Growth and condition of the Greenshell mussel, Perna canaliculus, in Big Glory Bay, Stewart Island: relationships with environmental parameters

Intensive aquaculture of the Greenshell™ mussel Perna canaliculus has occurred in Big Glory Bay, Stewart Island since 1987. The objective of this study was to investigate possible reasons for the reduction in mussel growth rates in the Bay that affected farmers during the late 1990's. The first part of the study was aimed at assessing temporal and spatial differences in water properties along a seven station transect from Foveaux Strait (FS), through Paterson Inlet (PI) and into Big Glory Bay (BGB). The second part of the study aimed to relate monthly changes in the growth and condition of mussels at three farm sites and two depths in BGB with measured water properties. Measurements made of the mussels included live weight, shell weight and length, wet and dry tissue weight. Water properties were measured by CTD profiler (temperature, salinity, chlorophyll-a), from laboratory analysis of water samples (dissolved reactive phosphate (DRP), ammonium, total organic nitrogen (TON), chlorophyll-a), and water clarity was measured using a Secchi disc. Monitoring occurred on a monthly basis for thirteen months from July 1999 to July 2000. There were clear seasonal cycles of temperature, chlorophyll-a, and TON at all stations· along the transect. There were also clear inter-annual differences in the temperature, salinity, chlorophyll-a, ammonium and TON at all stations between the samplings in July 1999 and July 2000. Statistically significant differences were found between the measured properties in FS, PI and BGB for many months of the study, illustrating that although BGB is reliant on PI and FS for water exchange, the relationship is limited and BGB shows signs of being self-regulating and independent. Comparisons between data collected in the present study and previous investigations in the area found no measurable change in chlorophyll-a and TON concentrations. Mussel growth during the study was very slow (10 mm in 12 months) due in part to their age (3 years) and size (80 mm) at the outset of the study, but also due to the seeding density (-400 mussels/ m). Mussels from 15 m depth grew faster than mussels at 5 m, and there were significant differences in growth rate between the three farm sites. Growth was fastest over the summer months, related to water temperature, but primarily food availability. Mussel condition increased through spring and summer, peaking in early autumn (March). Rapid loss of condition from March to June appears related to the reproductive cycle and decreasing levels of food. Mussel condition correlates strongly with water temperature and TON levels, which are co-variants in the relationship between mussel condition and phytoplankton productivity. Although inter-annual variability in the BGB system is high, this study did not find a significant change in environmental conditions that could account for the reduction in growth rate of mussels in BGB in the late 1990's. This would suggest that the dramatic increase in the number of farms in the bay, but more specifically seeding density and individual farm management, might be responsible for the slow growth rates experienced in the past few years