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 si ze-specific rates of growth and survival, their variability, and seaso nal and interannual variation in recruitment. Simulation of size distr ibutions can be used to gain insight into the underlying processes tha t give rise to observed size structure of organisms in the field, but the utility of this approach depends critically on underlying assumpti ons. Incorrect judgment of the significance of assumptions can lead to erroneous conclusions concerning the causes of bi- or polymodal distr ibutions. Using the Brody-Bertalanffy growth model and a constant surv ival rate, bi- and polymodal distributions can be generated when recru itment is pulsed. Even with as many as 10 recruitment episodes per yea r, 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 s pecies, pulsed rather than continuous recruitment would be the better assumption when interpreting the shapes of size distributions. Our sim ulations differ from those conducted by Barry & Tegner (1990) who assu med continuous and constant recruitment and focused on changing growth and survival parameters to explain bimodal size structure. These auth ors also suggested that their analysis was appropriate for interpretin g 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 continu ous settlement; rather, both species had pulses of settlement in sprin g 1990 and 1991. Although age-specific variation in growth or mortalit y parameters can result in bimodal size distributions, it is more like ly that such distributions are caused by seasonal pulses of recruitmen t.