Condition cycle of the green-lipped mussel Perna canaliculus in New Zealand

Abstract

The condition of the green-lipped mussel Perna canaliculus (Gmelin) from 7 sites around the northern half of New Zealand was determined monthly between December 1973 and February 1975. Seven separate estimates of condition were calculated for each sample; 4 condition indices, 2 percentage glycogen analyses and 1 estimate of percentage solids. All showed a similar annual cycle, with minimum values in winter (June/ August) rising to peak levels in spring (October/ December). The 7 sites were divided into 3 groups, (i) a central New Zealand group, characterised by high levels of condition index (CI) throughout most of the year >10 CIweight for 8 to 11 months, average to high percentage glycogen (annualmeans 25 to 35% dry weight), and high percentage solids (annual means > 20% dry weight); (ii) an intertidal group, having low levels of CI (<10 CIweight for 10 to 11 months), low percentage glycogen(mean <20%), and low percentage solids (mean <18%); and (iii) a northern group, showing intermediate values for each of the estimates of condition. Difficulties in defining the causes of changes in the condition cycle are discussed. There is an inverse relationship between CI and mussel size, resulting from changes in body proportions. An inverse relationship is also suggested between CI and water temperature. A precise index, $$CI_{weight} = \frac{{100 \times dry meat weight}}{{whole{\text{ weight - shell weight}}}}$$ is recommended for biological studies on mussels and a rapid simple index, $$CI_{commercial} = \frac{{100 \times wet meat weight}}{{whole{\text{ (live) weight}}}}$$ for use in the field and in mussel farming practice.