Abstract
Mussels (Perna canaliculus) (Gmelin, 1791) collected from mid and low tide rocky platforms along the east coast of the South Island of New Zealand differed in dry weight condition index between shore levels. Mussel condition was c. 15% lower at mid tide level than the low tide level and there was greater variation between shore level than between sites. Feeding experiments were conducted at 15°C with mussels collected from mid and low tide levels using Isochrysis galbana as the food. Clearance rate depended upon body size, shore level, and season. During summer clearance rates of mid and low tide mussels were similar. However in winter, increased filtration rates were recorded in mussels from the mid tide level compared with those from the low tide. In addition, for mussels from both shore levels, winter rates were higher than those recorded in the summer. Nitrogen excretion by mussels also showed seasonal and shore level differences. High values for ammonia excretion of mussels collected from the mid tide during winter suggests they utilise metabolic reserves to maintain their basal metabolism. Although our results suggest that increased filtration rates for mid tide mussels might compensate for reduced feeding time, in the winter, larger mussels from the mid tide level would be unlikely to obtain sufficient energy for maintenance or growth. Large mussels were infrequent in the mid tide population, where the mean length was 62.5 mm compared with 75.1 mm from the low tide. It is concluded that nutrient limitation, as a result of reduced feeding time can act as a significant stress factor limiting the growth potential of mussels settled at the mid tide level.