Tracking the source of wild mussel spat for aquaculture using shell microchemistry and biophysical models

Abstract

The spatial and temporal uncertainty in availability and distribution of wild mussel seed is a major obstacle to sustainable production in mussel aquaculture globally. An improved understanding of patterns of dispersal of wild mussel spat could help to resolve this uncertainty to progress the management of wild spat resources and increase mussel seed supply. In this study, the natal locations and patterns of dispersal for recently settled green-lipped mussels, Perna canaliculus, at an offshore mussel farm in northern New Zealand were estimated using a combination of analysis of shell microchemistry of arriving spat and biophysical modelling. The shell microchemistry from mussel spat settling on the offshore mussel farm were compared with those from mussels cultured in situ at 22 surrounding sites (∼2–6 km apart) arranged around the Bay of Plenty, New Zealand, over two consecutive months. Six elemental signatures (Li, Co, Ni, Ba, La and U) were the primary drivers for correctly reassigning shells back to their individually generated sites with an overall 57.4% of Leave-one-out cross-validated (LOOCV) cases correctly assigned, ranging from a low of 10% for one coastal site (near Hikuwai Beach) and up to 78% for an offshore mussel spat collection site (near mussel farm). The microchemistry signatures were subsequently employed to backtrack ∼160 spat arriving on the mussel farm to two main natal sites (i.e., 7.5 and 8.0 km away) near Ōhiwa Harbour and known to have wild mussel reefs, which accounted for 85% of the mussels arriving on the mussel farm. Forward and backward simulations of movements of mussel spat using high resolution (i.e., 1 km) hydrodynamic particle tracking models indicated that most spat arriving on the mussel farm during the sampling period originated from the same stretch of coast identified by the shell microchemistry. The concordance of results from the two methods of determining natal sources provide mutual validation of the results and highlight the importance of managing coastal populations of wild mussels that are under increasing pressure from human activities, especially where they provide a critical seed source for aquaculture.