INFERRING DEMOGRAPHIC-PROCESSES FROM SIZE-FREQUENCY DISTRIBUTIONS - EFFECT OF PULSED RECRUITMENT ON SIMPLE-MODELS

The shape of a size-frequency distribution is the result of age- or size-specific rates of growth and survival, their variability, and seasonal and interannual variation in recruitment. Simulation of size distributions can be used to gain insight into the underlying processes that give rise to observed size structure of organisms in the field, but the utility of this approach depends critically on underlying assumptions. Incorrect judgment of the significance of assumptions can lead to erroneous conclusions concerning the causes of bi- or polymodal distributions. Using the Brody-Bertalanffy growth model and a constant survival rate, bi- and polymodal distributions can be generated when recruitment is pulsed. Even with as many as 10 recruitment episodes per year, size distributions show several modes. A sampling of the literature indicates that most fish and marine invertebrates have pulsed rather than continuous recruitment; thus, when very little is known about a species, pulsed rather than continuous recruitment would be the better assumption when interpreting the shapes of size distributions. Our simulations differ from those conducted by Barry & Tegner (1990) who assumed continuous and constant recruitment and focused on changing growth and survival parameters to explain bimodal size structure. These authors also suggested that their analysis was appropriate for interpreting the dynamics of red sea urchins Strongylocentrotus franciscanus. We have been documenting settlement of both red and purple (S. purpuratus) sea urchins. At La Jolla, California, neither species showed continuous settlement; rather, both species had pulses of settlement in spring 1990 and 1991. Although age-specific variation in growth or mortality parameters can result in bimodal size distributions, it is more likely that such distributions are caused by seasonal pulses of recruitment.