Small-scale mussel settlement patterns within morphologically distinct substrata at Ninety Mile Beach, northern New Zealand

Abstract

Microscale settlement patterns of juveniles of the mussel Perna canaliculus were investigated within drift material at Ninety Mile Beach, northern New Zealand. Size- and site-specific selectivity on various morphologically distinct algal and hydroid species were identified within drift material and corroborated in laboratory experiments with similar artificial substrata. Mussel spat densities were greater within fine-branching natural (similar to28-57%) and artificial materials (similar to13-20%) compared to medium- and coarse-branching natural (similar to7-8%) and artificial (similar to2-3%) materials. Size-frequency distributions of mussel spat within natural and artificial materials suggested a relationship of increasing mussel size with decreased branching of substrata. Field and laboratory investigations indicated higher settlement of 1.5-2.0 mm mussel size classes in coarse-branching substrata, whereas fine-branching substrata had greater settlement of mussels within the <0.5 mm size class. Mussel settlement comparisons within node and internode areas of all substrata in the field and in the laboratory indicated a strong preference of settlement in node areas over inter-node areas. The microscale settlement patterns observed in this study are argued to be indicative of a life strategy to maximize juvenile mussel survival within the dynamic environment of drift material in oceanic currents, before the potential arrival and re-settlement to rocky coastal areas. The present study is the first to elucidate settlement patterns of Perna canaliculus on drift material that washes up on Ninety Mile Beach, where >70 tonnes/year of this material is collected and supplied to the New Zealand aquaculture industry to seed mussel farms.