Measuring size and choosing category size for a transition matrix study of the seaweed Ascophyllum nodosum

A nondestructive method for measuring the size of the brown alga Ascophyllum nodosum (L.) Le Jol. has been modified in order to esbmate the dry weight of tagged ind~viduals for a size-based demographic study. There is a very strong positive correlation between the dry weight (dw) of an individual and a crude measurement of its volume. A total of 786 individuals were sampled on 3 occasions in 2 populations and the maximum length (1) and circumference (c) (in cm), and dry weight (in g) were measured. A model I1 regression was used to obtain the relationship between dry weight and lc2, and since there was a significant difference between spring and autumn measurements, 2 predictive equations were needed; for spring log,(dw) = 0.7900 log,(lc2) -2.8547 (r = 0.99), and for autumn log,(dw) = 0.8605 log,(lc2) 3 . 3 7 6 9 (r = 0.99). It is also possible to use this method on other species of seaweeds. Two methods for choosing the width of size classes were tested. One allowed the division of a population into subpopulations and the use of more than 2 census dates, and was found to be the most appropriate for this study. The first size-class was biologically distinct and was set to 0-5 g, which includes almost all nonfertile individuals. Four more were found by the algorithm and their lower limits are 5, 15, 54, and 190 g and these are used in the size-based demographic study.