Effect of water flow and oxygen concentration on early settlement of the New Zealand green-lipped mussel, Perna canaliculus

Early settlement patterns of the mussel Perna canaliculus were investigated within water tanks exposed to different water flow regimes and oxygen concentrations. Hatchery-reared larvae and wild juvenile mussels (0.5-3.0 mm in maximum length) collected from the surf zone and subtidal habitats at Ninety Mile Beach, northern New Zealand, were used for the laboratory settlement experiments. Combinations of three water flow regimes (∼1, 5, and 10 cm/s) and three oxygen concentrations (∼6, 9, and 12 mg/L) were established within replicated tanks, each containing standardized, artificial filamentous substrata. The mortality and settlement of larvae and re-settlement of juveniles were recorded for each tank during a 24-h period. Mortality was highest at low water flows, and decreased consistently with increasing water flows for larvae (∼55-5%) and juvenile mussels (∼70-50% for subtidal, and 23-4% for surf zone mussels), but increasing oxygen concentration caused a decrease only in the mortality of the larvae, not the juveniles. For larvae, settlement increased with increasing water flow, both on the filaments and on the tank rims at the water surface, and higher oxygen concentrations appeared to enhance larval settlement. Hourly larval settlement counts within filaments suggest that exploratory behavior (i.e., settlement and re-settlement) takes place within low and medium water flows, but not under high water flows. However, settlement always was higher within filaments compared to the tank rims for larvae, and for juveniles collected from the surf zone and subtidal habitats. While oxygen concentration did not significantly affect settlement of juveniles, mussels collected from the surf zone had slightly higher settlement rates than those collected from subtidal habitats. This study further illustrates the complexity of larval and juvenile settlement and re-settlement processes for P. canaliculus. In particular, pre-harvesting history/environment, as well as the physical conditions present after harvesting, may determine survival and re-settlement of mussels within natural and farm conditions.