Determining the Environmental Drivers of Greenshell Mussel (Perna canaliculus) Spat Settlement

Abstract

Globally, mussel aquaculture production is often constrained by a limited supply of spat, the juvenile stage of mussels, used to initiate production. In some instances, the harvesting of wild spat on spat-collection ropes has the potential to quickly solve this acute shortage. However, the settlement of mussel spat on collectors is known to be highly variable and influenced by a range of environmental factors. Therefore, this study aimed to evaluate the variability of mussel spat settlement in relation to key environmental drivers, particularly those pertaining to larval supply and delivery (i.e., sea surface temperature and chlorophyll-α concentrations, significant wave height, and wind speed). A single length of 6-m spat-collection rope was deployed at a spat-collector farm in Whangapē Harbour, northern New Zealand, at 6 weekly intervals from August 2022 to December 2024 to monitor the settlement of Greenshell mussel (Perna canaliculus Gmelin, 1791) spat and the little black mussel (Xenostrobus pulex Lamarck, 1819). Greenshell spat settlement ranged from 750 to 39,000 spat m⁻¹ with no clear pattern in the abundance of settlers over time. Little black mussel settlement remained low (<650 m⁻¹) throughout the study, except in a single instance where settlement peaked at 14,000 spat m⁻¹. Comparison of the settlement abundance with four environmental variables (i.e., sea surface temperature, chlorophyll-α concentration, significant wave height, and wind speed) isolated sea surface temperature as the most robust predictor for settlement in both species, with settlement increasing with decreasing temperature. Specifically, Greenshell spat settlement was nearly three times higher when the mean sea surface temperature was below 16°C, likely reflecting seasonal spawning cycles of mussels. For operators of spat-collector farms, this means that tracking seasonal seawater temperature changes can help identify peak spawning periods and guide the timing of collector deployments to maximise spat catches.