Elemental signatures in the shells of early juvenile green-lipped mussels (Perna canaliculus) and their potential use for larval tracking

The seed for New Zealand's US$150 million green-lipped mussel (Perna canaliculus) aquaculture industry is almost entirely supplied by harvesting wild early juveniles (spat). The location of the broodstock populations producing these seed mussels is unknown because the larvae may have travelled >100km before settling. Identifying these broodstock populations via larval tracking is critically important to assuring the sustainability of this mussel industry, yet larval tracking is not easily performed with currently available methods. However, deducing elemental signatures of juvenile shells via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has the potential to identify the natal origins of P. canaliculus larvae, yet validation of the technique work must be performed for each species before this can be achieved. To this end, eleven elemental ratios were analysed for early juvenile P. canaliculus shells collected from six sites from the west coast of the Northland and Auckland regions of northern New Zealand. Of these 11 ratios, seven (Zn:Ca, Mn:Ca, B:Ca, Sr:Ca, Mg:Ca, Ba:Ca and Cu:Ca) exhibited sufficient spatial variation for discriminate function analyses (DFA) to assign the juvenile mussels back to their region and site of collection 63–100% of the time. However, among open coast sites DFA analyses were unable to distinguish between juvenile mussels taken from sites that were 11km apart, revealing that there are limits in the resolving power of elemental signatures for P. canaliculus. Collections of early juveniles at one site (Maori Bay) at four different times over six months revealed temporal stability in elemental signatures, with early juveniles able to be correctly assigned to the collection location regardless of month of collection. Now that such information has been obtained we are better positioned to track dispersing P. canaliculus larvae via elemental signatures and ultimately locate the source wild broodstock populations currently supplying the majority of the spat for New Zealand's mussel industry.