THE RELATIONSHIP BETWEEN FARMED MUSSELS, PERNA-CANALICULUS, AND AVAILABLE FOOD IN PELORUS-KENEPURU SOUND, NEW-ZEALAND, 1983-1985

Condition index [CI= (meat weight x 100)/(total weight-shell weight)] of green mussels grown on longlines was monitored monthly for 2 years at 12 commercial farms in Pelorus-Kenepuru Sound. Water temperature and salinity, total particulate matter, chlorophyll a and particulate carbon were recorded at approximately 2-monthly intervals at six of the 12 CI stations. CI showed an annual cycle (high in autumn/winter/spring and low in summer) with year-to-year differences in amplitude, and a regional pattern (lowest at the most seaward stations, highest towards the upper reaches of the Sound). The environmental data paralleled the condition index data by showing gradients along the length of the Sound. Anomalies in the regional patterns resulted from localised environmental conditions occurring in major embayments off the main channel. On an industry-wide scale abiotic factors (temperature and salinity) were more highly correlated with mussel condition than factors measuring available food (chlorophyll a, carbon and particulates). However, at lower ambient food levels changes in food concentration were strongly associated with changes in CI. A food threshold, at about 200-mu-g C l-1. was identified, below which CI declined significantly. The variation in condition between years, months, and farms represents differences in meat yield which are economically significant.