A biological study of Durvillaea antarctica (Chamisso) Hariot and D. Willana Lindauer in New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The biology of Durvillaea Antarctica and D. willana, two New Zealand bull-kelp seaweeds, is discussed and the taxonomy and nomenclature of all Durvillaea species reviewed and clarified. Four species are recognised: D. potatorum (Labill.) Areschoug, D. antarctica (Chamisso) Hariot, D. willana Lindauer and D. chathamica n.sp. Morphological variation, population structure, and the effects of harvesting were studied by sampling primarily D. antarctica and secondarily D. willana. Reproductive periodicity, growth, mortality and longevity were studied by regularly monitoring marked plants. Morphology is extremely plastic and largely determined by wave force and grazing. There is little seasonal changed in the size structure of D. antarctica populations, although this varies with wave exposure. In New Zealand both species have a winter (April-August) fruiting season, with a midwinter peak. Evidence suggests a latitudinal lengthening of the reproductive season. Individual plants produce one crop of conceptacles annually. It takes at least two years for plants to become fertile. Regrowth of D. antarctica on experimentally cleared sites corresponded with the period of gamete release. Regrowth was primarily caused by fresh recruitment and not by rapid growth of previous dormant small or microscopic plants. Regrowth on sites cleared outside the Durvillaea fruiting season was very slow because most available space became occupied by other algae. Growth rates are highly variable. Net growth is fastest in spring and summer, i.e. after the fruiting season. Plant damage and mortality was highest in winter. Both species may live ten years or longer. Culling increase the growth rate of understorey plants, although fastest regrowth was obtained by harvesting when recruitment was imminent. Mean standing crop of D. antarctica was 123.5 tonnes/km of shore. Values varied from 47 tonnes/km on steep shores to 190 tonnes/km on wide flat reefs. There is insufficient D. antarctica on the South Island east coast to provide the minimum commercial requirement of 30,000 wet tonnes per annum. Methods of small-scale harvesting are discussed.