relation to historical records of wind speed and direction, tidal range, water temperature, and modeled swell height and direction. For the long-term data set, spatfall events and the amount of spatfall increased markedly with strong offshore winds. On days with high tidal range, there tended to be an increase in the amount of spatfall, but this trend was not significant statistically. Daily and seasonal water temperature records did not show a significant effect on the timing or the scale of spatfall events. However, low swell height in the onshore direction was associated with a significant increase in spatfall events and amounts. Within the 9 year data set, storm events (wind speeds > 20 m s − 1 ) were most frequent during May to October. An average lag time of 4 months was found between peak storm events and the subsequent peak in spatfall events and amounts of spatfall occurring in September to October. Years with a greater number of storm events were also associated with significantly higher number of spatfall events and amounts of spatfall. Storminess and water temperature are associated with El Niño/La Niña episodes, which greatly influence the wind climate of New Zealand. During El Niño periods mussel farm managers could greatly reduce their risk of a shortfall in natural spat supply interrupting mussel production by securing sufficient spat to stock their farms from the large but less frequent spatfall events. Overall, the results provide valuable insight into possible ecological and oceanographic processes involved in spat arrival and will help with better utilization of the spat resource for this major mussel aquaculture industry.