Spatial variability in reproductive behaviour of green-lipped mussel populations of northern New Zealand

Abstract

The reproductive behaviour of four intertidal and two subtidal mussel populations were investigated within two spatial scales at Ninety Mile Beach, northern New Zealand, where large quantities of seed mussels are collected for aquaculture. Reproductive cycles were compared between intertidal and subtidal populations and among intertidal populations along a 90-km coastal area. Histological sections of mussel gonads were used to identify the reproductive state of individual mussels collected monthly between July 1998 and April 2000. Gonad indices were calculated for populations each month at each site. Frequency distributions of different gametogenic stages were plotted and differences in reproductive cycle among populations were identified. Mussel shell sizes were also recorded and compared among populations. Although all intertidal and subtidal populations had similar temporal variations in reproductive behaviour, spatial patterns among various sites revealed clear differences in reproductive cycles among populations. In particular, mussel shell size and gonad indices were higher for subtidal sites compared to their intertidal counterparts. Among intertidal mussel populations, the northernmost population had the highest maximum shell length, gonad indices and percentage ripe and spawning individuals, followed by two populations at the southern end of Ninety Mile Beach. A small mid-beach mussel population had the smallest mussels with the lowest gonad indices and few spawning mussels during the reproductive season. A prolonged spawning period was observed within the most reproductively active populations (two subtidal populations at the south end and one intertidal population at the north end of the beach) between June and December. However, the reproductive cycle of two intertidal populations at the south end of the beach indicated 2–3 spawning events during the same period. Differences in mussel shell size and reproductive behaviour among sites are likely to be related to variations in environmental conditions along Ninety Mile Beach.