Corresponding authorAs a result of the deliberate introduction of Undaria pinnatifida (Harvey) Suringar into the Rance Estuary for aquaculture purposes, this kelp has now established itself naturally in the coastal area around St Malo. A population of U. pinnatifida in the Dinard area was subjected to a monthly monitoring programme, conducted in the lower eulittoral zone during low water of Spring tides. Tagging of thalli and punching of holes on the blades were methods used to estimate growth rate. Five parameters were measured: total length, stipe length, midrib width, sporophyll number and sporophyll width. During the 20 months of this study, 4 series of thalli (two per year) developed successively, in spring and autumn. Thalli appeared throughout the year, although very few were present during a period of three weeks, from the end of August to mid-September. Maximal daily growth rates occurred in MarchϪApril (21 mm/day) for spring thalli appearing from January to March, and in November (18 mm/day) for those appearing in October. The highest growth rate was observed both in the spring rise and autumnal fall of temperature and illumination. Thalli that appeared in autumn and winter produced the widest sporophylls. Thalli that appeared in spring and early summer gave rise to sporophylls less wide but more numerous. The maximal life-span noted in the field was 222 days (about 7.5 months); but most had a life-span of about 6 months. Maximal size observed was 1.90 m, with 1.470 g wet weight and 190 g dry weight.

No abstract