Composition of beachcast material containing green-lipped mussel (Perna canaliculus) seed harvested for aquaculture in New Zealand

A globally unique aspect of the substantial mussel aquaculture industry in New Zealand is its major reliance on wild juvenile mussel seed that is harvested from beachcast material sporadically arriving at a single beach in the north of the country. Hundreds of tonnes of the seed material are harvested from Ninety Mile Beach each year and transported around the entire country to seed Greenshell (TM) mussel (Perna canaliculus) farms, which have a combined production that is valued at well over US$250M a year. Despite the enormous importance of this fished resource for supplying around 80% of the seed used by this aquaculture industry, relatively little is known about this natural seed source. In an effort to better understand the composition of the seed material and how it varies through time, harvested spat material was sampled during three annual harvesting seasons (2004-2005, 2014 and 2016) and sorted into seven fractions (i.e., mussels, macroalgae, sediment, plant material, shells, hydroids and other material), weighed, and the size composition of the mussels determined. Over the three years of sampling, macroalgae was the dominant component of the harvested material comprising on average 48.4 +/- 2.6% (S.E.) of the wet weight of the samples, followed by mussel seed (18.1 +/- 12.7%), sediment (16.2 +/- 1.8%), land plant material (7.7 +/- 1.6%), hydroids (5.8 +/- 1.3%), other material (2.7 +/- 0.5%) and shells (1.0 +/- 0.3%). The proportion of macroalgae by wet weight was similar among the three years of sampling (49.3%, 48.2% and 47.7% for 2004-2005, 2014 and 2016 respectively). The majority of the mussels collected in 2004-2005 were in the size class of 0.76-1.0mm in shell length, whereas in 2014 and 2016 the majority of the mussels were in the size class of 1.01-3.0mm in shell length. However, the components of the harvested material and the size of mussels it contained were highly variable both among and within years. Overall, the results provide valuable insight into possible ecological and environmental processes involved in spat arrival and will help with improving the management of this important wild mussel seed resource.