Evaluation of four models used to estimate kelp productivity from growth measurements

Four models have been proposed in the literature to estimate production of kelp blade material from length increments. In order to evaluate them, we obtained direct measurements of blade production by weighing Laminaria longicruris plants before and after a 24 d field experiment. The best model was that of Mann & Mann (1981) who proposed that blade production was equal to growth times unit blade weight in the area of maximum blade biomass. This model however overestimated actual production by 20 to 40% depending on whether overall sample production or individual blade production was considered. Similar results were obtained with a model based on unit weight of entire blades. The estimates generated by 2 models based on weight-length relations exceeded actual production by up to 100%. We discuss the influence of blade size and blade productivity on the performance of each model.